Little did the learned Dutchman Leeuwenhoek dream when, more than two hundred years ago, he recorded, in his Arcana NaturÆ, that he had found "viva animalcula" in his saliva, that this, the first beginning of bacteriology, would lead, a couple of centuries later, to the inauguration of a new era in the treatment of disease, in which these so-called animalcula, from being considered as curiosities, would come to be regarded as powers for good and evil of the first importance. Protective inoculation or serum therapy, of which the public have lately heard so much in connection with diphtheria, is the direct outcome of bacterial investigations which during the last two decades have been pursued with such zeal in every part of the globe.

The vast domain of immunity, which until recently was an undiscovered country, is now being bit by bit annexed, and in all directions workers are engaged upon opening up new tracts, in overcoming difficulties, in changing chaos into order.

The problems which surround immunity are of so complex and subtle a character that their mastery is by no means either easy or rapid, and many recondite researches appear at frequent intervals on this subject in foreign and other scientific journals, rendering it a difficult matter to keep pace with the new discoveries and the latest theories.

The interest in this country in toxins and anti-toxins not unnaturally centres round that branch of the subject which deals with diphtheria, this disease having of late years figured so prominently in our mortality tables, whilst the production of diphtheria and other anti-toxic serums has been so finely elaborated abroad that it already constitutes an article of commerce, and doubtless helps to swell the exports of our great continental commercial rival.

In this connection the following statistics, published by Dr. Jalzer, of the MÜlhaus Hospital, are of interest regarding the mortality from diphtheria before and after the introduction and application of diphtheria anti-toxin. The death-rate from this disease, writes Dr. Jalzer, which in 1892 and 1893 was fully 50 per cent., fell in 1895 to 38·5 per cent., in 1896 to 28·8 per cent., in 1897 to 16 per cent., to 20 per cent. in 1898, 15·15 per cent. in 1899, and 18·75 per cent. in 1900.

So far the efforts which have been made to mitigate human suffering have attracted most attention; but it will be remembered that Pasteur, before he commenced the study of hydrophobia, had already won his laurels in combating disease in the victory he gained over anthrax, the ravages of which so frequently decimated the herds of the French farmer and robbed him of his well-earned return on his capital and labour.

In summoning the brilliant Director of the German Imperial Board of Health to South Africa to investigate the nature of rinderpest, and, if possible, discover a means of protecting cattle from its onslaught, the Cape Government afforded another opportunity for the scientific study of a disease associated with animals, upon the successful mastery and limitation of which the agricultural prosperity of South Africa is so largely dependent, being as it is one of the most fatal and contagious maladies to which cattle are subject. Apart from the great commercial importance attending Dr. Koch's discovery of a device whereby cattle can be immunised or protected from contracting rinderpest when exposed to its contagion, this discovery is of great scientific interest, inasmuch as it has inaugurated a new departure in methods of immunisation.

The previous methods in vogue for inducing immunity in animals from a particular disease consisted in converting the virus itself into a vaccine, as was done by Pasteur in his classical investigations on anthrax and its prevention; and secondly, the employment of anti-toxic serums, in which the virus is not directly inoculated into the animal to be protected, but in which an intermediary is employed between the virus and its victim. This intermediary, or living machine for the generation of the anti-toxin, is usually a horse, which is artificially trained by being given gradually increasing doses of the virus or toxin, until it ultimately withstands doses which in the first instance would infallibly have killed it. When the animal has arrived at this satisfactory stage or condition of complete immunity, some of its blood is from time to time drawn off, and the serum thus obtained constitutes the anti-toxin which now figures so prominently in modern therapeutics. Besides diphtheria-anti-toxic serum there are also those of tetanus, or lock-jaw, plague, the famous anti-venene serum, about the discovery and preparation of which greater detail is given later on, and many others which are still the subject of experimental inquiry.

Now Koch's method for the compassing of rinderpest differed from both the systems above mentioned, inasmuch as he neither employed artificially weakened cultures of the virus, or an anti-toxic rinderpest-serum; instead he took one of the natural secretions of an animal infected with rinderpest, and by injecting this into a healthy animal it was discovered that the latter, as is the case with a vaccine, suffered only local and temporary discomfort, and acquired pronounced immunity from the active virus. The secretion selected by Dr. Koch and his assistant, Dr. Kolle, for this purpose was the gall, and it might be supposed, from the fact that its inoculation into healthy animals did not communicate the disease, that the rinderpest bacteria were absent from the gall. But this is not so, for Dr. Kolle has succeeded in isolating the latter from the gall of infected animals, and, moreover, has proved them on isolation to possess their full complement of virulence. Further investigations made by Koch and Kolle have shown that the explanation of this seeming anomaly is to be found in the fact that the gall of an animal suffering from rinderpest contains a substance which prevents the migration of the rinderpest bacteria, with which it is associated, from the point of inoculation. Hampered in their movements by the controlling influence of this special substance which has been generated in the gall, the bacteria remain rooted to the spot where they are first situate, and only a passing and exceedingly slight local affection results, which on its departure leaves the animal with an immunity from rinderpest lasting some four months. A number of interesting investigations have not unnaturally been stimulated by this remarkable discovery, and researches on the properties inherent in the gall of healthy animals of various kinds have been recently carried out by Dr. Neufeld, of the Institute for Infectious Diseases in Berlin, which are, however, of a too technical nature to deal with here.

As an illustration of the practical use to which Koch's gall immunisation method may be put in dealing with outbreaks of rinderpest, reference to a recent report furnished by the Health Officer of Shanghai may be of interest. Dr. Arthur Stanley describes the outbreak as follows:—

Dr. Stanley points out that ten of these animals, judging by the time which elapsed after the injection, when they showed the first symptoms of the disease, must have been already infected when the injections were made; the eleventh animal, however, undoubtedly contracted the disease after and in spite of the injection.

