364 



HUMAN PHYSIOLOGY 



A bundle of involuntary muscle fibers, as 

 seen under the microscope. 



knobs or plates on the muscle cells; 

 thus, the action of every muscle fiber, 

 perhaps every cell, is under the control 

 of the nervous system. 



Structure of Involuntary Mus- 

 cle. — Muscles in which the movement 

 is not controlled by the will are called 

 involuntary. The most apparent dif- 

 ference between voluntary and invol- 

 untary muscle fibers (such as those 

 found in the wall of the stomach and 

 intestines of man) is that the latter do 

 not show the cross striation character- 

 istic of the former. The involuntary 

 fibers are supplied with blood vessels 

 and nerves. The latter, however, 

 come from what is known as the sym- 

 pathetic nervous system. Of this we 

 shall learn more later. Fibers from 

 the sympathetic system go to most 

 organs in the body 

 cavity of man — the 

 heart, the food tube 

 and different glands 

 connected with it ; 

 and to the muscles of 



the blood vessels. The act of breathing and the heart-beat 

 are examples of an automatic muscular action where ner- 

 vous control comes largely from the sympathetic system. 

 The churning movements of the stomach and intestines 

 are examples of muscular action entirely beyond control 

 of the will. 



Structure of Heart Muscle. — Heart muscle differs 

 slightly in appearance from both voluntary and involun- 

 tary muscle fibers. The cross striation is seen here as in 

 voluntary muscle. The outline of the individual cells can 

 be made out in heart muscle. (See diagram.) 



Rate of Muscular Contraction. — A wave of muscu- 

 lar contraction moves rather slowly in an involuntary 

 muscle; a little more rapidly in heart muscle; and most 

 rapidly of all in voluntary muscle fibers. 



Muscle Tissue and its Uses. — As we have seen 

 in the frog, muscles evidently form a large part of 



?lii 



Striated muscular 

 fibers, from the 

 heart of man, di- 

 vided by trans- 

 verse septa into 

 separate nucle- 

 ated portions, 

 much magaified 



