In pancreatic digestion, proteids are exposed to the action of an enzyme of much greater power than pepsin, one endowed with a far greater range of activity, and consequently proteolysis as it occurs in the small intestine becomes a broader and more complicated process. As you well know, the ferment trypsin manifests its power not only in a more rapid transformation of insoluble proteids into soluble and diffusible products, but there is a diversity in the character of the many products formed which testifies to the profound alterations this ferment is capable of producing. The primary and secondary products of pepsin-proteolysis, as well as unaltered proteids, are alike subject to these changes, and bodies of the simplest constitution may result in both cases from the series of hydrolytic changes set in motion by this proteolytic enzyme. The power of the ferment as a contact agent is astonishing, for in the case of trypsin no accessory body is necessary to bring out its latent power. Water, proteid, and the enzyme at the body-temperature are all that is necessary to call forth prompt and energetic hydrolytic action.

Moreover, hydrolysis does not stop with the mere production of soluble proteoses and peptones, but the hemi-portion of the latter is quickly broken down into crystalline bodies, such as leucin, tyrosin, lysin, lysatin, etc. This special characteristic of the ferment testifies in no uncertain manner to the existence of inherent qualities in the inner structure of the enzyme peculiar to the body itself. In general properties and reactions, pepsin and trypsin may be closely related; both are products of the katabolic action of specific protoplasmic cells, but the inner nature or structure of the two must be quite different. Pepsin, as we have seen, is powerless to produce any change in proteid bodies unless acids are present to lend their aid. Furthermore, pepsin is limited in its action to the production of proteoses and peptones, while trypsin gives rise to a series of hydrolytic cleavages which result in the ultimate formation of comparatively simple bodies.

Trypsin, however, in its natural environment is dissolved in an alkaline medium. Its proteolytic action is therefore carried on, under normal circumstances, in an alkaline-reacting fluid containing 0.5 to 1 per cent. sodium carbonate, and the proteolytic power of the ferment is unquestionably manifested to the best advantage in such a medium. At the same time, it will act, and act vigorously, in a neutral fluid, and likewise in a fluid having a weak acid reaction, provided there is little or no free acid present. Thus, in experiments168 on blood-fibrin it was found that, while a solution of trypsin containing 0.5 per cent. sodium carbonate, digested or dissolved 89 per cent. of the proteid in three to four hours at 40°C., a perfectly neutral solution of the ferment, otherwise under exactly the same conditions, digested 76 per cent., and a 0.1 per cent. salicylic acid-solution of the enzyme converted 43 per cent. of the proteid into soluble products.

With hydrochloric acid, trypsin is quickly destroyed, unless there is a large excess of proteid matter present,169 which obviously means that the acid in such case exists wholly as combined acid. Indeed, experiments made in my laboratory have shown that as soon as free acid, especially hydrochloric acid, is present in a solution containing trypsin, then proteolytic action is at once stopped. When, however, acids, especially organic acids, are present in a digestive mixture containing an excess of proteid matter, so that the solution contains no free acid (or better, with the proteid matter only partially saturated with acid) then trypsin will continue to manifest its peculiar proteolytic power, although to a considerably lessened extent. Hence, it is evident that the ferment may exert its digestive power under the three possible sets of conditions which, under varying circumstances, frequently prevail in the small intestine.

In considering the general phenomena of proteolysis by trypsin, one is especially impressed by the large and rapid formation of peptone which almost invariably results from the action of a moderately strong solution of the ferment, on nearly every form of proteid matter. To be sure, primary products are first formed, but these are quickly converted into peptone, and a little experience in studying the action of pepsin and trypsin soon reveals the fact that the latter is especially a peptone-forming ferment. In other words, it is peculiarly adapted to take up the work where it has been left by pepsin and, if necessary, carry forward the hydrolytic change even to the extent of a conversion of the entire hemi-moiety into crystalline products.

The primary products of trypsin-proteolysis, however, are not exactly identical with those formed by pepsin. Thus, protoproteoses and hetero­proteoses seldom appear in an alkaline trypsin digestion; the proteid matter being in most cases, at least, directly converted into soluble deutero­proteoses,170 which are then transformed by the further action of the ferment into peptones and other products. Hence, we may express the order of events in the trypsin digestion of a native proteid as follows:

In the digestion of fresh blood-fibrin with trypsin, there is plainly a preliminary solution of the proteid without any marked transformation or cleavage occurring, the soluble product being apparently a globulin, coagulating at about 75°C.,171 viz., at approximately the same temperature as serum-globulin. This body, however, quickly disappears, giving place to true deutero­proteoses as the ferment-action commences; for it is not probable that this globulin is a product of enzyme-action, but rather represents a simple solution of the fibrin by the alkaline fluid and salts. In any event, this globulin-like substance is not formed in the pancreatic digestion of coagulated-albumin, serum-albumin, or vitellin, and hence cannot be considered as a true product of trypsin-proteolysis.

The fact that deutero­proteoses are the primary products of trypsin-digestion again emphasizes the natural adaptability of this ferment to the part it has to play in the digestive process. Its natural function is to take up the work where left by pepsin, and carry it forward to the necessary point; and hence, when acting upon a native proteid the primary products of its action correspond to the secondary products of pepsin-proteolysis. Trypsin is thus equally efficient in the digestion of all native proteids, but the products of such action are always deutero­proteoses, peptones, and crystalline amido-acids. It is to be remembered, however, that in trypsin-proteolysis the deutero­proteoses and the amphopeptones must necessarily be represented by bodies in which there is a preponderance of anti-groups. In pepsin-proteolysis, as we have seen, the hemi- and anti-groups of the proteid molecule remain more or less united, but in pancreatic digestion, the formation of amphopeptone is quickly followed by the breaking down of a portion of the hemipeptone into leucin, tyrosin, etc. thus leaving a larger proportion of the anti-moiety in the remaining amphopeptone.

Theoretically, at least, in the-case of a vigorous and long-continued pancreatic digestion, all of the hemipeptone formed from any native proteid can be converted into crystalline and other products, thus leaving a true antipeptone resistant to the further action of trypsin. Hence, we are prone to speak of the peptone of pancreatic digestion as antipeptone, although, as can be readily seen, the exact nature of the peptone, i.e., the relative proportion of hemi- and anti-groups it contains, will obviously depend upon the length of the digestion and the strength of the ferment. Again, it is possible, as certain facts seem to suggest, that the amido-acids which are so readily formed from hemipeptone may come in part directly from the hydration of a portion of the hemi­deutero­proteose, without passing through the preliminary stage of hemipeptone. If so, we have another source of variation in the relative proportion of hemi- and anti-moieties in the deutero­proteoses and peptones of pancreatic digestion. Still again, it is to be remembered that in normal digestive proteolysis, as it occurs in the living intestinal tract, the proteid matter to be acted upon has already passed through certain preliminary stages in, its transit through the stomach, as a result of which still further variations in the proportion of hemi- and anti-groups may be possible.

It is thus plainly evident, in view of the ready cleavage of the hemi-group into amido-acids, that the primary products of trypsin-proteolysis, the proteoses and peptones, must necessarily be composed in great part of those complex and semi-resistant atoms which we include under the head of the anti-group. However much one may be skeptical about the real existence of so-called hemi- and anti-groups, there is no gainsaying the fact that a given weight of native proteid, like egg-albumin or blood-fibrin, cannot be converted wholly into crystalline or other simple products by trypsin; indeed, it is quite significant that at the end of a long-continued treatment with an alkaline solution of the pancreatic ferment, there is usually found about fifty per cent, of peptone, while the other fifty per cent. of the proteid is represented mainly by more soluble products, such as the amido-acids. It is also significant that the peptone obtained from an artificial pancreatic digestion, where the proteolytic action has been long-continued and vigorous, resists the further action of the ferment. In other words, it is the so-called antipeptone. In line with this result is the fact that the peptones formed in pepsin-proteolysis, when treated with an alkaline solution of trypsin, are converted into amido-acids and other bodies of simple constitution to the extent of about fifty per cent. This is easily explainable on the ground that the hemi-portions of the above peptones are broken down into simple products, while the anti-portions remain unchanged, being resistant to the ferment and thus leading to a separation of the two groups, or at least to the isolation of the anti-molecules.