In the domain of immunity there is, however, no more fascinating or interesting story than that which deals with the discovery and elaboration of a cure for snake-bites, a discovery which, while attracting but comparatively little attention in this country, should prove of paramount importance to our fellow-subjects in the great Indian Empire. The significance to India of Professor Calmette's discovery of a specific cure for snake-poison may be gathered, indeed, from the statistics which have been compiled of the number of deaths attributed by Indian officials to this cause alone, amounting, it is said, to some 22,000 annually.

The Pasteur Institute in Paris has despatched many pioneers of science to various quarters of the globe, but perhaps no scientific missionary has produced more fruitful results than has Dr. Calmette. It was while acting in the double official capacity of MÉdecin de 1st Classe du Corps de SantÉ des Colonies and Director of the Bacteriological Institute of SaÏgon, in Cochin China, in the autumn of 1891, that Calmette first commenced his experiments on the neutralisation of serpent venom in the animal system.

He had, indeed, exceptional opportunities in the matter of serpent venom wherewith to carry out his investigations, for during the rainy season a village in the neighbourhood of Bac-Lieu (Cochin China) had been attacked by a band of most venomous serpents.

These creatures, driven by the floods into the very huts of the natives for shelter, created a terrible panic, and no fewer than forty individuals were bitten by them. The panic was certainly not without justification, for these serpents belonged to the species known as naja tripudians, or cobra de capello, renowned for the deadly nature of their venom, and widely distributed over India, Burmah, Sumatra, Java, Malacca, and Cochin China; but until Calmette set to work to systematically study the nature of this reptile's venom but little precise or reliable information had been obtained as to its character.

The governor of the district gave orders that as many as possible of the reptiles were to be captured alive and forwarded to the Director of the Bacteriological Institute, and a plucky Annanite actually succeeded in securing ninety specimens, which were forwarded in a barrel to Dr. Calmette.

This formidable gift was received with enthusiasm by the director, who realised the importance and scope of the inquiry, which he at once set himself to systematically work out.

Forty of these reptiles arrived alive, and several were at once sacrificed to secure their venom glands. Each gland, resembling both in size and shape a shelled almond, contains about thirty drops of venom, and in this transparent limpid liquid is embodied a toxin of extraordinary strength. It was, of course, necessary in the first instance to ascertain, within as narrow a limit as possible, the exact degree of toxic power inherent in the venom, and to determine, if possible, the precise lethal dose in respect of each variety of animal experimented upon.

A correct calculation of the quantity of venom required in every case was, however, found to be quite impossible, for so virulent is the poison that a single drop of an emulsion produced by pounding up eight glands in 300 grammes of distilled water is sufficient, when introduced into the vein of a rabbit's ear, to kill it in five minutes. All the mammals to which Calmette administered this cobra venom, such as monkeys, dogs, rabbits, guinea-pigs, rats, succumbed more or less quickly, according to the size of the dose.

Small birds and pigeons die very rapidly, but the domestic fowl is more fortunate, being somewhat less susceptible. Frogs also fall a prey to the venom, but they are far more refractory than rabbits, for it takes thirty hours to kill a frog with a dose of venom which would infallibly destroy a rabbit in ten minutes. Toads, curiously, do not enjoy to the same extent this power of resisting its toxic action, for they die more quickly than frogs, whilst it makes short work of lizards and chameleons. Fish form no exception to the rule, and even invertebrates, such as leeches, are killed by minute traces of venom.

Whilst Calmette has found that the venom of different kinds of reptiles exhibits marked differences in its toxic character, he has also discovered that the venom secreted by one and the same serpent varies considerably, according to the length of time the animal has fasted. He describes how he kept a naja haje (Cleopatra's asp) in his laboratory, which during the whole eight months that it lived never took any food whatever, although it was offered the most diverse dainties. On its arrival it was made to bite on a watch-glass, this being one method adopted for collecting the venom; the liquid was at once dried, and 0·7 milligramme was found to kill a rabbit weighing nearly four pounds in four hours. Two months later on, when the venom was again collected, 0·25 milligramme proved a fatal dose. On the death of the animal, at the end of eight months, the venom extracted from the glands was so toxic that it only required 0·1 milligramme to kill a rabbit of about the same weight as the previous one. The same curious fact was noted in the case of a cobra's venom. Another circumstance which appears to control the degree of toxicity inherent in serpent venom is the interval of time which elapses between two successive bites. The longer the interval the more virulent is the venom; and Calmette points out that these observations are in accordance with what has for a long time been known in France with respect to indigenous vipers—that their bites are far more dangerous and far more fatal in the spring, after the winter period of torpor is over, than in the autumn.

Until quite recently it was thought that the only creatures which could resist the fatal action of this poison were serpents, both poisonous and non-poisonous. Calmette was led to this conclusion because, although he inoculated large doses, as much as ten drops, into cobras, they suffered absolutely no inconvenience, and the same results were obtained with harmless snakes. On repeating these experiments, however, and using much larger quantities of venom, Calmette has found that they do ultimately succumb. That their susceptibility in comparison with other animals is very slight, may be gathered from the fact that a lethal dose of venom for reptiles is roughly estimated to amount to as much as three times the quantity of venom normally present in their respective poison glands. These animals, therefore, although very refractory, are not absolutely immune from the action of venom-toxin.

There are, however, other animals which enjoy a relative although not absolute immunity to snake poison, and amongst these may be mentioned swine, hedgehogs, and the mongoose. Swine, it is well known, will greedily devour reptiles, and in some countries they are specially trained up and employed for this purpose. Of particular interest, however, are some experiments which were carried out to test the traditional immunity towards this toxin ascribed to the mongoose. These animals are very useful in sugar plantations, and are largely employed to keep down the serpents and rats with which they abound, for the carnivorous little mongoose is extremely partial to such prey. Attempts have been made by sugar planters to introduce them into Martinique, where they are not found in the wild state, as in the island of Guadeloupe.

