CHAPTER VI 



THE MOTIONS OF ANIMALS, AMD THE SKEL- 

 ETON AND MUSCLES 



Motions and locomotion. — Our attention is usually 

 first attracted to an animal by the movements it makes. 

 These are the plainest proof that it is alive. For the 

 animal itself the ability to move is essential to existence. 

 Most animals move in search of food, to escape from their 

 enemies, to find and build their homes, to seek their 

 mates, and care for their young. In the higher forms the 

 organs of motion constitute the great bulk of the body. 

 The shape and size of such an animal are determined 

 largely by these organs. 



The heart and blood-vessels, the lungs and digestive 

 system, are principally concerned with supplying the 

 organs of motion with materials necessary for their 

 working, and by far the larger part of the work of the 

 sense organs and nervous sj'stem is to put these organs 

 into action, and to direct and control them. We can 

 see therefore that they have much to do with both the 

 structure and physiology of animals. Indeed the most 

 marked difference between animals and plants is the pos- 

 session by the former of the organs of motion and their 

 controlling organs, the sense organs, the brain and 

 nerves. True, plants have the power of motion and arc 

 sensitive to light, heat, and other influences as are 

 animals, but to a far less tlegrce. 



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