CHAPTER IX 



THE MUSCLES 



205. It has been learned that motion is one of the 

 physiological properties of protoplasm, along with sensi- 

 bility and the power of assimilating 



food. The white blood cells have 



a distinct motion, and certain cells 



that line the windpipe and other 



air passages are able to contract 



and wave their cilia, or hairlike 



appendages, to and fro. With these 



two exceptions, the muscle cells are 



the only cells of the body that have 



a marked power of contracting and 



producing motion. As feeling or 



perceiving through the nerves, and 



moving or exerting energy through 



the muscles, seem to be the two 



chief purposes of the organism, we see how important the 



muscles are for the purposes of life. The body has been 



compared to a steam engine but the only object of an 



engine is to set free energy. 



206. Kinds of Muscles. Muscular tissue occurs in 

 nearly every organ. We found that it helps to form the 

 walls of the blood vessels, and assists in the circulation 

 of the blood ; the eyeballs are moved by six sets of 

 muscles; the act of swallowing is performed by muscular 

 contraction in the gullet; the contraction of the muscles 

 in the walls of the stomach produces the motion by which 



119 



FIG. 103. A Portion of 

 Two Striated Muscle Fi- 

 bers, highly magnified. 



, nucleus. 



