28 



ANIMAL PHYSIOLOGY. 



human subject, are the collar bones or clavicles, which imite 

 the shoulders with the skeleton of the trunk. The clavicle 

 has no existence in many mammals, such 

 as the horse, the ox, and the sheep ; 

 while in others it is rudimentary and 

 without function, as in the cat and the 

 dog; and in all such instances the shoulder 

 and fore limb are united to the rest of the 

 skeleton by mere muscular connections; 

 but in the animals in which it exists foi 

 example, squirrels and monkeys it is the 

 fulcrum on which the arm moves when 

 stretched out from the body or approached 

 to the middle line. The outer end of the 

 clavicle articulates with the scapula or 

 shoulder-blade, and the two bones together 

 form the shoulder-girdle. '* 



The joints at the outer and inner ends 

 of the clavicles permit the shoulder-blades 

 to be moved upwards, downwards, for- 

 wards, or backwards at will, while they 

 continue to glide on the conical walls of 

 the upper part of the chest. The part 

 of the shoulder-blade with which the 

 clavicle articulates is called the acromion, 

 and is the expanded extremity of a spine 

 which arises from the back of that bone, 

 and is directed outwards and upwards. 

 At a little distance from its outer end, 

 the clavicle is likewise united by strong 

 ligaments to another process of the 

 shoulder-blade called the coracoid, against 

 which it rests when the shoulders are 

 pushed upwards. 



The humerus or arm bone articulates 

 by a rounded head with a surface of the 

 scapula called the glenoid fossa, distinct 

 Fig. 9. VERTEBRAL from both acromion and coracoid pro- 

 COLUMN. cesses, and this articulation is the shoulder 



joint. It permits greater freedom of motion than any other 