Six specimens of the mongoose were forwarded to Calmette from Martinique, and these particular animals, it was stated, had never been set at liberty since they were imported, so that they had had no previous experience of snakes or venom. On arriving at the laboratory, one of these little creatures was placed in a glass cage along with a large cobra. The cobra, at once rising up and dilating its neck, darted with fury upon the mongoose; but the latter, thanks to its extraordinary agility, escaped being caught, and took refuge, stupefied and terrified for the moment, in a corner of the cage. This stunned condition, however, did not last long, for just as the incensed cobra was preparing to make a fresh attack upon its insignificant little victim, the latter, with wide-open mouth, rushed and jumped upon the head of its enemy, viciously bit through its upper jaw, and broke its skull in a few seconds. Thus, although in size but a little larger than a squirrel, this tiny creature was more than a match for a cobra two yards long.

Artificial inoculations of cobra venom into the mongoose fully substantiated all the observed facts as to its remarkable immunity from this poison. A dose sufficient to kill a large rabbit in three hours was absolutely without effect; only when the venom was introduced in quantities amounting to as much as eight milligrammes was it followed by fatal results. Thanks, therefore, to their extraordinary agility and remarkable power of resisting the effects of this lethal toxin, these little animals are able to battle successfully with the most dangerous reptiles.

The rapidity with which serpent venom becomes absorbed by the system is almost incredible, and is well illustrated by the following experiment. A rat was inoculated with venom near the tip of its tail. One minute later the latter was cut off a short distance above the point of inoculation; but this operation was quite unable to save the animal's life, for even in that brief interval the poison had accomplished its fatal work, and a few hours later claimed its victim.

This rapid diffusion of the venom helps to explain the difficulty which is experienced in arresting the course of the poison by local treatment, for its passage is too rapid to permit of its being overtaken by superficial measures of even the most stringent character. But Calmette points out that local precautions are not to be neglected, for although they cannot nullify the action of the venom, they undoubtedly do delay its progress, and thus create a longer interval or respite, during which an opportunity is afforded for administering the anti-toxin. Before, however, passing on to the investigations which have culminated in the production of a specific antidote for this terrible toxin, there are a few more details which Calmette has furnished as to its character which are of interest. Serpent venom is characterised not only by its intensely virulent properties, but also by the tenacity with which it retains them under diverse circumstances. Thus it may be stored up for a whole year, and yet at the end of that time be as active as ever; and even after several years, although its toxic powers are somewhat reduced, it still retains them to a very appreciable extent.

Unlike the bacterial toxins, this venom toxin can stand exposure to considerable temperatures without injury to its activity, and that of the cobra only suffers after it has been submitted to 98° Centigrade for twenty minutes. Sensitiveness to temperature varies, however, with the snake from which the venom is derived. Thus the venom of the so-called "tiger-snake" of Australia will stand being exposed for ten minutes to from 100° to 102° degrees Centigrade, and its virulence only disappears when this temperature has been applied for twenty minutes. The venom of the "black snake," another Australian variety, loses its toxicity at a temperature of between 99° and 100° Centigrade; whilst an exposure to only 80° Centigrade for ten minutes is sufficient in the case of viper venom, according to Messrs. Phisalix and Bertrand, to profoundly modify its lethal action. A continuous exposure for a fortnight to a temperature of 38° Centigrade does not affect cobra venom in the least; but if during that same time it has been placed in the sunshine, it entirely loses all its lethal properties. Thus, a pigeon was inoculated with about thirty drops of venom which had been exposed to the sun's rays for fourteen days, and it survived; whilst another pigeon was inoculated with a little over six drops of similar venom which had been kept during this time in the dark, and it died in a quarter of an hour.

All these elaborate researches as to the character of serpent venom were essential to enable the next step to be taken in the elaboration of the antidote. Before this great achievement could be accomplished it was necessary to first succeed in artificially immunising animals against the effects of this powerful toxin, so that the serum of such animals could be applied for the protection and cure of other animals from the effects of snakebites.

It may be readily conceived that the task of artificially rendering animals immune from snake poison was not an easy one, for the process depends upon training the animal to gradually withstand larger and larger doses of the venom; and considering the intensely toxic character of the substance which had to be handled, the danger was ever present of the animal succumbing to venom poison before its serum had acquired the requisite pitch of protective power to render it of service as an anti-toxin. Dr. Calmette tells us that he carried out a very large number of experiments before he met with success. But it is not necessary here to discuss his various efforts; suffice it to say that at length his labours were rewarded, and the following extract from one of his memoirs describes the methods which he adopted for this purpose:—

An immense number of animals have been vaccinated by this method at the Pasteur Institute at Lille, where Dr. Calmette is now director; and in one of his memoirs we are told that they have horses there which have yielded during a period of eighteen months serum extremely active against venom. These horses receive in a single inoculation, without suffering the least inconvenience, doses of venom sufficient to kill fifty horses fresh to the treatment.

Large quantities of this serum have been forwarded from the Lille Institute to various parts of the world where venomous serpents are most frequently met with, and already important evidence has been collected as to its efficacy in cases of human beings bitten by dangerous reptiles. So impressed with its importance are Indian medical authorities, that its preparation has been included in the work which the new great bacteriological institute at Agra is carrying on.

The importance of the production in situ of this anti-venomous serum has been recently demonstrated by the experiments which have been conducted in the Plague Research Laboratory, Bombay, by Mr. Lamb and his colleagues, on the keeping properties of such serums in India. From the careful investigations which have been made on this subject, these gentlemen state that anti-venomous serum undergoes a progressive and fairly rapid deterioration when stored in hot climates, and that this deterioration is greater and more rapid the higher the mean temperature to which it is subjected.

The protective potency of this horse-serum may be gathered from the fact that it suffices to inject a rabbit, for example, with a quantity amounting to about one two-hundred-thousandth of its weight to ensure the latter acquiring complete immunity from a dose of venom capable of otherwise killing it in twelve hours.

