MOVEMENTS OF THE ALIMENTARY CANAL. 651 



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 

 itself or whether it is due mainly to the action of the walls of the 

 abdomen. A forcible spasmodic contraction of the abdominal 

 muscles takes place, as may easily be observed by any one upon 

 himself, and it is now believed that the contraction of these muscles 

 is the principal factor in vomiting. Magendie found that if the 

 stomach was extirpated and a bladder containing water was sub- 

 stituted in its place and connected with the esophagus, injection 

 of an emetic caused a typical vomiting movement with ejection of 

 the contents of the bladder. Gianuzzi showed, on the other hand, 

 that upon a curarized animal vomiting could not be produced by an 

 emetic because, apparently, the muscles of the abdomen were 

 paralyzed by the curare. There are on record a number of ob- 

 servations which tend to show that the stomach is not passive 

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



