CHAPTER XX. 



THE EVOLUTION OF A BURSA. 



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, bursse 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 life- 

 time 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 bursse 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 anaesthetic 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 



