THE EVOLUTION OF A BURSA 179 



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. 



Bursae 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 trabecule 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 sug- 

 gested 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 par- 

 ticular uses and regions. 



The description of bursse given by Macalister is too clear and 

 good not to be given in his own words. 1 



" Synovial membranes are found either as the lining of 

 joints, or as Bursce, which are closed sacs (a) between con- 

 tiguous soft parts, or (6) beneath soft parts which glide tensely 

 over a bone. Bursse 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 thecce or sheaths, and the tendons are con- 

 nected to the sheaths by synovial reflections. Sometimes 



1 Text Book of Human Anatomy. A. Macalister, 1889, p. 48. 



n2 



