CHAPTER III 

 THE MUSCLES 



A 72. Spontaneous Movements in Animals. Trees and 

 stones never move unless acted upon by external force, 

 while the infant and the tiniest insect can execute a great 

 variety of movements. When the higher animals are at 

 rest it is possible to observe some kind of motion in them. 

 Even in the deepest sleep the beating of the heart and the 

 motion of the chest never cease. In fact, the power to 

 execute spontaneous movements is the most distinctive property 

 of living animals. 



All motion of our bodies is produced by means of 

 muscles. Not only the limbs are moved by them, but even 

 the movements of the stomach and of the heart are caused 

 by muscles. Every part of the body which is capable of 

 motion has its own special set of muscles. 

 _3 73. The Two Great Kinds of Muscles. All the move- 

 ments of the body as a whole are performed by muscles. 

 These muscles make up the red flesh, well known to 

 every one as the lean of butcher's meat, which, together 

 with the fat, clothes the bony framework. We often call 

 these voluntary muscles, because they are more or less 

 subject to the control of the will. 



The motion of the internal organs, as those of digestion, 

 secretion, excretion, circulation, and respiration, are pro- 

 duced by muscles of another kind, that is, by muscles not 

 usually under the control of' the will. This work goes on 



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