A STUDY OF THE MUSCLES 201 



always touching the ground. In running there are instants 

 of time when neither foot touches earth. Kunning differs 

 from walking, too, in that the heel does not touch ground in 

 running ; for when the foot is thrust forward, one lights upon 

 his toes alone, and then the toes of the other foot give the 

 body a vigorous push forward. 



2. INVOLUNTARY MUSCLE 



Nerve Control. We have denned involuntary muscle as 

 tissue that contracts and relaxes without being controlled 

 by the will power. This does not mean that the nervous 

 system has no control over it, for, as we have already learned, 

 its action is directed by a special mechanism in the trunk of 

 the body called the sympathetic nerve system. 



7* 



FIG. 94. A Plain Muscle Fiber. 



/ = cell body. 



n = nucleus. 



P = granular substance near the nucleus. 



Functions. Involuntary muscle makes up most of the 

 thickness of the walls of the heart, of the alimentary canal, 

 and of the blood vessels. It is an experience common to all 

 of us that processes, like the winking of the eyes, breath- 

 ing, and walking, are carried on without conscious thought. 

 We can, however, close the eyelids when we wish, breathe 

 rapidly, slowly, or stop breathing for a time, and can con- 

 sciously direct the leg movements in walking. These 

 activities are, therefore, regarded as automatic, and the 

 muscles that carry on these movements are voluntary muscles. 

 Involuntary muscles carry on the functions that are beyond the 

 control of the will. 



Structure of Involuntary Muscle. If a piece of stomach 

 muscle is teased apart with needles and then examined with 

 a compound microscope, this tissue is found to consist of 



