394 



In the majority of cases it would seem that the descent of the 

 anterior end of the first rib, which also occurs between birth and 

 adult age, would neutralise the stretching effect of the descent of the 

 shoulder on the nerve (see Fig. V). The result of this investigation 

 has shown that great individual variation occurs in the descent of 

 both the shoulder and the anterior end of the rib (manubrium). Moreover 

 the descent of one structure bears no relation to the descent of the other. 



Again, the distance travelled by the surface of the rib which is 

 crossed by the nerve trunk is less than half that travelled by the 

 anterior extremity of the bone. For the nerve crosses posterior to 

 the mid-point of the length of the rib. In the present connection 

 it may be noted that the clavicle in the female attains a somewhat 

 greater proportionate length than it does in the male. 



Pasteau states that the clavicle has a length of 45.04 \ that of 

 the humerus in the female. In the male the length of the clavicle 

 equals 44.32 ^^/^ that of the humerus (6). 



Taken individually, each of these factors may be small, but 

 because of the independent way in which each acts, a combination 

 of forces is possible, which will have a stretching effect on the 

 lowest brachial trunk through pressure on the first rib. The present 

 investigation shows by its results that the combination detrimental to the 

 nerve trunk consists in: 



(1) Relatively great descent of the shoulder. 



(2) Relatively considerable growth in length of the clavicle. 



(3) Relatively slight descent of the anterior extremity of the first rib. 

 It is most likely produce symptoms in females at the commence- 

 ment of adult age — that is to say, at the period when the full 

 effect of the various factors comes into play. 



This is in harmony with clinical observation, as has been shown 

 in a previous paper (3). 



Material. 



In the foregoing research observations have been made on 

 100 patients at the Manchester Royal Infirmary and the Pendlebury 

 Children's Hospital. I would thank the Honorary Physicians and 

 Surgeons who have given me this opportunity to work in the wards. 



The observations on the cadaver have been carried out on 

 40 post-mortem examinations in the Manchester Royal Infirmary, 

 through the kindness of Professor Lorrain Smith, to whom I would 

 acknowledge my indebtedness. 



