MICTURITION 111 



into two parts, in the first part (ascending and transverse 

 colon, cecum) the most frequent movement is an antiperi- 

 stalsis which begins somewhere in the colon and passes toward 

 the ileocecal valve. These waves occur in groups, separated 

 by periods of rest. The ileocecal valve prevents the backward 

 passage of food into the small intestine. It requires about 

 two hours, in man, for food to pass from the ileocecal valve 

 to the hepatic flexure and four and one-half hours to reach the 

 splenic flexure. As the colon becomes filled some of the material 

 penetrates the second part of the large intestine, namely, 

 the descending colon. In this part peristaltic waves move 

 the contents toward the rectum. 



The intestines have a double nerve supply. The fibers of the 

 vagi carry chiefly motor impulses, while those of the sympa- 

 thetic, chiefly inhibitory impulses. The intestinal move- 

 ments are not altered by complete severance of all extrinsic 

 fibers, so that the latter probably have only a regulatory 

 influence. It is known the movements may be influenced 

 by psychical states, so that there are evidently connections 

 with higher centres. 



Defecation. When the undigested food has reached the 

 lower part of the large intestine and the rectum, sensory im- 

 pulses pass to the brain from the latter, giving rise to a desire 

 to defecate. The normal peristaltic movements of the rectum 

 are increased, while voluntarily a deep breath is taken, the 

 glottis is closed, and pressure is brought to bear upon the 

 abdominal contents. The external sphincter ani is voluntarily 

 relaxed, while the internal sphincter is inhibited. Both rectum 

 and sphincters have a double nerve supply, which in function 

 are motor and inhibitory. It has been shown that section of 

 the spinal cord of dogs in the lower thoracic region does not 

 prevent normal defecation. The centre probably lies in the 

 lumbar cord, but is connected with the brain so as to be under 

 voluntary control. 



Micturition. The urine as it is formed in the kidney is period- 

 ically carried to the bladder by a peristaltic action of the 

 ureters. Whether the peristalsis is due to the stimulus of the 

 contained urine or whether the ureters are automatically 

 rhythmic is not known. The urine is emptied into the bladder 

 in a series of spurts, and is prevented from flowing through 



