A bursa exercises a function in the animal body which is the direct opposite of that shown to belong to the flexures of the hand and foot. Whereas the latter are adapted to the preven­tion of slipping in the act of prehension, bursÆ are delicate contrivances for producing the maximum effect of sliding, within certain limits, between two opposed surfaces, either between the skin and a hard surface beneath it, between two muscles, or a tendon as it moves over a bone. As they are very variable and most of them are inherited and congenital, while some are produced only in the lifetime of the individual, they are useful for considera­tion in regard to the questions of transmission of modifications and of the origin of initial variations. Their degree of utility ranges, for example, in man, from that of the prepatellar bursa without which no useful movement of the knee-joint is imaginable, to the insignificant bursa which may or may not be found on the dorsal surface of a phalangeal joint of the foot. The principle laid down by Lyell, to which allusion has been made elsewhere, that is, of “explaining changes in the surface of the earth by reference to causes now in action,” is applicable in this small department of the evolution of a minor structure of the animal body. As man furnishes the largest of all collections of these lubricating organs, his skeleton and skeletal muscles will form the main subject of this chapter, and I venture, if one may say so, to “Lyell” them. None of the sections of this book except that on the mammalian hair affords so simple and easy a field for watching in operation certain mechanical forces. We may here go down to the potter’s house and watch him moulding his clay, or the cobbler his leather. So much are bursÆ in the human body under the power of extraneous forces that I venture to say that if some young surgeon of an inquiring mind were to choose a place and time when the Honourable and Vigilant Stephen Coleridge was out of the way, and were to produce in a young chimpanzee under an anÆsthetic a “greenstick fracture” of his radius and ulna, immobilising it at a right angle for a month, the animal would exhibit at his death some years later a highly developed bursa over the bony protuberance nearly as good as the olecranon bursa on the uninjured side, and better than that of the injured limb. As I have reason to know the meticulous vigilance of this professional and expert humanitarian I hasten here to say in advance that I do not recommend this experiment, not because it would not be entirely justifiable, but because nature herself in the highest Primate has produced many undesigned experiments of nearly equal value, as I hope to show.

BursÆ Described.

Broadly considered a bursa is a sac lined by synovial membrane, and an extreme example of the simplest form in which it is found may be said to be that of the condition found in a domestic dog. Under its skin, except on such regions as the snout, the tail and the feet, there is hardly a place where a bursal surface does not exist. Here and there trabeculÆ may divide the great sac imperfectly, but from the protective and selective point of view this mechanism under a dog’s skin may be compared to the oil with which an Indian criminal lubricates his naked body so as to elude capture. To us who are too familiar with dog-fights (to which the Hon. Bertrand Russell likened the recent Great War, as we all remember) and who know how much noise and ferocious attempts are made by the warriors to bite one another, and how little success they achieve, the beautiful adapta­tion of nature in the dog far surpasses that of the Indian criminal. Indeed the latter may well have been suggested by the former.

Between such a simple and undifferentiated bursal surface as this and another such as the small but essential bursa under the tendo achillis there are endless variations adapted to particular uses and regions.

The descrip­tion of bursÆ given by Macalister is too clear and good not to be given in his own words.67

“Synovial membranes are found either as the lining of joints, or as BursÆ, which are closed sacs (a) between contiguous soft parts, or (b) beneath soft parts which glide tensely over a bone. BursÆ are formed around and beneath tendons in the neighbourhood of joints; and the hard part on which the tendon plays is often invested with a layer of cartilage over which the synovial membrane does not extend. When they completely surround tendons, as in the finger and toes they are called thecÆ or sheaths, and the tendons are connected to the sheaths by synovial reflections. Sometimes bursÆ lie between exposed areas of skin and projecting bony points, such as the patella, olecranon, ankles, etc.

“Their (synovial) membrane differs from the synovial membrane of joints in not having so continuous or definite an endothelial lining; indeed, while some bursÆ, such as that beneath the ligamentum patellÆ, have a more or less regular lining of regular endothelium, others have only elongated connective cells forming an imperfect lamella, and there are all possible gradations met with between the regular saccular bursa, and a loose meshwork of areolar tissue of which the bursa is only a specialisa­tion. BursÆ may be (1) subcutaneous, (2) subfascial, (3) between two tendons, or (4) between tendons and subjacent ligaments or bone. Of these, some communicate with the neighbouring joints always, some occasionally, and some never. BursÆ underlying parts which have an extensive range of motion are unilocular, with a single cavity. BursÆ spread over an extensive surface, and whose walls move but little on each other, are often divided by imperfect fibrous septa, and are called multilocular. Almost all the lesser bursÆ are unilocular, most of the subcutaneous bursÆ are multilocular.”