The rapidity with which it acts is also extremely remarkable. Thus, if a rabbit receive two cubic centimetres (about fifty drops) of anti-venomous serum in the marginal vein of one of its ears, it will suffer with absolute impunity an injection of venom into the marginal vein of the other ear capable of killing it under ordinary circumstances in a quarter of an hour. Its curative powers are not less remarkable, for it is possible to inject venom sufficient to kill an animal in two hours, and to let one hour and three-quarters elapse before administering the antidote, and yet at this late stage to save the victim's life, although it is necessary where such a long interval has occurred between the respective venom and serum injections to employ the latter in larger quantities than is usually required. Dr. Calmette believes that the anti-toxin may be applied at an even more advanced stage of the disease if it is employed in yet larger doses. Another novel and important feature about this anti-venomous serum is the fact that it not only protects animals from one species of very active venom, such as that of the cobra and other poisonous snakes, but it also affords protection from the dreaded venom of scorpions. This is a very remarkable and significant discovery, for hitherto the opinion has been stubbornly held that each toxin requires its specific anti-toxin for its correction. Dr. Calmette has, however, frequently indicated by his researches that this view cannot be considered so completely proven as is claimed by its supporters, and his latest investigations support the theory that particular toxins may be counteracted by several anti-toxins of different origin. Thus it has been shown by Calmette and Roux that rabbits hyper-vaccinated against rabies acquire the power of resisting venom-poison, and that the serum of horses vaccinated against tetanus or lock-jaw also nullifies the action of serpent venom.

The practical bearing of this discovery is obvious, and the hope is justified that the at present cumbrous appliances required for the elaboration of anti-toxins of such varied origin will ultimately give way to simpler and less costly methods, which will admit of these new antidotes being more widely circulated and applied.

We have seen that although most animals fall an easy prey to serpent venom, yet there are a few notable exceptions, amongst which may be mentioned hedgehogs, swine, and the mongoose. Now the very natural question arises why, if these animals are already in such a high degree immune from this poison, should not they be employed to furnish forth protective serum, instead of laboriously training up susceptible animals to become artificially immune and supply this venom anti-toxin?

This brings us face to face with one of the many problems connected with the subject of immunity which so far have successfully eluded all attempts made to solve them. Experience has shown repeatedly that although artificially acquired immunity from a particular poison can be handed on by means of an animal's serum, yet the natural immunity from a given poison enjoyed by one species of animal cannot be similarly transferred to less-favoured varieties.

This fact has long been recognised in the case of poisons of bacterial origin. Thus, white rats are absolutely immune from diphtheria, but Wassermann showed some years ago that the serum of these animals has no power whatever to counteract the action of diphtheria-toxin in other animals. Guinea-pigs were inoculated with fatal doses of diphtheria toxin along with white-rat serum; but although other guinea-pigs treated with the same toxin mixed with the ordinary artificially elaborated anti-diphtheritic serum survived, those which received the rat serum died in every case.

Now very similar results have been obtained by Calmette in respect to the serum of animals naturally immune from serpent venom. The serum of the refractory little mongoose, as well as that of the hedgehog, is wholly unable to save other animals from the lethal effect of venom poison, and similar results have been noted in respect to swine serum. But a very curious fact has also been discovered by Calmette—i.e. that these so-called naturally immune animals very frequently are quite incapable of being artificially trained to elaborate a serum possessing protective powers which can be transferred to another animal.

How splendid a domain for beneficent research lies before the scientific investigator is apparent to all, and the important work already accomplished is but an augury of yet greater discoveries awaiting the labours of such leaders as Calmette. It is not surprising, therefore, that the scientific interest in toxins and anti-toxins shows no signs of abatement. On the contrary, the competition for obtaining and working the new "claims" which pioneer research enthusiasts are constantly engaged in "pegging out" remains as keen as ever.

Despite, however, the extraordinary interest which this subject has aroused in scientific circles all over the world, nearly ten years elapsed before any notice was taken of the curious discovery made by two brothers that the blood of eels contained a highly poisonous principle, and the memoir containing this remarkable announcement remained until comparatively recently buried in the Italian journals where it was first published.

Calmette was, we believe, the first to call attention to this discovery of the brothers Mosso and give it the prominence it deserves, and both he and other investigators have not only fully confirmed it, but have greatly added to our knowledge concerning the character of the poison contained in eel serum.

Now the venerable Izaak Walton, in one of his quaint and most fascinating discourses, which although written more than two centuries ago have a freshness as if penned but yesterday, waxes enthusiastic over the eel, and supplies an elaborate recipe for its preparation for the table, telling us "it is agreed by most men that the eel is a most dainty fish; the Romans have esteemed her the Helena of their feasts, and some the queen of palate-pleasure." The announcement that the blood of eels is poisonous will hardly, despite its scientific interest, form a comfortable subject for reflection to the modern votaries of this novel Helena. Indeed, in the present timid temper of the public, this article of diet would not improbably share the ill-odour which befell the unfortunate oyster and be practically banished from our tables; but although the oyster is perhaps justifiably at present ostracised from our menus, taking the majority of its breeding-grounds into consideration, it would be the height of injustice to measure out similar drastic treatment to the eel.

That the oyster bred in sewage-contaminated beds may revenge itself upon its consumer by infecting him with the germs of typhoid has been repeatedly contended, but that the eel, although its unsavoury surroundings are proverbial, can be held responsible for poisoning those who eat it has never, we believe, been seriously maintained, although there is an old Italian saying which bids us "give eels and no wine to our enemies."

