THE ANIMAL MACHINE 15 



instance, terminate their muscular portions above the 

 knee, the tendon continuing throughout the entire length 

 of the cannon and the pastern before being inserted. 

 The origin of the muscle serves as the fixed point toward 

 _ which the bone on which the muscle has its movable 

 insertion is drawn by its contraction. However, other 

 muscles may fix the point of movable insertion when the 

 action of the muscle is reversed. The long muscle con- 

 cerned with knee action in the horse is an example in 

 point. It has its origin at the side of the head and upper 

 part of the neck and is inserted on the humerus of the 

 arm. With the fixed point above, its contraction elevates 

 and carries forward the arm and with it the leg, but with 

 the leg fixed as in the standing position, its contraction 

 may serve to incline the head and neck to one side. 



18. Muscular action. — Each muscle has a definite 

 action, depending upon its position with relation to the 

 joint and the nature of the joint surface itself. Hinge 

 joints have their articular surfaces so arranged as to pre- 

 vent motion in but one plane, as the elbow or hock joints, 

 while others, as the ball and socket joints of the shoulder 

 and hip, are capable of considerable freedom of motion, 

 even to the extent of rotation. The action of a muscle 

 is to flex or close the angle of a joint, when it is situated 

 in the angle and behind the joint ; to extend when in 

 front of the joint or over the summit of the angle ; to 

 abduct or adduct when on the side of the joint away from 

 or toward the median plane of the body ; to rotate if so 

 arranged as to cross from one side to the other in its 

 course. 



All muscles are not equally employed. The class of 

 animals and the particular use to which each is put have 

 much to do in determining which muscles are most fre- 



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