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THE MECHANICS OF DIGESTION 1021 



nerve fibers pass to the musculature of the rectum and the internal and 

 external sphincters. By means of these channels, this center is 

 brought into functional relation with different local receptors which 

 may either augment or inhibit its activity. In the latter case, the 

 sphincters are relaxed. It is connected with the cerebrum by means 

 of different afferent and efferent paths, so that volition, emotions, and 

 various sensory impulses, may be brought to bear upon it. 



The powerful band of smooth muscle tissue forming the internal 

 sphincter, receives its motor supply from the hypogastric plexus by way 

 of the nervus erigens, and its inhibitory supply from the same source 

 by way of hypogastric nerve. These nerves also embrace sensory 

 fibers from the same region, as well as sensory and motor fibers for 

 the rectum. When severed, the excitation of the central end of the 

 nervus erigens gives rise to an inhibitory effect which is made possible 

 with the help of the hypogastric nerve. Quite similarly, the stimula- 

 tion of the central end of the divided hypogastric nerve produces 

 motor results through the intervention of the nervus erigens.^ The 

 external sphincter is composed of striated muscle tissue and is inner- 

 vated by the nervi hemorrhoidals inferiores which are derived from 

 the nervus pudendus and sacral spinal nerves. This muscle acts in 

 unison with the levator ani and other perineal muscles, and aids in 

 restoring the everted mucous membrane of the anus after the completion 

 of defecation. 



1 Frankl-Hochwart and Frohlich, Pfliiger's Archiv, Ixxxi, 1900, 420. 



