62 HUMAN PHYSIOLOGY 



Movements brought about by smooth muscle are never 

 very rapid. In most cases they can fairly be called 

 gradual or sluggish. When the stomach of a living 

 animal is exposed to inspection creases are seen in its 

 surface and the position of these creases steadily shifts 

 toward the adjoining intestine, but the progress is so slow 

 that in the cat 30 seconds may be occupied in traversing 

 an inch. Contractions of smooth muscle are never 

 sharp twitches; their onset and their disappearance are 

 both gentle and slow. The reader will perhaps question 

 this as he recalls the convulsive character of vomiting 

 movements, but these are not true gastric contractions. 

 They are produced by skeletal muscles bearing upon 

 the outside of the stomach. 



Varieties of Muscular Movements. Muscular move- 

 ments of all varieties fall into two classes according to 

 their causation. Some are due to the influence brought 

 to bear through the central nervous system. This is 

 true of those made by the skeletal muscles. Every 

 breath that we take is the result of a distinct act on the 

 part of a certain cell-group in the brain. This is not 

 so at all in the case of the beating heart. Here the 

 successive contractions testify to an independent rhyth- 

 mic tendency resident in the cardiac muscle. In other 

 words, the heart would continue to beat even though 

 disconnected from the central nervous system. We ex- 

 press this fact by saying that cardiac muscle is automatic 

 and the same may be said in a general way of smooth 

 muscle. 



A ring cut from the stomach of a frog and suspended 

 so that its contractions shall lift a light lever will, under 

 favorable conditions, shorten and relax with a slow 

 rhythm during many hours. The same behavior has 

 been described for the urinary bladder of a cat. The 

 automatic property is an important matter to be reck- 

 oned with in considering both the physiology and the 

 hygiene of the alimentary tract. One inference may 

 be drawn without delay: namely, that when we have to 