There is much that might be cited in further support of these views, but doubtless I have said enough to make it plainly evident that in the pancreatic digestion of any native proteid, not more than one-half can at the most be transformed into crystalline products, while the other half will be represented mainly by a peptone incapable of further change by trypsin. Similarly, the products of pepsin-proteolysis exposed to the action of trypsin may undergo a like separation, the hemi-groups only breaking down into simple products. Hence, the whole theory of the hemi- and anti-moieties of the proteid molecule means simply that of the many complex atoms composing the molecule, one-half are easily decomposable by the pancreatic ferment, while the other half are more resistant and make up the so-called anti-group.

In any active pancreatic digestion of either a native proteid, or of the products of pepsin-proteolysis, the anti-group is represented mainly by antipeptone, although there is often found a small amount of a peculiar antialbumid-like body, insoluble in the weak alkaline fluid. Antipeptones, thus far studied, when entirely free from proteoses, are characterized by a low content of carbon, like the amphopeptones from pepsin-proteolysis. The following table shows the composition of a few typical examples:

COMPOSITION OF ANTIPEPTONES.

From these data it is evident that, while each individual peptone may have a composition peculiar to itself, they are all alike in possessing a relatively low content of carbon. The antialbumid, however, split off in these hydrolytic changes, like the antialbumid formed by the action of dilute acids at 100°C., is characterized by a correspond­ingly high content of carbon and a low content of nitrogen. As an illustration, may be mentioned the myosin-antialbumid formed in the digestion of myosin from muscle-tissue by an alkaline trypsin-solution. This body contains 57.48 per cent. of carbon, 7.67 per cent. of hydrogen, 13.94 per cent. of nitrogen, 1.32 per cent. of sulphur, and 19.59 per cent. of oxygen.177 It is only necessary to compare these figures with those expressive of the composition of myosin-antipeptone, to appreciate how wide a gap there is between these two products of trypsin-proteolysis, and both members of the anti-group. Antialbumid, however, is a peculiar product, one which is liable to crop out somewhat unexpectedly, and with varying shades of resistance toward the proteolytic ferments. As formed in pepsin-proteolysis, it is more or less readily soluble in sodium carbonate, and in part readily convertible into antipeptone by trypsin. Still, the same substance, or at least a closely related body, makes its appearance in the form of an insoluble residue whenever a native proteid is digested by trypsin. At times, the amount of this insoluble product may be quite large, even reaching to one-fourth of the total proteid matter;178 but when so formed in the intestine it must entail a heavy loss of nutriment, for whenever the anti-group is split off after this fashion it becomes very resistant to the further action of the ferment. Separating in this manner from an artificial digestive mixture, it may be dissolved in dilute caustic alkali, reprecipitated by neutralization, and then once again brought into solution with dilute sodium carbonate. In this form, it will yield some antipeptone by the further action of trypsin, although even then a large amount of the antialbumid is prone to separate out as a gelatinous coagulum, more or less resistant to the further action of the ferment.

These two bodies are therefore representatives of two distinct groups or radicals present in the hemi-portion of the proteid molecule; the first belonging to the fatty acid series, the second to the aromatic group from which come such well-known bodies as indol, skatol, benzoic acid, and other substances prominent in proteid metabolism. Moreover, these two hydrolytic products of trypsin-proteolysis are formed in considerable quantity, at least in an artificial digestion. Thus, KÜhne has reported the finding of 9.1 per cent. of leucin and 3.8 per cent. of tyrosin as the result of a typical digestion, and I have tried many similar experiments with like results. Further, we know from observations made by different investigators that both leucin and tyrosin may be formed in considerable quantities in trypsin-proteolysis as it occurs in the living intestine. But to this point we shall return later on.

Besides leucin and tyrosin, aspartic acid and glutamic acid have long been known as decomposition-products of the vegetable proteids. Thus, both acids were discovered by Ritthausen and Kreusler179 in the cleavage of such proteids by boiling dilute acid. Hlasiwetz and Habermann180 likewise obtained aspartic acid in large quantity by the breaking down of animal proteids under the influence of bromine. Further, Siegfried181 has recently obtained glutamic acid as a product of the decomposition of the phosphorus-containing proteid, reticulin, from adenoid tissue. As products of trypsin-proteolysis, Salkowski and Radziejewski182 found aspartic acid in the digestion of blood-fibrin; and v. Knieriem183 likewise obtained it in the digestion of gluten from wheat. Both of these acids belong to the fatty acid series, the aspartic acid being a dibasic acid, COOH.CH₂CH(NH₂).COOH, or amido-succinic acid, while glutamic acid, COOH.C₃H₅(NH₂).COOH, is likewise a dibasic acid, known as amido-pyrotartaric acid.

Of more interest physiologically, are the recently discovered nitrogenous bases lysin and lysatinin, or lysatin. These two bodies were first identified by Drechsel184 and his co-workers as products of the decomposition of various proteids, when the latter are boiled with hydrochloric acid and stannous chloride. They were first obtained by Drechsel as cleavage products of casein.185 Later, Ernst Fischer,186 working under Drechsel’s direction, separated them as decomposition-products of gelatin; while Siegfried187 obtained them as products of the cleavage of conglutin, gluten-fibrin, hemiprotein, and egg-albumin, by boiling with hydrochloric acid and stannous chloride. In all of these cases it is obvious, from the method of treatment pursued, that the two bodies result from a simple hydrolytic cleavage of the proteid molecule. Hence, it might be assumed that these two bases would likewise be formed in trypsin-proteolysis. This assumption, Hedin,188 working in Drechsel’s laboratory, has proved to be correct, and furthermore he has shown that the amount of these bases formed in pancreatic digestion is not inconsiderable. Thus, as products of the digestion of three kilos. of moist blood-fibrin with an alkaline solution of trypsin, 28 grammes of pure platino-chloride of lysin were obtained, and sufficient lysatinin to establish its identity.

Lysin has the composition of C₆H₁₄N₂O₂, being a diamido-caproic acid, a homologue of diamido-valerianic acid. Hence, this body, like leucin or amido-caproic acid, is a representative of the fatty acid group, the chemical relationship between the two bodies being plainly apparent from their constitution. The constitution of lysatinin is less definitely settled, but apparently it has the composition of a creatin, its formula being C₆H₁₃N₃O₂, in which case it might be more appropriately termed lysatin. The special point of interest, however, connected with this latter body as a product of trypsin-proteolysis is the fact that by simple hydrolytic decomposition, all chance of oxidation being excluded, it can break down into urea.189 For years, chemists have been seeking to trace out the line of cleavage or decomposition by which urea results in proteid metabolism. In the nutritional changes of the body, nearly all the nitrogen of the ingested proteid food is excreted in the form of urea, but chemists working with dead food-albumin have been heretofore unable to break down proteid matter directly into urea. This, however, Drechsel has now succeeded in doing, and it is to be especially noted that the line of decomposition or cleavage is simply one of hydration, in which the proteid molecule, either through the action of boiling dilute acids, or through the more subtle influence of the hydrolytic enzyme, trypsin, is gradually broken down into cleavage products, from one or more of which comes lysatin. The very resemblance of this body to creatin suggested that, since the latter breaks down into urea and sarcosin when boiled with baryta water, lysatin might possibly behave in a similar manner. This, as has been previously stated, was found to be the case, and Drechsel obtained from ten grammes of a double salt of lysatin and silver one gramme of urea nitrate, by simple boiling with baryta water.

