650 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. Here the 

 nearly solid material stimulates by its pressure the sensory nerves of 

 the rectum and produces a distinct sensation and desire to defecate. 

 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 

 seems to be thrown into a condition of tonic contraction until 

 the act of defecation begins, when it is relaxed. The sphincter is 

 composed of involuntary muscle and is innervated by fibers having 

 the general course given above for the nerves of the large intestine. 

 The external sphincter ani is composed of striated muscle tissue and 

 is under the control of the will to a certain extent. When, however, 

 the stimulus from the rectum is sufficiently intense, voluntary 

 control is overcome and this sphincter is also relaxed. The act of 

 defecation is in part voluntary and in part involuntary. The 

 involuntary factor is found in the contractions of the strongly 

 developed musculature of the rectum, especially the circular layer 

 which serves to force the feces onward, and the relaxation of the 

 internal sphincter. It would seem that these two acts are mainly 

 caused by reflex stimulation from the lumbar spinal cord, although 

 it is probable that the rectum, like the rest of the alimentary tract, 

 is capable of automatic contractions. The rectal muscles receive a 

 double nervous supply, containing physiologically both motor and 

 inhibitory fibers. The former come probably from the nervus 

 erigens by way of the pelvic plexus ; the latter from the lumbar cord 

 through the corresponding sympathetic ganglia, inferior mesenteric 

 ganglion, and hypogastric nerve. It has been asserted that stimu- 

 lation of the nervus erigens causes contraction of the longitudinal 

 muscles and inhibition of the circular muscles, while stimulation of 

 the hypogastric nerve causes contraction of the circular muscles 

 and inhibition of the longitudinal layer. This division of activity 

 has not been confirmed by recent experiments. 



The voluntary factor in defecation consists in the inhibition of 

 the external sphincter and the contraction of the abdominal muscles. 

 When these latter muscles are contracted and at the same time the 

 diaphragm is prevented from moving upward by the closure of the 

 glottis, the increased abdominal pressure is brought to bear upon the 

 abdominal and pelvic viscera, and aids strongly in pressing the 



