MOVEMENTS OF THE ALIMENTARY CANAL, ETC 387 



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 it' 

 a deep inspiration is first made and then maintained during the contraction 

 of the abdominal muscles. Although the act of defecation is normally initiated 

 by voluntary effort, it may also be aroused by a purely involuntary reflex when 

 the sensory stimulus is sufficiently strong. Goltz 1 has shown that in dogs 

 in which the spinal cord had been severed in the lower thoracic region defe- 

 cation 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. A 

 curious result of these experiments was the fact that the external sphincter 

 eventually regained its tonic activity. 



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

 reflex. The physiological centre for the movement probably lies in the Lumbar 

 cord, and has sensory and motor connections with the rectum and the muscles 

 of defecation, but this centre is in part at least provided with connections with 

 the centres of the cerebrum through which the act may be controlled In- 

 voluntary impulses and by various psychical states, the effect of emotions 

 upon defecation being a matter of common knowledge. In infants the essen- 

 tially involuntary character of the act is well seen. 



Vomiting. — The act of vomiting causes an ejection of the contents of the 

 stomach through the oesophagus and mouth to the exterior. It was long 

 debated whether the force producing this ejection comes from a strong contrac- 

 tion 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 extir- 

 pated and a bladder containing water was substituted in its place and connected 

 with the oesophagus, injection of an emetic caused a typical vomiting movement 

 with ejection of the contents of the bladder. Giauuzzi 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, however, a number of observations which tend to show thai 

 the stomach is not entirely passive during the act. < >n the contrary, it may 

 exhibit contractions, more or less violent in character, which while insufficient 

 in themselves to eject its contents, probably aid in a normal act of vomiting. 

 According to < )penchowski, 2 the pylorus is closed and the pyloric end of the 



1 Archiv fiir die gesammte Physiologie, 1874, Bd. viii. 8. 460; also Bd. lxiii. S. 362 



2 Archivfur Physiologie, 1889, S. 552. 