It is thus evident that a certain amount of urea may come from the more or less direct hydrolysis of proteid matter in the intestinal canal, all but the last steps in the process being the result of the ordinary cleavage processes incidental to trypsin-proteolysis. This fact affords additional evidence of the profound changes set in motion by this proteolytic enzyme. It is not, of course, to be understood that all the urea formed in the body has its origin in this manner. Such a method of decomposition taking place in the intestinal tract would be exceedingly unphysio­logical and wasteful, but we can readily see how such a line of cleavage might result in inestimable gain to the economy in cases where excess of proteid food has been ingested. Under such circumstances, a portion of the surplus might be broken down directly in the intestine into this urea-antecedent, and thus quickly removed from the system with a minimum amount of effort on the part of the economy. Drechsel estimates that about one-ninth of the urea daily excreted may come from the direct decomposition of lysatin, the latter obviously having its origin in trypsin-proteolysis.

Another product of trypsin-proteolysis which has long been recognized, although its real nature has not been known, is tryptophan or proteino­chromogen. This body is not only a product of the pancreatic digestion of proteids, but it is also formed whenever native proteids are broken down through any influence whatever, the substance coming presumably from the hemi-moiety of the molecule. It is especially characterized by the bright-colored compound it forms with either chlorine or bromine, so that for a long time it went by the mystical name of the “bromine body.” When brought in contact with either of these agents, it immediately combines with them to form a new compound of an intense violet color, termed proteinochrome. This constitutes the usual test for its presence, a little bromine water, for example, quickly bringing out a violet color when added to a fluid containing the chromogen. The body is readily soluble in alcohol, and hence can be easily separated from the primary products of trypsin-proteolysis, such as the proteoses and peptones. Krukenberg considered the substance not a true proteid, but rather a body belonging to the indigo-group; but Stadelmann, who has given the matter a very thorough investigation, comes to the conclusion that it is truly a proteid body, in part closely related to peptone, although in many ways quite different.

The following composition of bromine proteinochrome, as determined by Stadelmann,190 shows the general nature of the compound formed when bromine combines with the chromogen:

From the average of the several results obtained, it would appear that the proteino­chromogen, which could not be isolated by itself in sufficient purity for analysis, must contain approximately 61.02 per cent. of carbon, 6.89 per cent. of hydrogen, 13.68 per cent. of nitrogen, 4.69 per cent. of sulphur, and 13.71 per cent. of oxygen. As a proteid-like body, it is thus especially characterized by an exceedingly high content of carbon and a high content of sulphur. As a product of trypsin-proteolysis, it must presumably come from the cleavage of hemipeptone, which, however, contains only 0.75 per cent. of sulphur. But as we have seen, this latter body breaks down by further cleavage into substances such as leucin, tyrosin, lysin, etc., which contain no sulphur whatever, and as there is no elimination of sulphur in this process through formation of hydrogen sulphide gas or otherwise (putrefaction being excluded by the presence of either chloroform or thymol), it follows that this surplus sulphur must accumulate somewhere. The high content of carbon, however, in proteino­chromogen is sufficient evidence that the substance cannot have its origin in a simple cleavage of hemipeptone. On the other hand, everything points to a synthetical process, in which two or more cleavage products of the proteid molecule combine and form a new body, such as proteino­chromogen, containing all the sulphur cast off from the hemipeptone in the production of the crystalline bodies, and having in itself properties common to peptone and to a body of the indigo-group, the latter obviously coming from some aromatic antecedent.

In view of the apparent complexity of the processes attending trypsin-proteolysis, it is not strange that even simpler substances than those already described should make their appearance. Thus, when it was suggested that ammonia, NH₃, might be formed under the influence of trypsin, it was not considered at all improbable, for in the hydrolytic decomposition of proteids by boiling dilute acid, as well as by baryta water, it had long been known as a prominent product. Obviously, in trypsin-proteolysis, the one thing to be guarded against in proving the formation of ammonia is the contaminating influence of bacteria. Hirschler,191 however, with a full recognition of this danger, made digestions of blood-fibrin with trypsin extending only through four hours and at a temperature of 32°C., and yet he obtained plain evidence of the formation of ammonia. Stadelmann,192 with still greater precautions to exclude all bacterial agencies, using boiled fibrin as the material to be digested and thymol to prevent any possible infection of the digestive mixture, proved conclusively that ammonia was formed as a result of trypsin-proteolysis. Thus, in the digestion of 35 grammes of boiled blood-fibrin with 60c.c. of a pancreas infusion for three days, 20.8 milligrammes of NH₃ were developed, presumably coming from the liberation of a certain amount of nitrogen attendant upon the formation of such bodies as leucin and tyrosin, which contain considerably less nitrogen than their direct antecedent hemipeptone, or the original proteid. We thus have striking proof of the ability of this peculiar proteolytic enzyme to set in motion hydrolytic changes which may extend even to the production of such simple substances as ammonia, thus making still more striking the parallelism between trypsin-proteolysis on the one hand, and the artificial hydrolysis produced by boiling dilute acids on the other.

In view of all these facts regarding the nature of the products obtainable by pancreatic proteolysis, it is very evident that many chemical changes may take place side by side in a vigorous pancreatic digestion of proteid matter. We know without a shadow of doubt that all of the bodies enumerated as products of pancreatic digestion are the results of trypsin-proteolysis, and not the products of putrefactive changes. Bacteria, it is true, are able to produce many like products, and in the living intestinal tract exercise an important influence, especially in the breaking down of resistant forms of proteid matter, and in the decomposition of surplus material which has escaped the pancreatic ferment. But all the bodies described above are readily obtainable by trypsin-proteolysis under conditions which exclude all possibility of bacterial action.

Granting, then, as we must, that these various bodies are all products of pancreatic proteolysis when the process is carried on in beakers or flasks, we need to consider next how far such bodies appear in the natural process as it takes place in the living intestine. We know indeed that the natural and the artificial processes are very much alike so far as the qualitative results are concerned, but what differences there may be between the quantitative relationships in the two cases is less certain. One might naturally reason that, with the facilities for rapid absorption that exist in the small intestine, trypsin-proteolysis would rarely proceed beyond the peptone stage, yet we have ample evidence that, under some circumstances at least, both leucin and tyrosin are formed in considerable quantities in the intestine.

It obviously makes a very great difference to the economy in what form the proteid matter ingested leaves the intestine on its way into the blood-current. It has been more or less generally assumed that, under the ordinary circumstances existent in the intestinal tract, the crystalline and other bodies coming from the more profound changes incidental to trypsin-digestion are rarely formed, mainly on the ground that such trans­forma­tions would entail great loss of nutritive material to the blood. Years ago, Schmidt-MÜlheim193 made a series of experiments on the changes which proteid foods undergo in different portions of the alimentary tract, from which he concluded that leucin and tyrosin are formed in such small quantities in natural pancreatic digestion that they represent only a very small part of the nitrogen absorbed from the intestine. This conclusion has been more or less generally accepted, especially as several observers have reported finding only small amounts of these bodies in the intestine under what might be assumed to be favorable circumstances for their formation. In artificial digestions, on the other hand, as we have seen, leucin and tyrosin, together with the other simple bodies described, may appear in large quantities. Obviously, two suggestions present themselves as explanatory of this difference; either there is such a rapid absorption of these crystalline products from the intestine that they cannot be detected other than as mere traces, or else the natural process takes a different course from the artificial, owing to the rapid withdrawal from the intestine of the antecedent of the leucin and tyrosin, viz., the hemipeptone.

