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.