MUSCULAR AND SKELETAL SYSTEMS. 



227 



bral column in different classes are very important in 

 geology. 



2. Structure of Shoulder Joint and Fore Limb. — There 

 is a progressive change from man to the more specialized 

 mammals in the position of the shoulder joint, its mov- 

 ableness, the presence and movableness of the two bones 

 of the forearm, and the free use of the hand. All these 

 are strictly correlated, and all reach their highest point 

 in man, because all are in him connected with the erect 

 attitude and the consequent liberation of the fore limbs 

 from the function of support and locomotion for higher 

 uses. In man the shoulder joint and the arms are on 

 the side of the body, being kept wide apart by the clavi- 

 cle, and the motion of the arm is the extreme of free- 

 dom. For this freedom firmness of the joint is sacrificed. 

 The two bones of the forearm roll the one on the other, 

 carrying the hand with it, and the hand has the most 

 perfect capacity for grasping. Now, as we go down the 

 scale of mammals, the collar bone disappears, the fore 

 limbs are brought together in front for support, the two 

 bones of the forearm become less and less movable on 

 one another, and the paw loses its power of grasping, un- 

 til, finally, in the hoofed animals the extreme is reached ; 

 the limbs are brought closer together in front and used 

 only for support, and therefore restricted in motion ; 

 the two bones of the forearm are consolidated into one, 

 and therefore lose entirely all rotary motion ; the paw 

 is no longer a paw, much less a hand, but a hoof, wholly 

 incapable of grasping. 



Motion and Locomotion. — We have explained the 

 function of muscle and of skeleton. We must now 

 show how they co-operate to produce motion and loco- 

 motion. 



1. Limb Motion. — Remember, then, that a muscle has 

 two skeletal attachments — viz., the origin and insertion. 