Concerning this point, Lea194 has recently reported some experimental evidence obtained by a comparative study of artificial pancreatic digestion as carried on in a flask, with similar digestions carried on in parchment dialyzer tubes, the latter so arranged that the diffusible products of proteolysis can pass from the tube into the surrounding fluid. As Lea justly says, this whole question of the formation of leucin by proteolysis is a very important one, since it bears closely upon one of the possible methods by which urea may be quickly formed from proteid food. Thus, we have evidence that when leucin is administered to mammals a portion of its nitrogen, at least, quickly reappears as urea and uric acid in the urine.195 Further, there is a certain amount of evidence that this transformation takes place in the liver, viz., in the organ where leucin absorbed from the intestine would naturally be first carried.196

Obviously, the main point to be gained in a dialyzer-experiment is the removal of the soluble products of digestion as soon as they are formed; but peptones are not rapidly diffusible, and the process, as noted under the head of gastric digestion, cannot be considered in any sense as yielding the same results as might be obtained in the living intestine. Still, the method offers a closer approach to the natural process than when carried on in a flask, and the results are of interest. Thus, Lea finds in the first place that in a dialyzer-digestion the proteid is more quickly dissolved, and that there is far less tendency for the formation of an insoluble antialbumid with its natural resistance to the ferment. Still, it is to be noticed that the amount of this antialbumid-residue formed by trypsin-proteolysis in a flask is mainly dependent upon the strength of the ferment solution, and the character of the proteid undergoing digestion. If the latter is in a fairly digestible form, and the enzyme solution reasonably active, then even the flask-digestion may show almost no residue of antialbumid. Yet there is at least a shade of difference in the two cases, which may be expressed by the statement that trypsin-proteolysis, as carried on in a dialyzer-tube, is prone to give less insoluble antialbumid than a corresponding digestion in a flask. Further, the amount of leucin and tyrosin formed in a flask-digestion is always greater than in a dialyzer-digestion, other conditions being equal. Naturally, these results help us very little in drawing any conclusions regarding the extent to which leucin and tyrosin may be formed in the intestine. They merely emphasize the fact that the withdrawal of a certain quantity of hemipeptone from the digestive mixture tends to reduce by so much the yield of leucin and tyrosin. It is hardly to be assumed, however, that the rate of withdrawal of peptone from the intestine can keep pace with its formation, especially when it is remembered that the proteid matter coming into the small intestine, owing to its preliminary treatment in the stomach, is in a comparatively digestible condition. Further, the pancreatic juice is a remarkably active fluid, and proteolysis under its influence must make rapid strides. I can easily conceive that proteolysis by trypsin, when carried on in a flask, may lead to the formation of much larger amounts of leucin and tyrosin, and of other bodies as well, than occurs in the natural process; but there is certainly no ground for the belief that leucin and tyrosin are wholly wanting in pancreatic proteolysis as it occurs in the intestine.

With a view to obtaining some positive evidence on this point I have tried a few experiments on animals, the results of which have convinced me that, in the case of dogs, at least, both leucin and tyrosin may be formed in natural pancreatic digestion in considerable quantities. Thus, in one experiment a good sized dog, kept without food for two days, was fed four hundred grammes of chopped lean beef at 8 A.M. At 2 P.M. the animal was killed and the intestine ligatured close to the pylorus. The lower end of the small intestine was likewise ligatured. The portion inclosed between the two ligatures was then removed from the body, and the contents of the intestine pressed and rinsed out with distilled water. In the stomach, was found a small amount of semi-digested matter weighing about fifty grammes. The material obtained from the intestine was strained through mull, the fluid rendered faintly acid with acetic acid, and heated to boiling. The clear filtrate from this precipitate was concentrated to a very small volume, and while still hot precipitated with a large amount of ninety-five per cent. alcohol. A small gummy precipitate resulted, which was thoroughly extracted with boiling alcohol and the washings added to the alcoholic filtrate. The precipitate contained some deutero­proteose and a small amount of true peptone.

The alcoholic fluids were evaporated to a small volume and set aside in a cool place. As a result, quite a separation of leucin and tyrosin occurred in the characteristic crystalline forms. No attempt was made to effect a quantitative separation of the two bodies, but the mixed precipitate finally obtained weighed, after re­crystal­liza­tion, over three-fourths of a gramme. Leucin was plainly in excess, but considerable tyrosin must have been left in the alcoholic precipitate, owing to its greater insolubility in this menstruum. This experiment is almost a counterpart of one reported by Lea,197 and like his indicates that both leucin and tyrosin may be formed in not inconsiderable quantities by pancreatic proteolysis as it occurs in the intestine. This being so, one is naturally called on to explain “the physiological significance of a process which at first sight appears to result in a degradation of the potential energy of proteids, under conditions such that the energy set free can be of little use to the economy.”198 But it is quite possible, as Lea has suggested, that these amido-bodies have an important part to play in some of the synthetical or other processes of the organism, and that their formation is consequently necessary for the well-being of the body. Whether this is so or not, we may well consider the formation of these amido-acids in pancreatic proteolysis as a means of quickly ridding the body of any excess of ingested proteid food, with the least possible expenditure of energy on the part of the system. This has always seemed to me the probable purpose of the profound changes which the pancreatic ferment is capable of inducing.

The primary object of both gastric and pancreatic proteolysis is to render the proteid foods more easily available for the needs of the economy, viz., to aid in their absorption and consequent distribution to the master tissues and organs of the body. This is doubtless fully accomplished by the formation of the so-called primary and secondary products of proteolysis, i.e., the proteoses and peptones which are, comparatively, not far removed from the mother-proteid, except in solubility and other minor points. In the ferment trypsin, however, we have a special agent endowed with the power of carrying on the hydrolytic cleavage to a point where exceedingly simple bodies result, and through whose agency any excess of proteid material in the intestinal canal may be quickly broken down into a row of products easily removed from the system. It is to be remembered, however, that the very nature of the proteid molecule precludes the possibility of anything like a complete decomposition into crystalline or other simple products. Full fifty per cent. of the peptone formed must be antipeptone, which cannot be further changed by trypsin under any circumstances, so that, whether the amount of proteid in the intestine be large or small, or whether it is exposed for a longer or shorter period to trypsin-proteolysis, there will always be a fairly large amount for absorption. This may well be considered as one of the reasons for the peculiar structure of the proteid molecule, the anti-group being always available for the direct nutrition of the body, while the represen­ta­tives of the hemi-group, especially when proteid is present in excess, can be quickly and readily broken down into simple products. In other words, the direct formation of these simple bodies in the intestine furnishes a short path to urea, thus leading to the rapid elimination of any excess of proteid material.

We may well attribute to the epithelial cells of the intestine the power, under normal circumstances, of regulating and controlling, even though indirectly, the order of events in the intestine. Just as the so-called secreting cells of the tubuli uriniferi may lose for a time their power to pick out from the blood material destined for the urine, being clogged or exhausted by continued effort, so the epithelial cells of the intestine, which play such an important part in the absorption of proteid matters from the alimentary tract, may, in the presence of an excess of proteid matter, become temporarily exhausted, and, refusing passage to the proteoses and peptone formed by proteolysis, render possible further hydrolytic cleavage into leucin, tyrosin, lysatin, etc.; bodies which, by one method or another, can be readily transformed into urea. At the same time, as already stated, it seems more than probable that some formation of these amido-acids always occurs in the intestine, and that these bodies have some specific part to play in the normal processes of metabolism going on in the body. The more one studies the processes of nutrition in general, the more one is impressed with the view that there is a purpose in everything, and that the formation of even such bodies as leucin and tyrosin may be connected with hidden processes, the key to which has not yet been found. We see an analogous case, perhaps, in the action of the inorganic salts in nutrition, some of which, at least, neither undergo change themselves nor induce changes in other substances, and yet we know their presence is indispensable for keeping up the normal rhythm of the nutritional processes of the body.

