MOVEMENTS OF THE ALIMENTARY CANAL. 729 



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 def- 

 ecation begins, when it is relaxed. The external sphincter ani is 

 composed 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. hemorrhoidals inferiores, arising from the N. pudendus and 

 eventually from the sacral spinal nerves. This muscle, therefore, 

 Uke 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. Both 

 the internal and the external spliincter 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. The internal sphincter, like 

 the rest of the musculature of the rectum, receives a double nerve 

 supply, one set of fibers coming to it through the pelvic nerve 

 (sacral autonomics) and one set thi'ough the hypogastric nerve 

 by way of the inferior mesenteric ganghon (thoracic autonomics). 

 The action of the efferent fibers in these nerves has been a matter 

 of dispute, and possibly the results may differ in different animals. 

 According to one account the sphincter responds in the same way 

 as the rest of the circular muscle of the rectum, that is to say, 

 it is thrown into contraction by stimulation of the pelvic nerve, 

 and is inhibited by stimulation of the hypogastric nerve. The 

 act of defecation as it occurs normally is partly a voluntary and 

 partly an involuntary act, the part taken by each varying with 

 circumstances. The involuntary act consists in peristaltic con- 

 tractions of the rectum or, indeed, of the whole colon, together 

 with an inhibition of the sphincters, and in a strong call to defeca- 

 tion this act is effective in emptying the bowel without voluntary- 

 aid other than the relaxation of the external sphincter. Under 

 ordinary conditions the desire to defecate is aroused by the 

 passage of feces from the pelvic colon into the rectum. A vol- 

 untary act then follows, consisting in a contraction of the ab- 

 dominal muscles, or, according to Hurst,* a contraction of both 

 the diaphragm and the abdominal muscles, the effect of which is 

 to greatly increase the pressure in the abdomen, since the glottis 

 is kept closed, and thus force more of the feces into the rectum. 

 Through the stimulus thus produced in the rectum the involun- 

 tary peristalsis is liberated, the movement starting well back in 

 the colon and including as part of the reflex an inhibition of the 

 internal sphincter. 



* Hurst, Loc. cit. 