Now if one were not engaged upon such a problem as that of initiative in evolution and in trying to give examples of it there would be no Gordian knot to cut, and the condensed statement of Macalister might be simply taken as an accepted account of the manner in which reading between the lines a bursa is formed in the animal body. But, when an hypothesis such as the present is in question, one may not cut the Gordian knot in this way, and must produce briefly certain observations of the process, not only those known in man by anatomists and surgeons but also some found in lower Primates.

Human BursÆ Enumerated.

The following is a list of bursÆ in man of which some are normal or always present, and others which are both occasional in their appearance and often imperfectly developed.

Front of Neck.

Pharynx. A small central pit constituting a single bursa the bursa pharyngea.

Behind the angle of the lower jaw. One.

On the symphysis of the chin. One.

On the Acromion process. One.

Beneath the deltoid and the acromion process, one large bursa often opening into the shoulder-joint.

Elbow.

Wrist.

Hand.

Region of hip.

Thigh.

Knee-joint. The prepatellar bursÆ.

In the Ham.

Tibia.

Ankle.

Foot.

I calculate that there are at least fifty-two separate bursÆ (about one hundred on the two sides of the body) in the normal or fully developed state, though of these many will be found either absent or with very little of the full structure of a bursa. One small but significant point may be referred to here. We are all familiar with the prominence of the knuckles of the hand and the very efficient bursÆ which cover them, but most persons do not recognise that the foot has no such knuckles (or prominent metatarso-phalangeal joints) and no bursÆ over these joints, except that of the great toe which happens to be very much more exposed to friction and has a much greater range of action than the other four metatarso-phalangeal joints. This might be called by some persons a beautiful bit of adapta­tion for locomo­tion and by others an equally admirable bit of adapta­tion produced by locomo­tion.

Examination of Two Still-born Children.

Some further light may be thrown upon the human bursÆ by an examina­tion of two still-born children I dissected in 1908 in Lewisham infirmary, and give here the results as to the more important subcutaneous bursÆ.

Male Child: full term.

This example of a still-born, but otherwise normal infant illustrates well the previous statement that certain bursÆ are congenital and others of less functional importance are formed after birth. Whereas the olecranon, wrist, patellar, ankle and tendo achillis bursÆ are fully formed, those under the acromion processes, one of those of the condyles of the femur, and the digits of the hand, those over the superior anterior spines of the ilium and those of the foot are little if at all developed in this case.

Another still-born child at seven months was also dissected and this had well-formed prepatellar bursÆ, scanty ones over the olecranon processes, also over the small joints of the hand and foot where they were difficult to isolate and over the malleoli they were only slightly developed.

A foetus in spirit I examined and found no commencement of a prepatellar bursa.

Examination of Living Primates.

Anthropoid Apes.

Eight of these I examined during life at the London Zoological Society’s gardens in 1908, four chimpanzees, two orangs and two gibbons. These afforded the opportunity of ascertaining by means of touch the presence, and in a minor degree the size and efficiency of the main subcutaneous bursa, just as one can do this in a human subject. The chimpanzees were A, aged thirteen; B, aged seven; C, aged three; and D, aged two-and-a-half years; the orangs E, aged thirteen; F, aged three years; the gibbons G and H both two to three years.

These eight specimens possessed good examples of the leading subcutaneous bursÆ over the olecranon process, the styloid process of the ulna, the patella and both malleoli.

The smaller and less definite bursÆ gave the following results.

Chimpanzees.

Orangs.

Gibbons.

Dead Specimens.

I also examined the hands and feet after death of certain lower Primates in 1909:—

Hapalemur Griseus H. Hands. No bursÆ on styloid processes of radius and ulna, and no localised bursÆ on any metacarpo-phalangeal or phalangeal joints.

Hapale Jacchus I.

Cercopithecus Callitrichus J.

Cercopithecus Mona K.

Macacus Rhoesus L.

Further Undesigned Experiments.

The preceding facts as to the natural history of bursÆ in man and some lower Primates, even if they stood alone, are enough to produce convic­tion as to the manner in which bursÆ of all degrees of perfec­tion are formed by function, and point to the origin of the initial stages of these structures. But they do not stand alone, for in man there have been carried out certain undesigned experiments in a similar direction, comparable to those described in the sections on direction of hair and arrangement of papillary ridges. These demonstrate the fact that frequent friction of skin over a hard surface has the power of producing adventitious bursÆ in regions where they are not found in the normal state.

