PECTORALIS MAJOR AND DELTOID INSERTION 163 



of the lever, if the shortening of the muscles is equal. This can 

 be readily seen from the diagram (fig. 3) A —B represents 

 the humerus, its caput at A. C and D correspond to the two 

 most extreme points of insertion of the pectoralis major. 1 and 

 2 indicate the two appertaining muscles. Should the latter 

 shorten by the same amounts C — C' = D' — D' , than the lower 

 end of the humerus B is moved more extensively by muscle 1 

 (to B") than by muscle 2 (to B'), for instance the humerus is 

 turned through a greater angle when the muscles are contracting 

 equally by the more proximal one, and consequently also more 

 quickly, A more distally situated pectoralis major would have 

 to contract more in order to pull the arm forward to a certain 



D'-/--..C' 



B ' * i — '^'-^^-'- A 



D C 



Fig. 3 Diagram of the movements of the humerus with equal shortening of 

 the pectoralis major muscle at different points of attachment. 



angle than would be necessary if it were more proximally at- 

 tached. With increased contraction, however, a muscle loses 

 in tension and consequently the greater shortening of the more 

 distal muscle diminishes the advantage of its favorable lever 

 arm. 



In the group of whites as well as in the negroes, the averages 

 of the index of position in the males exceed those of the females, 

 the differences being 2.07 and 1.50 respectively. Since the proba- 

 ble errors of these averages are only small, this sexual difference, 

 such as that the female possesses a more proximally attached 

 pectoralis major muscle, must be considered as a rather essential 

 and definite one. It is not only of interest in connection with the 

 above mentioned consideration of musculo-mechaniscs, but also 



