MOVEMENTS OF THE ALIMENTARY CANAL. 733 



the cord in the sacral nerves (second to fourth), form part of the 

 nervi erigentes and enter into the hypogastric plexus. When 

 stimulated these fibers cause contractions of the muscular coats; 

 they may be regarded, therefore, as motor fibers. As in the 

 case of the small intestine and stomach, we may assume that 

 these motor and inhibitory fibers serve for the reflex regulation 

 and adaptation of the movements. 



Defecation. The undigested and indigestible parts of the 

 food, together with some of the debris and secretions from the 

 alimentary tract eventually reach the sigmoid flexure and 

 rectum. Authorities differ as to whether the rectum normally 

 contains fecal material or not. According to the observations 

 of Hertz,* made upon man by means of x-rays, fecal material 

 is normally absent from the rectum except just before defeca- 

 tion. It seems probable that a distinct desire to defecate 

 is felt only when the feces have actually entered the rec- 

 tum and produced some distension. The fecal material is 

 retained within the rectum by the action of the two sphincter 

 muscles which close the anal opening. One of these muscles, 

 the internal sphincter, is a strong band of the circular layer of 

 involuntary muscle which forms one of the coats of the rectum. 

 When the rectum contains fecal material this muscle is thrown 

 into a condition of tonic contraction until the act of defecation 

 begins, when it is relaxed. The external sphincter ani is com- 

 posed of striated muscle tissue and is under the control of the 

 will to a certain extent. It is supplied by a motor nerve, the 

 Nn. hemorrhoidales inferiores, arising from the N. pudendus 

 and eventually from the sacral spinal nerves. This muscle, 

 therefore, like striated muscle in general, is innervated directly 

 from the spinal cord, but it possesses properties which are to 

 some extent intermediate between those of plain and of striated 

 muscle. For example, it differs from the latter and resembles 

 the former in the fact that it does not atrophy after section of 

 its motor nerve; it is much less sensitive to the paralyzing action 

 of curare than the typical striated muscle, and it is stated that 

 its curve of contraction, when it is stimulated through its nerve, 

 exhibits a long latent period and a slow contraction and relaxa- 

 tion. Both the internal and the external sphincter are normally 

 in tonus and unite in protecting the anal opening. The force 

 of the tonic contraction of the internal is somewhat less (30 to 

 60 per cent.) than that of the external sphincter.f The innerva- 

 tion and control of the internal sphincter is better understood 



* Hertz, "Guy's Hospital Reports," 61, 389, 1907. 



t Consult Frankl-Hochwart and Frohlich, "Archiv f. de ges Physiologie," 

 81, 420. 