Public confidence, however, in the eel as an article of food need not be shaken, for it is satisfactory to learn that researches which, on the one hand, condemn eels as living generators of a highly poisonous substance, on the other hand allay any alarm which they may have reasonably raised by showing that this toxic principle is entirely destroyed in the processes of digestion, and that, therefore, taken through the mouth it is rendered harmless, and only when introduced into the system by inoculation beneath the skin or injected into the peritoneum can it assert its dangerous properties. That the blood of eels is, however, justifiably to be in future classed amongst the toxins, the number of which has of late been so increased, is at once apparent when we learn that about a dozen drops inoculated into a dog weighing about fourteen pounds will destroy the latter in less than ten minutes, whilst pigeons, rabbits, and guinea-pigs similarly treated, only with smaller quantities, also invariably succumb to its lethal action.

Quite recently an endeavour has been made to determine precisely the degree of toxicity possessed by eel's blood, or, in other words, to standardise the poisonous principle contained in it, so as to afford a guide to those experimenting on the subject; and it has been asserted that one cubic centimetre, or about twenty drops, injected into the veins of a rabbit weighing four pounds, may be regarded as a fatal dose for such an animal. But many difficulties surround such an attempt to exactly define the degree of toxic action possessed by such a substance, for, in the first place, the blood varies in respect to this property in different eels, whilst it also differs widely in character at different stages of the life of the fish. This seasonable variation in toxic character has been noticed in the case of viper venom, which it will be remembered was shown to be far more lethal in action when collected from snakes in the spring of the year than in the winter months.

The toxic substance contained in eel serum was originally called by its discoverers, the Mosso brothers, ittio-tossina; and they record the fact that the blood of rabbits and frogs, which animals had succumbed to its action, did not coagulate after death, whilst, curiously, in the case of dogs this abnormal phenomenon was not observed.

There are various means which may be resorted to for destroying the poisonous principle contained in eel blood, and from a dietetic point of view it is satisfactory to know that heat-exposure for a quarter of an hour to a temperature of from 57·7° to 77·7° Cent. entirely removes it, whilst its virulence is greatly modified by submitting it for a longer period, twenty-four hours, to a much lower temperature, i.e. 37° Cent. It also gradually loses its toxic properties eight days after it has been collected, even when carefully shielded from light, a feature which contrasts favourably with viper venom, which can be kept for more than a year and remains as active as when first derived from the snake. We have seen also that its toxic properties invariably succumb to the processes of digestion, so that even if fashion or fad or advertising speculators, backed by scientific names, were to decree that a wealth of nourishment and support was contained in raw eel "juice," and the edict went out that it was a desirable and highly important article of invalid diet, the general public may, according to its wont, innocently accept the edict and in this case suffer no evil consequences.

But another and very remarkable method of mitigating the virulence of eel blood, and one which so far has received no explanation, is mentioned by Dr. Wehrmann, of Moscow, who has been lately studying the character of this fish's blood in Dr. Calmette's laboratory at the Pasteur Institute at Lille. Dr. Wehrmann found that if blood serum be taken from animals previously rendered artificially immune to the action of serpent venom, and if some of this so-called anti-venomous serum be injected under the skin of eels some hours before they are killed, the lethal properties of their blood after death are considerably reduced. Thus, an eel weighing about six ounces received subcutaneous injections of five cubic centimetres of anti-venomous serum; after the lapse of four-and-twenty hours it was killed and bled, and its serum inoculated into animals in the usual way. But whereas two cubic centimetres of normal eel blood sufficed to kill a guinea-pig, this eel's blood had to be administered in twice that quantity to produce a fatal result, so that its toxic character had been reduced to a very appreciable extent. The readiness with which eel serum parts with its lethal properties, and the restricted conditions under which they can operate, sufficiently assure us that in the present state of our knowledge there is no danger to be apprehended from this fish, and in the absence of any experiments to show what is the effect on human beings of subcutaneous inoculations of such blood, there is no call for this substance to be scheduled under the Poisons Act. We have, however, by no means exhausted the extremely curious properties which characterise this material, and these properties are brought to light in a remarkable manner in connection with the investigations which have been carried out to artificially protect animals from its lethal influence, and also in some interesting experiments which have been made to compare the toxicity of eel blood with that of vipers.

It is far from an easy matter to secure for experimental purposes an adequate supply of eel serum, for even a big fish weighing nearly five pounds is not capable of yielding more than about twenty-five cubic centimetres of blood, and from this only from ten to twelve cubic centimetres of serum are obtainable. Calmette has shown that not only the venom glands of reptiles contain toxic substances, but that the blood of such snakes also possesses lethal properties, only in a far less degree. Curiously, the serum of eels is no less than three times as toxic as the serum of the most vicious viper, and, moreover, produces far more discomfort and pain to the animals into which it is introduced than accompanies the injection of viper blood. In the case of viper blood its introduction is followed by no symptoms of discomfort, the animal remains quite quiet, growing more and more somnolent, a condition which is followed by an abnormal fall of temperature, ultimately ending in complete collapse, symptoms which in a much more modified degree characterise the injection of heated eel serum into animals. This heated eel serum, which we have seen is deprived of the objectionable characteristics of ordinary eel serum, produces but very transitory symptoms in animals, occasioning some degree of somnolence, and now and again a reduction in temperature, a condition from which, however, the animals rapidly recover in from two to three hours. Animals, however, treated with this heated eel serum acquire a power of resisting the lethal action of unheated or ordinary eel serum, and this artificially induced condition of immunity continues for about three days after the completion of the treatment.

The protective properties of this heated serum are not restricted to animals subsequently inoculated with eel serum, but are extended also to animals which afterwards receive injections of viper serum; but of much greater interest and importance is the remarkable fact that heated eel serum, as well as weak doses of the latter not heated but diluted with water, are capable of protecting animals from the fatal consequences of the far more potent viper venom.

It is interesting to note that, although diluted eel serum can protect an animal from so deadly a poison as viper venom, the serum of vipers is quite unable to afford any such service in the case of animals inoculated with ordinary eel serum. The full complement of protective power obtainable from this treated eel serum is only able to slowly assert itself, for it is necessary for a period of as long as twenty-four hours to elapse after its introduction to ensure the animal's system being thoroughly impregnated with it and enable it to withstand a lethal dose of viper venom.