These adventitious bursÆ are the following:—

In the first place certain normal bursÆ in important situations are frequently so much enlarged by the constant irrita­tion of pressure and friction that they become considerably enlarged. This enlargement may go on to definite pathological changes and thus come under the care of surgeons.

These may be called “occupa­tion-bursÆ” and may be classed with three other well-known adventitious bursÆ which are formed on the shoulder in “deal runners,” on the scalp in “fish porters” and in the back of the neck in Covent Garden porters, known as a “hummy.” Entirely new bursÆ are formed also over the cuboid bone in talipes equino-varus, over the internal condyles of the femur in bad cases of knock-knee from friction of one joint against the other, over the prominent vertebrae in a humpback. A structure closely resembling a bursa and arising from similar causes to those producing adventitious bursÆ is found in unreduced dislocations or ununited fractures.

A small example of an adventitious bursa came under my notice. A woman, E.L., aged 49, had remarkable enlargement of the metatarso-phalangeal joint of her great toe of the left foot, and over this joint was formed a well-marked bursa on the dorsal surface. The right foot showed a much less prominent joint and only a very slight development of the corresponding bursa.

This instance of a bursa-like structure being produced in unreduced dislocations and ununited fractures suggests the concep­tion which I here propose, but do not attempt to verify that all joints in all animal forms from the lowest up to man have been evolved in a manner to which this pathological experiment may give a clue.

A remarkable case reported by Sir William MacEwen in the Royal Society’s Phil. Transactions, Series B, Vol. 199, pp.253, 279, is worth referring to in this connec­tion. It was a case of a growth of bone in muscle connected with an old injury to the thigh of a man 38 years old, and healthy. At the operation performed by the author of the paper the tumour was found to be movable, partly attached to the fascia lata of the thigh, and the upper part of the tumour moved on the lower. It was found that the tumour consisted of two parts, the upper three-and-a-half and the lower seven inches long, altogether a mass about ten inches in length. Muscular bundles of the vastus externus were included in this ossific formation, one passed through a tunnel in the bone through which it worked, and the sides of it were polished. At the point where the newly formed bone came in contact the surfaces fitted each other and were polished as if they were covered with cartilage, and were here surrounded by a capsule. (Italics not in original.) This fibrous covering when opened was seen to contain a thin serum, which, though not of the consistence of synovial fluid, still aided in lubricating the polished surfaces as they played over one another.

A similar case was reported also by Dr. C. Paterson, surgeon to the Glasgow Royal Infirmary.

A very interesting address by the Hunterian Professor, Mr. Jonathan Hutchinson, was given in February, 1917, on Dupuytren’s work, especially in the discovery of the cause and treatment of the contrac­tion of palmar fascia known by his name. Professor Hutchinson described his method of curing this by the removal of the head of the first phalanx, and showed excellent results and evidence of the formation of a perfect new joint to take the place of the old distorted one, and the fingers were as efficient as in the normal state in the exercise of flexion. He gives photographs of the hand some months after the operation showing it to be capable of easy and full extension as well as of flexion. This again agrees well with the cases of Sir W. MacEwen and Dr. Paterson of the formation of a functional joint by use and habit.

Another distinguished Hunterian Professor A. Keith, also gave two lectures in January, 1918, on the “Introduc­tion of the Modern Practice of Bone-grafting,” which, in its modern form, he assigns to the credit of Sir William MacEwen. He lays great stress on the important work performed in such cases by the osteoblasts without whose living and formative action these results could not be obtained. He explains how necessary it is that these living elements should be stimulated into action by work. They thrive only so long as they have work to do. Another surgeon, Ollier, “wondered why the fragments of bone which he had succeeded in raising from slips of periosteum planted beneath the scalp or amongst muscles ceased to grow and tended to disappear. These bony grafts withered because they were not subjected to the strains and stresses which rouse the activity of osteoblasts.” MacEwen, “by a fortunate chance, planted his tibial grafts in a situation where they soon became subjected to muscular strains and stresses. In a short time bony fragments gathered from the legs of six boys became intrinsic parts of the humerus of a seventh; from the moment of primary union the bone cells of the graft were brought under the stimulating impulses of the biceps and triceps. Osteoblasts are the obedient slaves of muscles; muscular dominance is their breath of life.” (Italics not in the original.)

