CHAPTEE XIII 



COMPARISON OF VOLUNTARY AND INVOLUNTARY MUSCLE 



THE main difference between voluntary and involuntary muscle is the 

 difference expressed in their names. Voluntary muscle is under the 

 control of that portion of the central nervous system the activity of 

 which is accompanied by volition. Involuntary muscle, on the other 

 hand, is, as a rule, also under the control of the central nervous 

 system, but of a portion of the central nervous system the activity 

 of which is independent of volition. There appear, however, to be 

 exceptions to this rule, and the involuntary muscle executes its con- 

 tractions independently of nervous control ; that is to say, it is 

 sometimes in the truest sense of the term really involuntary. This 

 is very markedly seen in the developing heart of the embryo, which 

 begins to beat before any nerve-fibres have grown into it from the 

 central nervous system. 



Another characteristic of involuntary muscle is a tendency to 

 regular alternate periods of rest and activity, or rhythmicality. This 

 is best exemplified in the heart, but it is also seen in the lymphatic 

 vessels, especially the lymph hearts of the frog, and the mesenteric 

 lymphatic vessels (lacteals) of many animals. It is seen in the 

 veins of the bat's wing, and in the muscular tissue of the spleen, 

 stomach, intestine, bladder, and other parts. 



A third characteristic of involuntary muscle is peristalsis. If 

 any point of a tube of smooth muscle such as the small intestine is 

 stimulated, a ring-like constriction is produced at this point. After 

 lasting some time at this spot it slowly passes along the tube at the 

 rate of 20 to 30 millimetres per second. This advancing peristaltic 

 wave normally takes place in only one direction, and so serves to 

 drive on the contents of the tube. 



Involuntary muscle nearly always contains numerous plexuses of 

 non-medulla ted nerve-fibres with ganglion cells ; so that much dis- 

 cussion has taken place on the question whether the phenomena of 

 rhythmicality and peristalsis are properties of the muscular tissue 

 itself or of the nerves mixed with it. The evidence available (namely, 



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