In this respect, what may be designated treated or protective eel serum differs very markedly from anti-venomous serum, which we have seen is serum derived from animals trained up to withstand fatal does of serpent venom, for anti-venomous serum acts immediately, and at once confers immunity on an animal from the lethal effects of such venom.

The rapidity with which it acts is indeed one of the most astonishing properties of this particular anti-toxin. Thus if two cubic centimetres of anti-venomous serum be inoculated into the marginal vein of a rabbit's ear, it at once confers upon the latter complete immunity from snake poison. Immediately after the injection of the serum, venom sufficient to destroy an ordinary rabbit in a quarter of an hour may be injected with impunity into the vein of the other ear. But not only are the protective powers of this serum so remarkable in their degree, but its curative powers, a much more difficult property to establish in a substance, are extraordinarily intense, as may be gathered from the following example. Four rabbits were inoculated with a quantity of venom calculated to destroy them in the space of two hours; one of these four animals was abandoned to its fate, but the other three received, practically at the eleventh hour, viz. just fifteen minutes before the expiration of the calculated two hours' respite, an intravenous injection of a small quantity of anti-venomous serum, only amounting to one four-hundredth part of the weight of each animal respectively. The rabbit which received only the venom died at the end of two hours, whilst the other three remained in perfect health.

But although eel serum can be persuaded to part with its poisonous character and even exercise protective powers over otherwise doomed victims, it is not able to stretch forth a healing hand to the afflicted, for, when once the poison has been introduced, whether it be eel or viper blood, or the venom of snakes, it is absolutely powerless to mitigate or stop in any way the deadly progress of the toxin. Thus whilst eel blood may acquire protective properties it cannot acquire curative properties, and, therefore, treated eel serum cannot be legitimately enrolled with the anti-toxins which have been elaborated, as, for example, anti-venomous serum, for, to be worthy of such rank, a substance must be capable of wielding both protective and curative powers.

But, although eel serum may under certain conditions protect from the lethal action of serpent venom, eels are not themselves under ordinary circumstances endowed with any power to withstand the influence of this poison, for a good-sized eel will succumb to a dose of venom which is sufficient to kill a guinea-pig.

Considerable interest is attached to the fact that anti-venomous serum not only acts as an anti-toxin towards serpent venom, but also towards a poison of quite a different character, such as that present in the normal blood of eels, for this fact tends to confirm the view upheld by some authorities, that specific toxins do not necessarily only yield to specific anti-toxins, and that a particular anti-toxin may act as such towards divers toxins of varied origin and character. Calmette has brought this point out very clearly in his later investigations on the vegetable poison abrine, a very powerful toxin, furnished by the active principle of the seeds or beans of a leguminous plant common in India and South America, and frequently used, as already mentioned, by the natives in India to revenge themselves on their enemies in poisoning their cattle. Immunising serums of various kinds were selected for testing their protective action on animals poisoned with abrine, and it was found that anti-tetanic, anti-diphtheritic, anti-anthrax, and anti-cholera serums all individually exerted a decided immunising action with regard to this powerful vegetable poison. The hope is, therefore, perhaps not beyond the realm of possibility, that at some future time the complexity of drugs which now figure in the chemists' pharmacopoeia may be replaced by a few substances the application of which will come within the means and understanding of all. So far we have not dealt with the artificial immunisation of an animal from the action of eel poison, but this apparently offers very little difficulty, and is accomplished by introducing very small and gradually increasing doses of eel serum into the system, care being taken to proportion the quantity given according to the weight and general condition of the animal to be immunised. A rabbit, for example, treated in the above manner, subsequently yielded a serum which was proved to possess both preventive and curative powers in respect to both eel poison, and viper venom and blood, entitling this so called anti-eel serum to take its place amongst the anti-toxins, and furnishing yet another instance of a substance exercising its immunising influence over various toxins.

This process of gradually acclimatising, as it were, animals to a particular poison by repeated doses of the same poison, recalls the old proverb, "Seek your salve where you got your sore," and brings us to a consideration of some of the primitive antecedents of a practice which, at the present time, promises to bring about so profound a revolution in the art of medicine. The modern system of inoculation has, however, arisen quite without reference to such antecedents, which latter were not based upon any scientific laws or considerations, but owed their evolution to local customs and experience handed down from age to age by tradition, and in many cases preserved through a simple faith in the superstitions which surrounded them.

To such a category must be added the curious superstitions indulged in by the native population of Tunis regarding methods of preventing hydrophobia in persons bitten by rabid animals. Dr. Loir refers to these primitive ideas on the art of healing in a report of the work carried out at the Anti-rabic Institute at Tunis, one of the many centres for the prevention of rabies by Pasteur's method which have been established in every quarter of the globe except Great Britain, the inhabitants of this "great conservative island-Empire," as a renowned foreign scientist describes it, still preferring a trip to Paris to countenancing the establishment of an anti-rabic institute in their own country. The Arab physicians in Tunis have from time immemorial sought to specially identify themselves with cures for this disease, which is so prevalent as to be a veritable scourge to the country. A much-vaunted remedy advocated by the profession consists in pounding up the charred head of a rabid dog with vinegar, and administering an emulsion of the same to the patient. The dung of camels is also highly prized as a remedy, as also the water of certain wells which the simple faith of the natives has endowed with supernatural curative properties. But the strangest prescription of all consists in broth made from lambs a year old, to which is added a peculiar kind of beetle, but in such a small quantity that the latter ingredient only equals the weight of a grain of corn. This concoction is given to the unfortunate patient twenty-three days after he has been bitten. In the urine, according to the Arabian doctors, seven small worms should be found which represent the embryos of dogs engendered by the virus in the human body, and which when once got rid of the patient recovers!

