THE SHOULDER-GIRDLE. 



217 



THE SHOULDER-GIRDLE. 



The upper extremity is joined to the trunk by the shoulder- girdle, which is 

 composed of the clavicle and scapula. The main movements are anteroposterior, 

 as in swinging the arm, those of abduction and adduction, as in raising and lower- 

 ing it sidewise, and rotation. 



The scapula is the more important bone; it is present in all mammals, and the 

 humerus articulates with it. 



The scapula in the mole and many other animals may be a comparatively 

 slender bone, but when, as in man, it is necessary to rotate the arm, then the 



Acromion process 



Clavicle 



Spine 



Scapula 



FIG. 227. Shoulder-girdle of man. 



scapula is large so as to form a strong support for the infraspinatus, supraspinatus, 

 teres minor and major, and subscapularis muscles. 



The clavicle is developed mainly from membrane, partly probably from carti- 

 lage, and is the first bone in the body to ossify. It keeps the shoulder out away 

 from the body and increases the range 

 of motion of the upper extremity. It 

 owes its existence to the function of 

 abduction. Without a clavicle abduc- 

 tion is practically wanting and when in 

 man the clavicle is broken, he is tem- 

 porarily reduced to the condition of those 

 animals which have no clavicles; he is 

 able to move the arm backward and for- 

 ward but not to elevate it properly, and 

 this is an important diagnostic symptom 

 of that injury. 



The clavicle is lacking in the ungu- 

 lates or hoofed animals. These have an 

 anteroposterior movement, but little abduction. A horse or cow moves its fore-legs 

 back and forward, but not out away from the body. Hence its helplessness when 

 these movements are essential. It is also lacking in seals and whales. In the 

 carnivora, as the lion and the tiger, which possess rudimentary clavicles, sufficient 

 adducting power is present to enable them to hold their food while tearing it apart. 

 In man, apes, bats, rodents, and insectivora the clavicle exists as a well-formed bone; 



FIG. 228. Shoulder-girdle of man, showing how 

 the clavicle acts as a prop to keep the shoulder out away 

 from the chest. 



