CH. xiV.J INVOLUNTARY MUSCLE. l6l 



CHAPTER XIV. 



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. Involun- 

 tary 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 contractions 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. 



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-medullated nerve-fibres with ganglion cells ; so that much 

 discussion 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, that portions of muscular tissue entirely free 

 from nerves act in the same way as those that possess nerves) 

 indicates that it is the muscular rather than the nervous tissues 



K.r. M 