ABSORPTION OF THE MAIN PRODUCTS OF PROTEOLYSIS.

In ordinary proteolytic action, both in the stomach and intestine, it is very apparent that the primary products of proteolysis, the proteoses and peptones, are the chief products formed, and that under normal circumstances the greater portion of the proteid food finds its way from the alimentary canal into the blood, after transformation into one or more of these two classes of products. At the same time, it must be borne in mind that even the acid-albumin formed by pepsin-hydrochloric acid may be absorbed without undergoing further change. The view once held, that the rate of absorption from the alimentary tract stands in close relation to the diffusibility of the products formed, and that non-diffusible substances are incapable of absorption, is no longer tenable. Absorption from the intestine is to be considered rather as a process involving the vital activity of the epithelial cells of the intestinal mucous membrane, where chemical affinities and other like factors play an important part in determining the rate and order of transference through the intestinal walls into the blood and lymph. Thus, we have abundant evidence that native proteids which have not undergone proteolysis may be absorbed from the intestine, at least to a certain extent, provided they have been dissolved; i.e., converted into acid-albumin, or alkali-albuminate, by the gastric or pancreatic juice. We have a practical demonstration of this possibility in the early experiments of Voit and Bauer,199 as well as in many later ones that need not be mentioned here. Further, the recent experiments of Huber200 have given us quantitative data on the rate of absorption of fluid egg-albumin when introduced into the large intestine in the form of a clyster, showing that even fairly large amounts of a natural proteid may be absorbed without undergoing proteolysis if mixed with a neutral salt, like sodium chloride. To be sure, the rate of absorption is greatly increased when the albumin has been peptonized, but still absorption of the native proteid is possible without the agency of proteolytic enzymes. When, however, large amounts of egg-albumin are introduced into the intestine, albuminuria may result, as you very well know.

Moreover, it is well known that the proteids of muscle-tissue, in the form of syntonin, may be absorbed from the large intestine without undergoing further hydration. When introduced into the rectum of a hungry dog, the excretion of urea may be at once increased and the animal brought into a condition of nitrogenous equilibrium; absorption taking place from a portion of the large intestine, where proteolysis is never known to occur.201

Again, Neumeister202 has shown that the direct introduction of syntonin, alkali-albuminate, crystalline phytovitellin, as well as pure serum-albumin, into the blood of the jugular vein is not attended with the appearance of albumin in the urine. On the contrary, the proteid matter so introduced appears to be assimilated and utilized for the needs of the organism. Evidently, then, these substances are not to be considered as foreign bodies, for if so the kidneys would undoubtedly make some effort to remove them from the circulation. It is to be noted, however, that all native proteids are not assimilated in this manner, as casein,203 gelatin,204 and especially egg-albumin. Thus, J.C. Lehman,205 working under KÜhne’s direction, observed that the injection of a carefully filtered solution of egg-albumin into the veins of a dog was always accompanied by albuminuria, while similar injections of LieberkÜhn’s sodium albuminate, or of syntonin from frog’s muscle, failed to show any such result.

While these observations tend to show that some native proteids may be absorbed from the alimentary tract without previously undergoing proteolysis, it is not to be understood that any considerable quantity is so absorbed under normal circumstances. Doubtless, when small amounts of proteid food are taken, its de­natural­iza­tion by the primary action of the gastric or pancreatic juice, viz., its conversion into syntonin or alkali-albuminate, may be sufficient to insure its partial absorption, but digestive proteolysis is unquestionably a necessary preliminary to any general absorption, and there can be no manner of doubt that the greater portion of the proteid food is absorbed as proteoses and peptone. Peptones, as we have seen, are possessed of a higher endosmotic equivalent than the proteoses, but we need to keep continually in mind the possibility that the selective power of the epithelial cells of the intestinal mucosa may lead to as rapid transference of the proteoses as of the more diffusible peptones. It is not to be understood by this, however, that diffusibility is of no consequence in determining the rate of absorption. Surely, everything else being equal, the more diffusible the substance the more rapid will be its passage from the intestine into the blood-current. The more the process of absorption is studied, however, the more clearly do we see its dependence upon the functional power of the living epithelial cells, a fact which plainly emphasizes the physiological nature of the process.

Further, as already stated, absorption of proteid food-stuffs, or their products, from the alimentary tract, is, under ordinary circumstances at least, limited to the intestine; from the stomach there is comparatively little absorbed, and if necessary we might advance this fact as an important argument against the theory of general absorption of proteids in the form of acid-albumin. Even such indifferent fluids as water, or physiological salt solution, are absorbed with extreme slowness from the stomach;206 this organ showing very little ability to take up water even when the blood-vessels are dilated, as after the ingestion of food.

This brings us to a very important point in connection with the utilization by the system of the ordinary products of proteolysis. The latter, as we have seen, are mainly proteoses and peptones, and yet all the evidence points clearly to the fact that these substances are never present, at least in any quantity, in the blood or lymph, even when digestive proteolysis is at its height. Further, the very nature of the proteoses and peptones, their marked physiological action when they are introduced directly into the circulating blood, their rapid excretion, either as proteoses or peptones, by the kidneys when so introduced,207 all indicate that they are foreign substances, totally out of their natural environment when introduced into the blood-current. And yet we very well know that proteoses especially are possessed of high nutritive qualities; they are abundantly able to support animal life. Thus, Politzer208 found by feeding experiments with heteroalbumose, dysalbumose, and protoalbumose, that these bodies taken into the stomach have the same nutritive value as meat. Various feeding experiments with proteoses from different sources, carried out in my laboratory on young dogs, have shown conclusively that for short periods of time, at least, these hydrolytic cleavage products are fully as capable of sustaining the nitrogenous equilibrium of the body as the proteids from which they are derived. In fact, the results obtained favor the view that the proteoses, weight for weight, possess a higher nutritive value than fresh beef.209 It may be questionable, however, whether such a result would follow in experiments conducted over longer periods of time, but of this we may be certain, that the proteoses formed in the alimentary tract can be absorbed and utilized by the system without their exerting any toxic action whatever.

Consequently, we are forced to the conclusion that these primary products of proteolysis, so important in the nutrition of the animal body, must undergo some change during the process of absorption, by which they are converted into new bodies, less toxic in their nature, and better adapted for the direct nutritional needs of the organism. The same statement applies likewise to peptones.

