188 



SKELETON AND MUSCLES 



LABORATORY STUDY 



Study the skeleton, and examine long, flat, and irregular bones. How is 



the bone modified to do its work ? 



154. Muscles. — The muscles are the lean parts of the 



flesh of animals and are usually dark in color. Birds are 



an exception, for their breast meat 

 is generally white. Muscles are of 

 two kinds : voluntary (governed by 

 the will), such as those which we 

 use in walking, or in moving the 

 arms ; involuntary, such as those 

 which move the food along the 

 digestive tract or assist in breathing. 

 The voluntary muscles consist of 

 many long muscle cells (fibers) 

 bound together into a distinct 

 bundle. Usually the muscle bundle 

 is attached at each end to the bones. 

 A single muscle moves the arm in 

 one direction only, and in order to 

 lift the arm from the desk to the 

 head, for instance, 

 several muscles 

 must act together. 

 The cells of the 

 involuntary mus- 

 cles are unlike the 

 cells of the volun- 

 tary muscles. In- 

 voluntary muscle cells occur in la} r ers 



in the walls of the digestive tube, blood 



vessels, the bladder, and the like, and 



they are not under the control of the 



will. 



Figure 200. — Muscles 

 of Upper Leg. 



Note how they are ar- 

 ranged in bundles. 



Figure 201. — Vol- 

 untary Muscle 

 Cells. 



Showing how the 

 cells are bound to- 

 gether with connec- 

 tive tissue. At the 

 end of the muscle, 

 the cells of the con- 

 nective tissue form 

 the tendon. 



