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MANUAL OF PHYSIOLOGY. 



and it is probable that its great efferent channel for impulses is 

 reflected through the brain. 



On the other hand, the splanchnic nerves, which come from 

 the thoracic sympathetic, are said to be inhibitors of the myen- 

 teric plexuses. This may be explained by their effect on the 

 small vessels which they no doubt control causing a change 

 in the blood supply. Be this as it may, the splanchnic seem to 

 have considerable influence on the intestinal movements. When 

 stimulated they commonly check the intestinal motions, but may 

 sometimes (as when the movements have ceased after death) 

 give rise to new movements. 



On account of this double action, it has been said that there 



FIG. 59. 



FIG. 58. Meissner's plexus, low power. 



FIG. 59. Meissner's plexus (high power), showing cells grouped at nodal points. 



are two kinds of fibres, (1) inhibiting, which are easily excited, 

 and during life have greater influence, and (2), exciting, which, 

 though less excitable, retain their irritability longer. 



However, most of these effects may be explained by referring 

 them to vasomotor changes. 



With regard to defsecation, we know that a nerve centre exists 

 in the lumbar portion of the spinal cord, which governs the 

 sphincter, and seems to keep up its tonic contraction. This 

 centre may be either excited to increased action or inhibited, by 

 peripheral stimuli or by central influences from the brain. 



Thus the local application of warmth causes inhibition of the 



