OP THE SUB-CUTANEOUS SYNOVIAL BURS.E. 161 



and frequent motions; as between the skin and the patella, the 

 olecranon and the skin, on the trochanter, on the acromion, 

 before the thyroid cartilage; sometimes behind the angle of 

 the jaw; always between the skin and projecting side of the 

 metacarpal and metatarsal articulations, and between those of 

 the first and second phalanges. All these latter are generally 

 confounded with the neighbouring tendons. 



In order to have a good view of these membranes, we must 

 fill them with air. It is then seen that they form an obround, 

 multilocular cavity, that is, a cavity divided by incomplete, 

 but closed partitions, the air blown into it, remaining shut up 

 there, and not passing into the surrounding cellular tissue; the 

 walls of the cavity they form are very thin and weak. 



Their texture is very simple, like those of the serous mem- 

 branes generally, and appears to differ from that of the cellular 

 tissue, only by a little greater density. There are but very few 

 vessels in the thickness of these membranes: their free and 

 contiguous surface is humected by an unctuous or mucilagi- 

 nous liquid, in such small quantities, as to preclude the possi- 

 bility of its being properly examined. 



The local use of these membranes and the unctuous liquid 

 they contain, is evidently to facilitate the motion of the bones 

 under the skin. 



These bursae are developed at a very early period ; they exist 

 at the time of birth, and are then easily perceived on account 

 of the greater abundance of the liquid that moistens them. 



Their development augments in proportion to the exercise 

 of the parts they cover; that of the acromion, for instance, be- 

 comes more apparent in individuals who carry burdens on the 

 shoulder; that of the knee is most developed in those who are 

 habitually kneeling. 



203. They are formed accidentally, in those cases where 

 the skin exercises accidental rubbings. M. Brodie speaks of 

 a gibbosity, on which one was developed in consequence of 

 the continued sliding of the skin: the same thing is observable 

 in club-foot in the spot where the skin rubs against the salient 

 side of the tarsus; and again, the same result takes place after 