“Wolff was the first to devote thirty years of constant work and observa­tion to prove that the shape and structure of growing bones and adult bones depend on the stresses and strains to which they are subjected. By altering the lines of stress the shape of a bone can be changed.”

Wolff’s law is simply this: “Osteoblasts at all times build and unbuild, according to the stresses to which they are subjected.”

Professor Keith says further: “We are driven, as I have pointed out in a previous lecture, to look for the primary cause, not in the bones, but in the muscles, particularly in those which are tonically and constantly in action so long as we are standing.”

A terse expression of Wolff’s law is quoted from Dr. John B. Murphy, of Chicago: “The amount of growth in a bone depends upon the need for it.”

A remarkable illustra­tion of a similar process is given in the construc­tion of sponges by the scleroblasts and it is stated: “The soft walls of this sponge are constantly exposed to the force of moving waters, and we shall see that the spicule-builders—the scleroblasts—are endowed with the same properties as osteoblasts—the powers of fashioning and depositing the elements of the skeleton so that the sponge can best resist the forces to which it is habitually exposed.”

One more important quotation from this lecture will suffice. “No one who has watched the behaviour of scleroblasts and marked the design in their workmanship can doubt that they have acquired certain characteristic qualities, chief of which is a sensitiveness to vibrations—to stresses. We see them build the same form of spicules as their ancestors, and therefore must suppose that their building quality is a gift of inheritance. We see them alter their mode of building as stresses change; we must therefore suppose that their inherited powers can be changed by the circumstances under which they work.”69

In regard to the action of the scleroblasts of sponges I have only to point out that the cautious words of Professor Keith on the treacherous ground of inheritance amount to the very same concep­tion of personal selection and inheritance as are involved in the term “educability” of Sir E. Ray Lankester. Whether or not in the case of sponges this be a complete account of the matter it at any rate is a very important piece of evidence, if valid, for selection. Whether or not further it is a piece of evidence for a Mendelian factor implicit in the primordial sponges and released by some loss of inhibiting factors, as Professor Bateson would probably claim, is another and far more imaginative concep­tion. The mere neo-Lamarckian with the aid of personal selection fails to see any difficulty in realising the wonderful process described by Professor Keith.

An apology must be offered here to the patient reader for the introduc­tion under the heading of the “Evolution of a Bursa” of the apparently alien subjects of bone-grafts, artificial new joints and sponge-spicules, but I have hazarded the guess that all joints in all animals have been fashioned—“forged by the incident of use,” to employ a fine phrase of Professor Macdonald’s in another connec­tion—in slow but intelligible ways by use, and that in them, as elsewhere, function has preceded structure. This arose so simply out of the story of the bursÆ that I ventured to digress as aforesaid rather than make it the subject of a separate section.

The Significance of the Proceeding.

The foregoing slender contribu­tion to the comparative anatomy and physiology of bursÆ is sufficient to show that at certain important and “critical” points in the mammalian anatomy, efficient bursÆ are always present. One cannot indeed conceive the function of the parts involved being carried on at all without these ingenious contrivances, and no doubt can exist that in certain of the leading bursÆ selection guides and guards, while use and habit maintain them. Over such as these “dominance” or the appearance of mutations might perhaps be supposed to preside, and possibly some useful statistical results might arise from their study from these points of view. But, between these major bursÆ in man and lower Primates and the undifferentiated sacs which hardly deserve the name of bursÆ, there is a perfect little host of insignificant structures, which at the first attempt at dominion over them on the part of Mendel or de Vries would hoist the standard of revolt. These would even refuse allegiance to Personal Selection under the persuasive banner, “Educability,” which however valuable elsewhere, must stand aside in this little province of Nature. I have thus attempted to “Lyell” this body of facts. Basing the statement on an analysis of a considerable mass of small facts which no one disputes I claim that the modifications drawn from normal anatomy on the one hand and on the other adventitious structures, produced by acknowledged mechanical forces, are examples of the transmission of modifications, and illustrate the mode of formation of certain initial variations. In other regions where Plasto-diethesis, as I conceive it, is at work in producing adapted organisms, there may be included in the hyphenated area certain factors of heredity, Mendelian, mutational and others, but not in this group. This is merely an assertion of an opinion though I submit that there is good evidence for it. Not even the hardest hearted Weismannian, Mendelian or mutationist, and not even the biometrician can refuse to this poor little province the required time and mechanical forces, and, unless an opponent can offer some explana­tion more consistent with the facts than that here offered, the proof of causation is as sound as that shown in the larger one of the direction of hair.