SKELETON AND TEETH 



77 



Fig. 23. — Left clavicle of Man, front side. 



tiles and lower vertebrates generally, it is a large and im- 

 portant part of the shoulder-girdle and articulates with the 

 sternum. 



The collar-bone, or clavicle, is a complexly curved bar, 

 which, when present and fully developed, extends from the 

 forward end of the sternum to the acromion, the projecting 

 lower end of the scapular spine, supporting and strengthening 

 the shoulder-joint. In many 

 mammalian orders, notably all 

 existing hoofed animals, the 

 clavicle has become superfluous 

 and is lost more or less com- 

 pletely ; it may be said, in general, that the clavicle is devel- 

 oped in proportion to the freedom of motion of the shoulder- 

 joint and to the power of rotation of the hand upon the arm. 

 In arboreal animals, such as monkeys, in which the hand 

 rotates freely and the arm moves in any direction on the 

 shoulder, the clavicle is large and fully developed, as it also is 

 in Man. Many burrowing mammals (e.g. the moles) have 

 very stout clavicles. 



The posterior, or pelvic, girdle is composed on each side of 

 a very large, irregularly shaped bone, which is firmly attached 

 to one or more of the coalesced vertebrae which form the sacrum 

 and thus affords a solid support to the hind leg. Each half 



of the pelvis, or hip- 

 bone, is made up of 

 three elements, called 

 respectively the ilium, 

 ischium and pubis, 

 which are separate in 

 the very young animal, 

 indistinguishably fused 

 in the adult. The three 

 elements unite in a deep, hemispherical pit, the acetabulum, 

 which receives the head of the thigh-bone, a perfect ex- 



FiG. 24. — Left hip-bone of Wolf. //., ilium, 

 ischium. P., pubis, ac, acetabulum. 