In the face of such crude traditions upheld with so much tenacity by the native population, it is surprising that the Tunisian Anti-rabic Institute has met with such a large measure of support in the shape of applicants for admission, which, on an average, number over one hundred annually. The mortality amongst those treated closely approaches the satisfactory results obtained at the Paris Institute, where the death-rate amounts to about 0·38 per cent. of the persons treated.

There is perhaps no more interesting chapter in the history and literature of medicine than might be compiled by searching out the early uses of drugs and the primitive application of methods in the art of healing, and tracing their connection, if possible, with the practices which are in vogue at the present day. In the matter of toxins and anti-toxins, or in respect to the modern theories of preventive medicine, there would appear to be a curious link between the methods based upon elaborate scientific inquiries and those which arose through simple experience and expediency.

The idea of a poison, as the old proverb above tells us, being a corrective for itself is no new idea, for we read how in ancient times, for example, the Ophiogenes of the Hellespont were renowned for their immunity to snake poison, and one account of them states particularly that they fed upon serpents, and that to this diet they probably owed their reputed magical art in withstanding the action of serpent venom. Again, a traveller in Egypt, Hasselquist, tells us how the serpent-charmers there eat serpents, making them into a kind of broth, and that invariably before starting off to catch these reptiles they partake of some of it.

In a paper by Mr. T. R. Rao on the YÁnÁdÉs tribe of the Nellore district, Madras Presidency, the author mentions that these strange people have, amongst other characteristics, absolutely no fear in catching cobras, which they draw out of their holes without any alarm as to their fangs, and that they appear to protect themselves against the effects of snake-bites by swallowing the poison-sacs of snakes.

Bruce describes how he saw a serpent-charmer in Cairo who allowed himself to be bitten by a viper between the forefinger and the thumb, and made no endeavour whatever to apply remedies, neither did he exhibit the slightest anxiety as to the consequences. That this was no trick, and that the viper was really possessed of all its deadly faculties at the time it bit the man, was proved by the fact that a pelican subsequently bitten by the same animal died in thirteen minutes. Bruce also tells of a man who "with his naked hand took a viper from a number of others lying at the bottom of a tub. He put it on his head, then in his breast, and tied it about his neck like a necklace. Next it was made to bite a hen, which died in a few minutes; and, to complete the experiment, the man took it by the neck, and, beginning at the tail, ate it as one does a carrot or a stick of celery, without any seeming repugnance."

A most interesting account of snake-charmers is given by Drummond Hay, in his book on Western Barbary, in which he relates his experiences with some of these wonderful individuals belonging to the sect called Eisowy. Members of this sect, he mentions, frequently handled scorpions and poisonous reptiles without fear or hesitation, and they were never attacked by them. He was present at one of their exhibitions of feats with snakes in which they both allowed themselves to be bitten and provoked the snake to bite them. The charmer thus bitten then in his turn ate or chewed the reptile, which, he remarks, writhing with pain, bit him in the neck and hands till it was actually destroyed by the Eisowy's teeth.

In South Africa snake poison is actually taken as a protection against snake-bites, and if we turn to the Lancet of the year 1886, we shall find a letter from Mr. Alfred Bolton stating that his curiosity had been aroused by the fact that while in South Africa cattle and horses frequently died from the effect of snake-bites, the natives themselves seldom or never appeared to suffer any inconvenience from such injuries other than would follow any accident which would set up local inflammation. On inquiry he found that they were in the habit of extracting the poison gland from the snake immediately it is killed, squeezing it into their mouths and drinking the secretion, thereby apparently acquiring absolute immunity from snake-bites. So impressed was Mr. Bolton by what he observed that he adds: "I can no longer refuse to believe in the efficacy of the snake virus itself as a remedy against snake poison."

Savage tribes have learnt from bitter experience how to protect themselves from snake-bites, and it is well known that they have a method of inoculation which they employ with success. The Creoles of Surinam use an ointment as a protection against snake-bites, which is regarded as highly efficacious. It is reputed to consist principally of the pounded head of a rattlesnake, which concoction would therefore include the contents of the venom glands. This is then mixed with the juices of a certain plant, which addition probably mitigates the intensity of the venom by acting as a diluent. This substance is generally applied by making an incision in the wrist or forearm and rubbing it in, after which individuals thus treated appear to enjoy security from the venom of snake-bites.

What applies to serpent venom would also appear to hold good in regard to other poisons, such as that contained in the sting of a bee. This poison is extraordinarily tenacious of its irritant properties, and, unlike eel poison, retains its virulence even when exposed to high temperatures.

An interesting memoir on the immunity of the bee-keeper from the effects of bee poison was published a short time ago by Dr. Langer in a German scientific journal. He issued a number of circulars with questions to be answered, and sent these to more than a hundred bee-keepers in different parts of the country, with the result that a hundred and forty-four stated that they were now immune to bee poison, nine having been fortunately endowed with a natural immunity to this irritant, whilst only twenty-six out of the whole number applied to stated that they were still susceptible.

This condition of immunity to bee poison is obtained after a varying number of stings have been inflicted; in some cases thirty, at the rate of from three to four a day, are sufficient to ensure freedom from further discomfort, but the inoculations may have to be prolonged up to one hundred stings to secure complete immunity.

In experiments carried out on animals this immunity to bee poison has been also induced by repeated application of the irritant. It was formerly generally supposed that the irritant nature of a bee's sting was due to the presence of formic acid; but inasmuch as bee poison can retain its poisonous character in spite of being submitted to heat, which would effectually volatilise the formic acid present, this assumption must be abandoned, and opinion is more inclined now to regard this irritant substance as partaking of the nature of an alkaloid.

Before closing this brief review of some of the most recent discoveries which have been made in the domain of immunity, we must mention some extremely suggestive and important researches on the poison of tetanus, or lock-jaw, which have emanated from Dr. Roux's laboratory at the Institut Pasteur in Paris.