The fact that peptones are not discoverable in the blood and lymph, even during or after active digestion, was practically ascertained years ago by such well-known workers as Maly, Adamkiewicz, and others. The natural supposition following this observation was that the products of proteolysis underwent some change in the hepatic cells; but this view was soon shown to be untenable by examination of the portal blood, which was found to be as free from peptone as the blood of the hepatic vein. Neumeister,210 using the more modern methods of work and with the wider knowledge gained during these latter years, has shown conclusively that proteoses and peptones are never present in the blood, even when these substances are contained in the intestine in fairly large amounts. I can corroborate these statements from the results of my own experiments in this direction. Thus, I have taken a dog in full digestion, fed with an abundance of meat, and collecting the blood from the carotid artery have made a careful examination for peptone, by the following method: The blood was allowed to flow directly into a dilute solution of ammonium sulphate, sufficiently strong to prevent coagulation, and then shaken with ether to rupture the red blood-corpuscles. The solution, freed from ether, was next saturated with crystals of ammonium sulphate, by which the proteid matter was completely precipitated. The clear filtrate was then concentrated somewhat, the excess of the ammonium salt removed by filtration, and the filtrate carefully tested for peptone by addition of a large volume of a saturated solution of potassium hydroxide and a few drops of a dilute solution of cupric sulphate. The test was wholly negative, although the intestine of the animal showed the presence of both peptone and proteoses. This result, as I have said, is simply confirmatory of work done by others in this direction, notably Neumeister, and illustrates the statement that peptones are not to be found in the circulating blood, even after a full proteid diet. In this connection it is to be remembered that we have abundant proof of the rapid disappearance of both proteoses and peptones211 from the intestine, either by absorption or otherwise. They certainly disappear, and, as we have seen, are not to be found in the blood. Further, Neumeister has confirmed the original observation of Schmidt-Mulheim,212 that both chyle and lymph are practically free from proteoses and peptone, thus again forcing us to the conclusion that the primary products of proteolysis must undergo change prior to their passage into the blood or lymph.

Many observations lend favor to the view that a transformation of some kind takes place in the intestine itself, not indeed in the lumen of the tube, but somewhere in the walls, through which the peptones must pass before reaching the blood. Thus, peptones placed in contact with pieces of the isolated, though still living, intestine, after a time completely disappear from view,213 so completely that no reaction can be obtained even by the most delicate of tests. In support of this statement I may cite the results of several of my own experiments which certainly furnish evidence that true peptones undergo profound alteration by simple contact with the living mucous membrane of the small intestine. The method employed was similar to that made use of some years ago in a study of the influence of peptone on the post-mortem formation of sugar in the liver.214 A large, well-nourished rabbit was killed by severing the carotid artery and the blood collected and defibrinated. Of this, 50c.c. were mixed with an equal volume of 0.5 per cent. salt solution containing 1.25 grammes of pure amphopeptone, prepared from egg-albumin, the mixture obviously containing 1.25 per cent. of peptone. The fluid was transferred to a large, roomy flask, provided with a stopper having two holes, in one of which was fitted a long glass tube reaching below the fluid. The flask, with its contents, was then placed in a suitable water-bath at a temperature of 40°C.

The small intestine of the rabbit was carefully separated from the mesentery and from the pancreatic gland, and the upper portion cut open and quickly washed free from any contained matter or adherent secretions, by repeated immersion in 0.5 per cent. salt solution warmed at 40°C. This was repeated until the tissue was quite free from all impurities, after which it was cut into small pieces and immersed for a moment in a 0.5 per cent. solution of sodium chloride containing 1.25 per cent. of peptone. The tissue was then carefully collected on coarse muslin, allowed to drain, and then quickly transferred to the flask containing the warm blood and peptone. This mixture was kept at 40°C. for two hours, a slow current of air being bubbled through the fluid during the entire period. At the expiration of this time the fluid was separated from the pieces of tissue by nitration through muslin, and then saturated with ammonium sulphate after the usual method for the separation of albumoses, etc. On now testing a portion of the clear filtrate for peptone by the biuret test, not a trace of a reaction could be obtained. The entire amount of proteid matter present was precipitated by the ammonium salt, thus showing that the peptone originally added had been completely transformed into something precipitable by saturation of the fluid with ammonium sulphate. That this transformation of the peptone was accomplished mainly through the action of the intestine, was shown by a parallel experiment, in which all of the above conditions were duplicated, omitting only the pieces of intestine. Here, however, on testing the filtrate from the ammonium sulphate-precipitate, a strong biuret reaction was obtained, thus proving the presence of at least some unaltered peptone.

This experiment is almost a counterpart of one reported by Neumeister, and like his, testifies to the probability that the peptones formed in the alimentary tract, as a result of proteolysis, undergo retrogression through the agency of the epithelial cells of the intestinal walls during their absorption. I have tried similar experiments with deutero­proteose, notably with deuterocaseose, and have obtained corresponding results. The same method may be employed as that already outlined, although of course the deuterocaseose is in great part precipitated by saturation with ammonium sulphate. Still, this form of deutero­proteose, ߠdeuterocaseose, as I have elsewhere noted, is very slowly precipitated by the ammonium salt. Consequently, it is an easy matter to demonstrate that this proteose, on treatment with the intestinal mucosa in the presence of blood at the body-temperature, is transformed into something completely and readily precipitable by ammonium sulphate; the filtrate from the latter failing to show any biuret reaction, although the corresponding control experiment without the intestine gives a bright violet color with cupric sulphate and potassium hydroxide.

Hence, we are certainly justified in saying that both peptones and proteoses undergo some retrogression when in contact with the walls of the intestine. Moreover, there is some evidence that the proteoses, before undergoing such a transformation, are first converted into peptone by the action of the intestinal walls, a statement which will apparently apply to both the primary and secondary proteoses. This primary action of the intestinal walls is not considered as due to any adherent trypsin, or to possible traces of succus entericus, but rather as a part of the action of the living epithelial cells, or perhaps as connected with the possible presence of lower organisms not removable from the intestinal wall by ordinary washing.

The transformation of peptones by the substance of the intestine is apparently common to the intestinal tract of many animals, and perhaps to all, and indeed can also be accomplished by the liver.215 This latter fact is of some importance, since it adds weight to the supposition that this peculiar action of the intestine cannot be due to the possible presence of trypsin; a view which is strengthened by the fact that a glycerin-extract of the intestine has no action on amphopeptone. Certainly, the latter shows no diminution in the strength of the biuret reaction after long contact at body temperature with such an extract. Further, it has been shown that antipeptone, which is not affected by the pancreatic enzyme, suffers the same change as amphopeptone by contact with the intestine. Far more probable is it that retrogression or transformation of peptone by the substance of the intestine, is due to the vital activity of some or all of the epithelial cells of the intestinal mucosa; a characteristic possibly shared by some or all of the hepatic cells of the liver. The kidney-cells certainly do not possess this power, but we can see a special fitness in the liver-cells being endowed with the ability to quickly break down, or transform, any peptone or proteose that might by chance escape unaltered from the intestinal tract. Shore,216 however, inclines to the view that the hepatic cells do not possess this power to any great extent, in opposition to the older views of PlÒsz and Gyergai,217 as well as of Seegen218 and of Neumeister.

With reference to the action of the stomach-mucosa on proteoses, it has been shown219 that when relatively large amounts (5 grammes) are introduced into the stomach of a rabbit, the pylorus being ligatured, both proteoses and peptones may appear in the urine, thus indicating that while they may be absorbed to some extent under the above conditions, the proteoses are not readily transformed into native proteids without exposure to the intestine. Smaller amounts (2 grammes), however, may, under the above conditions, be completely transformed; at least Hildebrandt claims this to be the case, mainly on the ground that after the introduction of albumoses into the stomach, the pylorus being ligatured, no trace of them can be found in the urine. The same observer also claims that blood-serum, in the case of dogs, is able to transform albumoses into ordinary serum-globulin. Certainly, after intra-venous injection, proteoses disappear from the blood, but, as we shall see later on, a certain amount, at least, may be transferred to the lymph. It is also claimed that when albumoses are injected subcutaneously, neither albumoses nor peptones are to be detected in the urine. This, however, seems hardly probable in the light of what has been said, and especially in view of the fact that Neumeister’s experiments tend to show that even 0.1 gramme of albumoses introduced subcutaneously may give rise to temporary albuminuria.

