MOVEMENTS OF THE ALIMENTARY CANAL. 739 



garded, therefore, as motor fibers. As in the case of the small 

 intestine and stomach, we may assume that these motor and in- 

 hibitory fibers serve for the reflex regulation and adaptation of the 

 movements. In this connection attention may be called to the 

 difference in innervation between the stomach and small intestine, 

 on the one hand, and the colon and rectum on the other. The 

 former receive their extrinsic motor fibers mainly if not entirely 

 through the vagus (bulbar autonomic), while the latter receive their 

 motor fibers mainly, if not entirely, through the pelvic nerve 

 (sacral autonomies), arising near the extreme lower end of the cord. 

 The small intestine receives its inhibitory fibers through the 

 splanchnic nerve and superior mesenteric ganglion (thoracic 

 autonomies), while the colon and rectum get their inhibitory fibers 

 through splanchnic branches that connect with the inferior mesen- 

 teric ganglion (thoracic autonomies). 



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 z-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, 



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



