THE EVOLUTION OF A BURSA 187 



demonstrate the fact that frequent friction of skin over a hard 

 surface has the power of producing adventitious bursse in regions 

 where they are not found in the normal state. 



These adventitious bursse are the following : — 



In the first place certain normal bursse 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. 



They are Prepatellar bursse — " housemaid's knee." 



Olecranon bursse — " student's elbow " and " miner's 



elbow." 

 Tuber ischii bursse — " weaver's bottom." 



These may be called " occupation-bursse " and may be classed 

 with three other well-known adventitious bursas 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 bursse 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 vertebrse in a humpback. A structure 

 closely resembling a bursa and arising from similar causes to those 

 producing adventitious bursse 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 concep- 

 tion 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 