Assuming for the moment that the chief products of proteolysis, i.e., the proteoses and peptones, are, during the act of absorption, transformed through the vital processes of the epithelial cells of the intestine into serum-albumin, or globulin, and absorbed as such into the blood, we may well consider whether such transformation, i.e., a retrogression into a native proteid again, is inconsistent, or out of harmony, with the general character of the changes known to occur in the body. In attempting to answer this question we need not look far to find a perfectly analogous case. Thus, in the digestion of starchy foods by the amylolytic ferments of both the saliva and the pancreatic juice, the carbohydrate material undergoes hydration with formation of dextrins and maltose, the latter, at least, being quickly absorbed into the circulating blood. But large quantities of sugar in the blood are certainly inimical to the well-being of the organism, and we find in the liver a tendency for the sugar to undergo a transformation, i.e., a retrogression into glycogen, either through simple dehydration or otherwise. Further, with reference to the possible conversion of proteoses into peptone by the substance of the intestine, we have a perfectly analogous case in the behavior of the intestinal mucous membrane toward maltose, the final product of amylolytic action. Thus, according to the recent work of M.C. Tebb,220 the mucous membrane of the intestine has the power of transforming maltose into dextrose; simple warming at 40°C. of a solution of maltose in 0.5 per cent. sodium carbonate with a few grammes of the dried mucous membrane from the intestine, being sufficient to insure a marked conversion of maltose into the higher-reducing sugar, dextrose. This observation, I can confirm from experiments just completed in my own laboratory. This action is presumably due to a ferment, which, according to Tebb, is widely distributed throughout the body, being present not only in the intestine, but also in the liver, kidney, spleen, striated muscle-tissue, and, indeed, in the blood-serum; so that it would appear that nearly all the tissues of the body are endowed with the power of transforming maltose into dextrose. These statements being correct, it would seem that, while the amylolytic ferments of the several digestive juices transform, by hydrolytic action, starchy foods into maltose, the latter is exposed during its passage through the intestinal wall, as well as in the blood itself, to another ferment which carries the hydration still further, with formation of dextrose; and yet the latter product is destined, in part at least, to undergo retrogression into a starch-like body, i.e., glycogen, before it is completely utilized by the system. Thus, the analogy between these carbohydrate bodies and the products of proteolysis is complete, and we may well accept the statements already made regarding the ultimate fate of the proteoses and peptones formed during proteolysis, as in no way inconsistent with the general tenor of events going on in the body.

While we are inclined to believe that the chemical changes attending the absorption of proteoses and peptones occur mainly in the epithelial cells of the intestinal mucosa, and that there is a direct transference of the alteration-products to the blood, there are still other views that cannot be wholly ignored. Thus, the view originally advanced by Hofmeister,221 in which special stress is laid upon the functional activity of the leucocytes of the adenoid tissue surrounding the intestine, demands some consideration. The theory supposes that these cells not only have the power of taking up peptones, but also of assimilating and transforming them into the cell-protoplasm. This view being correct, it is plain that the so-absorbed proteid must pass into the circulating blood through the thoracic duct, and Hofmeister further considers it probable that the lymph-cells of the mesenteric glands can transform any absorbed peptone that may escape the leucocytes of the adenoid tissue.

In apparent harmony with this view is the fact that the leucocytes in the adenoid tissue of the intestine are greatly increased in number during digestion.222 Furthermore, it is a well authenticated fact that the proteoses or peptones found in pus are contained in the pus-cells themselves, and not in the fluid in which the corpuscles float.223 In support of the first statement, Pohl,224 in his recent study of the absorption and assimilation of food-stuffs, has emphasized the marked increase in the number of white blood-corpuscles in the circulating blood after the ingestion of proteid foods, especially such as meat, Witte’s peptone, and gelatin-peptone. It is to be noted further that the increase is most marked at about the third hour after the taking of food, viz., at a time when digestive proteolysis would naturally be at its height. Moreover, the maximal increase, according to Pohl’s data, is astounding, amounting as it does in many cases to a hundred per cent. Thus, in one instance, in the case of a dog, the number of white blood-corpuscles per cubic millimetre of blood was 8,689; yet two hours after the feeding of 100 grammes of meat the number increased to 17,296 per cubic millimetre, followed six hours after by a return to the original figure.

This indicates the general tenor of Pohl’s results, which have been taken, by some physiologists at least, as confirmatory of Hofmeister’s views; the interpretation naturally being that digestive proteolysis in the alimentary tract is accompanied by a rapid production of new leucocytes in the lymph-spaces surrounding the intestine, and followed by a rapid transference of the corpuscles from their point of origin to the circulating blood, from which they gradually disappear as their material is made use of in the different parts of the body. In harmony with this view, Pohl finds that there is a much larger number of leucocytes in the blood and lymph flowing from the intestine of an animal in full digestion, than in the arterial blood coming to the intestinal tract. Further, when due consideration is given to all the circumstances attending the circulation of the blood through the abdominal organs, in connection with the great increase in the number of leucocytes during digestive proteolysis, it seems not unreasonable to suppose that some proteid matter might be transferred from the intestine to the blood during the digestive period of six or eight hours. Moreover, if Pohl’s views are correct, we see that a portion, at least, of the proteid food-product may be transformed into organized material in the body of the lymph-cell prior to its passage into the blood, thus harmonizing with the statement already made regarding the utter lack of proteoses and peptones in the blood and lymph. This obviously means an upbuilding of the ordinary products of digestive proteolysis into the living protoplasm of the leucocytes in the intestinal walls, implying, however, that the transformation is accomplished solely by the leucocytes themselves, and not by the epithelial cells of the intestine.

I have given this brief summary of Pohl’s work because it is so closely in harmony with the original views of Hofmeister, and because it offers an easy explanation of one possible way in which some of the products of digestion might perhaps pass from the intestine into the blood. I am inclined to believe, however, that the so-called digestive leucocytosis, which unquestionably does exist, is not a direct result of digestive proteolysis in general, but rather an indirect result, coming from the stimulating action of the nuclein, contained especially in animal cells. Thus, it is a significant fact, as Pohl himself reports, that wheat-bread, with its fairly large amount of proteid matter, and which is fully capable of nourishing the animal body, fails to exert any influence on the number of leucocytes in the blood. Yet we know that the gluten and other proteids of wheat-flour are converted by digestive proteolysis into proteoses and peptones, with the same general properties as like products of animal origin.

In this connection we may note the experiments of Horbaczewski,225 which show that nuclein administered to a healthy man will give rise to a very marked increase in the number of leucocytes in the blood. Thus, a few grammes of nuclein may produce as striking a condition of leucocytosis as a large amount of proteid food, due no doubt to proliferation of the lymphoid elements of all the lymphatic tissues of the body. Horbaczewski has reported that the mere injection of 0.25 gramme of nuclein, in the case of rabbits, will cause marked enlargement of the spleen, with striking karyokinetic changes. Hence, it may be assumed that whenever nuclein is set free in the body, leucocytosis may result, provided the nuclein passes into the circulation and is not decomposed immediately after its liberation.

These facts, it appears to me, offer a more consistent line of explanation of digestive leucocytosis than that advanced by Pohl. All animal foods, especially meat of various kinds and milk, contain considerable nuclein or nucleo-albumins, which, by the action of the gastric juice, are liberated and partially digested, but the nuclein is certainly not dissolved. Nucleins, however, are soluble in weak alkaline fluids, and when exposed to the action of the alkaline pancreatic juice in the intestine, are in great part dissolved. Thus, Popoff226 has reported that different varieties of nuclein behave somewhat differently in the intestine, according to their origin. In young and tender tissues, solution of the contained nuclein through the alkaline fluids of the intestinal canal is fairly complete, while the older products are somewhat more resistant both to the pancreatic juice and to the putrefactive processes common to the intestine. However, experiments show that the greater portion of the nuclein of ordinary proteid foods is dissolved in the intestine, and absorbed as such in a practically unaltered form. Consequently, passing into the adenoid tissue surrounding the intestine, it has a marked stimulating action on the lymphoid elements, accompanied by a noticeable increase in the number of leucocytes, which are perhaps produced at the expense of a portion of the proteoses and peptones formed during proteolysis.