It will perhaps be remembered that Pasteur, when working at hydrophobia, experienced the greatest difficulty in exciting rabies in animals with certainty, and that it was only when the fact of its being a disease which essentially affects the nervous system of the animal was taken into account that it occurred to him to cultivate the virus in the medium for which it had seemingly the greatest affinity, viz. the nervous tissue of an animal; it was only on taking this step that he succeeded in invariably provoking rabies in the animals under experiment.

In the case of tetanus we have another disease affecting the nerve-centres of the body, and although many authentic cases have been cited in which the treatment with anti-tetanic serum has been entirely successful, a great many instances have occurred in which it has been of no avail at all, more especially when the disease has obtained a firm hold on its victim. Now Dr. Roux has not only been carrying out experiments to ascertain what is the result of directly attacking, as Pasteur did in the case of rabies, the nerve-centres of an animal with the tetanus toxin, but he has also taken another and very important step further, and has investigated, not only the action of the toxin, but also that of the anti-toxin on the nerve-centres of an animal suffering from tetanus.

In describing the cerebral inoculations which he has conducted on animals, Dr. Roux points out that the operation, in itself, is attended with no pain or even inconvenience to the animal in question, that subsequently it eats with its usual appetite, and shows no signs of discomfort.

First, as regards the infection of an animal with the tetanus virus introduced directly into the brain, it has been found that very much smaller quantities produce a fatal result than when subcutaneously inoculated. Thus, a rabbit which received two cubic centimetres of the poison under the skin took four days to succumb to tetanus, whilst one-twentieth of the quantity inoculated into the brain sufficed to kill another rabbit of the same size in less than twenty hours.

Another very instructive example of this susceptibility of the nerve-centres for certain poisons is afforded in the case of rats and the toxin of diphtheria. Rats possess a natural immunity from this substance, and can successfully withstand a dose of diphtheria poison introduced under the skin which would infallibly kill several rabbits. This state of immunity, however, entirely disappears when the toxin is brought directly in contact with nervous tissue, for a very small quantity of diphtheria poison—insufficient to cause under ordinary circumstances even a passing swelling at the seat of inoculation—will, when introduced into the brain of a rat, kill the animal.

Again, rabbits are generally credited with possessing high powers of resisting the action of morphia, a large dose of this substance introduced subcutaneously producing no result whatever. A cerebral inoculation, however, of a minute quantity of morphia causes an immediate reaction, and the animal, after remaining in a more or less dazed condition for several hours, finally succumbs to this drug. Dr. Roux is inclined to regard this difference in the susceptibility exhibited by animals to one and the same poison as being due to a good deal of the toxin, when subcutaneously introduced, failing to reach the nerve-centres, it having been destroyed or arrested in the system before it could attack them.

What is the nature of the subtle forces which may so beneficially intervene between the toxin and its victim has long been a problem which has excited the interest and ingenuity of some of the most brilliant scientific authorities of the day, and it is one which, even in the hands of men like Metchnikoff, is still awaiting a satisfactory solution!

The important point was next approached by Dr. Roux as to whether an animal, successfully trained to withstand large doses of the poison, as ordinarily introduced, could also resist it when directly inoculated into the brain. Is, in fact, the undoubted immunity to tetanus poison which may be possessed by an animal due to the nerve-centres having become insensible to this substance? The answer to this question would appear to be in the negative, for animals artificially protected from tetanus poison introduced under the skin succumbed to a small dose inoculated direct into the brain, which would otherwise have not produced even a slight passing tetanic affection of the limb where the inoculation was made. Immense numbers of experiments were made under varying conditions, but the result was fully confirmed, showing that the nerve-centres had not acquired any immunity to the poison, although the blood serum of the victims to such cerebral inoculations was proven over and over again to be endowed with strong protective properties against tetanus poison.

The endeavour was then made to, in Dr. Roux's words, "place the anti-toxin where the toxin is working," and preserve the vital force of the nervous tissue. To arrest tetanus by substituting cerebral for subcutaneous inoculations of the anti-tetanic serum was the next feat attempted. Several guinea-pigs and rabbits were inoculated subcutaneously with virulent doses of tetanus poison sufficient to kill them in about seventy hours; some were subsequently treated with anti-toxic serum introduced in the ordinary way under the skin, whilst others were inoculated with from six to seven drops of this protective serum direct into the brain. The results were extraordinarily successful. Although but a few drops of the anti-toxin were used for the cerebral inoculations, the animals survived the otherwise fatal doses they had received of the toxin; whilst out of seventeen guinea-pigs which received subcutaneous inoculations of the anti-toxin only two recovered, and the quantity of the anti-toxin employed reached as much as from ten to twenty cubic centimetres in some of the experiments, contrasting in a remarkable manner with the few drops which sufficed in the case of the cerebral inoculations.

Dr. Roux sums up this splendid result in the following modest words: "Il ne suffit pas de donner de l'anti-toxine, il faut la mettre au bon endroit."

The significance and far-reaching application of this most important discovery cannot easily be overestimated. Hitherto the preparation of an anti-toxin has been the chief point considered, but Dr. Roux and his able coadjutor, M. A. Borrel, have shown how great may be the results which attend its method of administration, and have opened up an entirely new direction for investigation.

Although the subject of immunity is not, as we have seen, by any means wholly a latter-day creation, yet its approach and consideration from a modern point of view, assisted by the resources and equipment provided by modern scientific methods, justifiably entitles the nineteenth century to claim it as its own discovery.

However brilliant and successful the labours may be of those who will follow in the future, subsequent generations will know how to venerate those great leaders of scientific thought, amongst whom we must rank Pasteur, to whose genius the world owes so great a debt of gratitude, and the vast extent of whose labours cannot be adequately measured at the present day by reason of the restricted scientific horizon which encircles public opinion in this country.

THE END

PLYMOUTH
WILLIAM BRENDON AND SON
PRINTERS