THE PRINCIPLES OF MYODYNAMICS. 9 1 



84. In general, the plane of the floor of the 

 glenoid cavity of the scapula, or its conjugate 

 diameter, runs somewhat backward from the 

 transverse plane of the body, while the transverse 

 diameter of this cavity is nearly parallel with the 

 long axis of the body. TJie surface of the cavity 

 looks foriuard and somewhat oidzvard. The head 

 of the humerus rests on this plane. Are there 

 any muscular components that tend to displace 

 the head of the humerus from the glenoid cavity 

 of the scapula ? In the meantime, let the mobility 

 of the scapula, in regard to the chest-wall, be 

 kept in mind. And let it also be kept in mind 

 that the extensive motion of the humerus is 

 facilitated by the comparatively large articular 

 surface of its head and by the comparatively 

 small articular surface of the glenoid cavity of 

 the scapula. 



85. Some of the muscles that span the 

 shoulder-joint have small displacing components 

 acting on the humerus : 



(i.) The supra spinatus has no displacing 

 components acting on the humerus. The an- 

 terior part and the posterior part of the deltoid 



