672 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 

 contents of the descending colon and sigmoid flexure into the rectum. 

 The pressure in the abdominal cavity is still further increased if 

 a deep inspiration is first made and then maintained during the 

 contraction of the abdominal muscles. Although the act of defeca- 

 tion is normally initiated by voluntary effort, it may also be aroused 

 by a purely involuntary reflex when the sensory stimulus is suf- 

 ficiently strong. Goltz* has shown that in dogs in which the spinal 

 cord had been severed in the lower thoracic region defecation was 

 performed normally. In later experiments, in which the entire 

 spinal cord was removed, except in the cervical and upper part of 

 the thoracic region, it was found that the animal, after it had re- 

 covered from the operation, had normal movement once or twice a 

 day, indicating that the rectum and lower bowels acted by virtue of 

 their intrinsic mechanism. An interesting result of these experi- 

 ments was the fact that the external sphincter suffered no atrophy, 

 although its motor nerve was destroyed, and that it eventually 

 regained its tonic activity. 



It would seem that the whole act of defecation is, at bottom, an 

 involuntary reflex. The physiological center for the movement 

 probably lies in the lumbar cord, and has sensory and motor con- 

 nections with the rectum and the muscles of defecation; but this 

 center is probably provided with connections with the centers of 

 the cerebrum, through which the act may be controlled by volun- 

 tary impulses and by various psychical states, the effect of emo- 

 tions upon defecation being a matter of common knowledge. In 

 infants the essentially involuntary character of the act is well 

 known. 



Vomiting. The act of vomiting causes an ejection of the con- 

 tents of the stomach through the esophagus and mouth to the 

 exterior. It was long debated whether the force producing this 

 ejection comes from a strong contraction of the walls of the stomach 

 *"Archiv f. die gesammte Physiologic," 8, 160, 1874; 63, 362, 1896. 



