MOVEMENTS OF THE ALIMENTARY CANAL. 741 



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. Hertz, on 

 the basis of his skiagraphic observations, insists that simul- 

 taneously with the contraction of the abdominal muscles and 

 the closure of the glottis the diaphragm is also contracted and 

 thus aids in bringing pressure to bear upon the pelvic organs. 

 Although the act of defecation is normally initiated by voluntary 

 effort, it may also be carried out as a purely involuntary reflex 

 when the sensory stimulus is sufficiently 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 recovered 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 move- 

 ment probably lies in the lumbar cord, and it has sensory and 

 motor connections with the rectum and the muscles of defecation. 

 But this center is probably provided also with intraspinal con- 

 nections with the centers of the cerebrum, through which the 

 act may be controlled by voluntary impulses and by various 

 psychical states; the effect of emotions upon defecation being 

 a matter of common knowledge. In infants the essentially in- 

 voluntary 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 stom- 

 ach 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 



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



