THE LIFE OF AN ANIMAL 295 



thicker, and pulls equally on the parts to which each end is 

 attached. Movement occurs in the part which offers least 

 resistence. The elbow is bent by a muscle attached at one 

 end to a bone below the elbow, and at the other to a bone 

 above. When this muscle contracts, the forearm and hand 

 move up, unless opposed by outside force. 



344. Antagonistic Muscles. A part of the body that has 

 been moved by a muscle is returned to its original position by 

 a pull exerted by another muscle. The second muscle is so 

 attached as to pull in an opposite direction from the first. 

 Such a pair of muscles are called antagonistic muscles. For 

 example, the knee is bent by the contraction of a muscle on 

 the back of the leg, attached at one end to a bone below the 

 joint, and at the other end to a bone above the joint. The 

 knee is straightened by the contraction of another muscle on 

 the front of the leg attached to the same bones. One of the 

 antagonistic muscles must relax when the other contracts, 

 if movement results; but there is nothing in the relaxing of 

 a muscle to exert a push and so to cause movement. Any 

 motion of any part of the body is caused by a pull. Of 

 the pair, the muscle which bends the joint is called the 

 flexor, and that which straightens it is the extensor. 



345. Organs of Locomotion. The organs which are 

 specially designed for the moving of an animal from place to 

 place are organs of locomotion. The fins of a fish, the wings 

 and legs of a bird, and the legs of a quadruped and of man are 

 organs of locomotion. Animals designed for rapid motion 

 have, as a rule, somewhat wedge-shaped bodies with the edge 

 of the wedge projected forward. The fish is propelled by a 

 movement of the tail fin and the hinder part of the body, 

 similar to the movement of an oar used in the stern of a boat 

 for sculling. 



The turning movements of a fish are brought about by 

 the use of the paired fins. One pair of fins corresponds to 

 the arms of man, to the forelegs of a dog, and to the wings of 