Thus, my interpretation of these results would lead me simply to the admission that, possibly, a portion of the products of proteolysis might pass from the intestine into the blood-current indirectly, through the bodies of the leucocytes formed in the adenoid tissue of the intestine. But even admitting this, we lack positive proof of any direct transformation of proteoses and peptones into the organized material of the white blood-corpuscles, for it may be that the above products are first transformed through other agencies into serum-albumin, or other like proteids. There are, indeed, many facts which are plainly opposed to any marked absorption and transformation of peptones by the leucocytes of the intestinal mucous membrane. Thus, Heidenhain227 has severely criticised the theory on the ground that there is very little increase in the flow of lymph from the thoracic duct during absorption, and further that the small percentage of proteid matter in the chyle (about 2.0 per cent.) cannot account for the large amount of proteid absorbed. Further, the objection is made that the leucocytes present in the intestinal mucosa, though numerous, are wholly inadequate to assimilate any large proportion of the ingested proteid food.

Still greater stress, however, may be laid upon the fact that experimental evidence points to the conclusion that lymph-cells cannot assimilate either peptones or proteoses. Thus, quite recently, Shore228 has studied the results following the introduction of a mixture of such products into the lymphatic system by secretion, by absorption, and by direct injection into a lymphatic vessel. Preliminary experiments on dogs showed that when peptone is introduced into the bile-duct it gradually appears in the lymph of the thoracic duct, consequently this method can be made use of as a means of ascertaining the fate of peptone so absorbed into the lymphatic system. The results obtained, using the ammonium sulphate method for the isolation of the peptone, showed that when peptone is injected into the bile-duct with sufficient force to overcome the low pressure under which bile is secreted, there is an increase in the rate of flow of lymph from the thoracic duct. Further, while peptone is somewhat slow in appearing in the lymph it eventually makes its appearance there, in from sixty to one hundred and forty minutes after its injection into the bile-duct. A certain amount of peptone naturally passes into the blood, but is then rapidly excreted through the urine. When, however, the renal vessels are ligatured, peptone still rapidly disappears from the blood, but then passes into the lymph, and under such circumstances can be detected in the lymph as early as thirty-eight minutes after its injection into the bile-duct. These results, therefore, do not accord with the view that peptones suffer marked transformation by contact with lymph-cells, for when only three-fourths of a gramme of peptone is introduced into the bile-duct, unaltered peptone can be detected in the lymph of the thoracic duct seventy to ninety minutes after its injection.

With reference to the fate of peptone when it passes by secretion into the lymphatic system, it will be remembered that Heidenhain229 has shown that the injection of peptone into the blood may be followed by a large increase in the rate of flow of lymph. Further, the amount of solids in the lymph, especially of proteids, is considerably increased. From these and other facts, Heidenhain is led to the view that the formation of lymph is a true secretion from the blood-vessels. Shore finds that when small amounts of peptone are slowly injected into the blood, there is generally only a slight acceleration in the flow of lymph, but the clotting power of the lymph is affected in a remarkable manner. Thus, about twenty minutes after the commencement of the injection the lymph loses entirely its power of coagulating. This continues for about twenty minutes, and then, in spite of the fact that the injection is being continued, the lymph rapidly regains its power of clotting, and finally coagulates quicker and firmer than before. This peculiar action of peptone on the clotting power of lymph may frequently be observed, even when the amount of peptone present is too small to be detected with certainty by chemical methods. Thus, when peptone in small quantity is injected very slowly into the blood, the greater part of it escapes through the urine, but a small fraction, sometimes too small to actually detect, passes into the lymph and shows its presence by its peculiar influence on the clotting of the fluid.

When, on the other hand, peptone is injected rapidly into the blood, 0.3 to 0.6 gramme per kilo. during two to ten minutes, it may disappear completely from the blood in five to ten minutes after the end of the injection. In such a case, the fall of blood-pressure induced leads to more or less arrest of the renal secretion, peptone appearing in large amount in the lymph; but there is no indication of any alteration of the peptone by the lymph, or its contained leucocytes. Thus, when there is no chance for the peptone to escape from the body, as on ligation of the renal vessels, the peptone injected into the blood is rapidly thrown into the lymph, and from the lymph in the tissues of the body it is gradually carried to the thoracic duct, and then again passes into the blood; all of which shows that there is little or no transformation of the peptone by the leucocytes of the lymphatic system.

Further, by direct injection of peptone into a lymphatic vessel, Shore has shown that even so small an amount as 0.049 gramme is not assimilated or transformed by the lymph in half an hour. Consequently, we seem to have strong evidence that peptones are not prone to direct alteration of any kind by the leucocytes of the lymphatic system. Further, it would appear that the lymphoid cells of the spleen are equally unable to assimilate small amounts of peptone injected into the splenic artery. Leucocytes, then, can play no direct part in the absorption of the products of proteolysis from the intestine; the lymph is normally free from both proteoses and peptones, and the leucocytes plainly have no power to transform these bodies into other forms. They can only utilize the proteid material elaborated from the products of proteolysis by other agencies.

Plainly, proteoses and peptones in the blood and lymph are foreign substances. When present in the circulation they give rise, as we have seen, to an increased flow of lymph and to a change in the coagulability of the blood. Further, not only is the flow of lymph augmented but there is likewise an increase in the amount of solid matter, while a corresponding decrease is noticed in the solid matter of the blood-plasma. This fact obviously gives support to the view that the increased formation of lymph after the injection of peptone is due to an active process of secretion by the endothelium cells of the capillary walls.230 Further, as we have seen, peptone disappears from the blood more or less rapidly after its injection, so that it is quite possible that the loss or alteration of the coagulability of the blood may not be due to the peptone itself, but rather to an altered condition of the blood induced by the peptone. Moreover, this altered condition of the blood may be the real cause of the increased transudation, or secretion of lymph so conspicuous after the injection of peptone. Starling, however, by carefully conducted experiments on dogs finds, in conformity with Heidenhain’s views, that peptone injected into the blood exercises a direct excitatory effect on the endothelial cells, causing thereby an increased flow of lymph; the increased flow being in no way caused by the change in the blood that is simultaneously produced. Further, it would appear, according to Starling’s views, that the change in the coagulability of the blood is not due to the effect of the peptone on the endothelial cells of the blood-vessels, or at least on their lymph-producing functions. Thus, the injection of peptone may result in an action on the endothelial cells of the blood-vessels, thereby increasing the flow of lymph, or on the blood itself with a destruction or diminution of the clotting power of the blood; the two results being more or less independent. Further, the rapid transferences of peptone from the blood to the lymph is effected by the selective activity of the endothelial cells of the vessel-wall, and according to Starling it is probable that a preponderating part is played by the endothelial cells of the renal capillaries.

In view of all these statements, it is very evident that proteoses and peptones once outside the limits of the alimentary tract may be passed about from organ to organ and from secretion to secretion, inducing changes here and there in their course, but suffering very little change themselves. The main efforts of the system are directed to the removal of these unwelcome strangers as speedily as possible, for their marked physiological action renders them somewhat dangerous visitors.

As normal products of digestive proteolysis, they are never found beyond the limits of the gastro-intestinal canal, but undergo retrogression in their passage through the epithelial cells of the intestinal wall, being presumably converted thereby into serum-albumin,231 which can be directly utilized for the nutrition of the body; a conversion which is plainly dependent upon certain inherent qualities of the living epithelial cells, and is doubtless of the nature of a dehydration.