THE PRINCIPLES OF MYODYNAMICS. 



93 



(2.) For reasons that are plain, the upper part 

 of the pectorahs major has a large inward and 

 forward displacing component. 



(3.) The teres major and the latissimus dorsi 

 act nearly in the same direction and have large 

 displacing components acting on the humerus, 

 downward, inward, and somewhat backward. 



(4.) When the arm is abdiuted the deltoid will 

 have a large displacing component acting on the 

 humerus. This results from the fact that then 

 the deltoid pulls nearly in a line with the con- 

 jugate diameter of the glenoid cavity of the 

 scapula. 



(5.) When the arm is abducted, the triceps 

 brachii, by means of its scapular origin, on ac- 

 count of the direction of the plane of the glenoid 

 cavity, will have a considerable displacing com- 

 ponent, acting forward, inward, and downward, 

 on the head of the humerus. 



(6.) Hence, it will appear that the shoulder -joint, 

 under some myo-dynaniic relations, is in a condi- 

 tio7t of remarkable instability. 



(7.) It may be noted that the arm can only be 

 abducted to a right angle with the body. 



