MOVEMENTS OF THE ALIMENTARY CANAL, ETC. 325 



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 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 l has shown that in dogs 

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

 cation was performed normally, the external sphincter being relaxed. 



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

 reflex. The physiological centre for the movement 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 by 

 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. Mageudie 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. 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, however, a number of observations which tend to show that 

 the stomach is not entirely passive during the act. On 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. 

 The act of vomiting is in fact a complex reflex movement into which many 

 muscles enter. The following events are described : The vomiting is usually 

 preceded by a sensation of nausea and a reflex flow of saliva into the mouth. 

 These phenomena are succeeded or accompanied by retching movements, which 

 consist essentially in deep spasmodic inspirations with a closed glottis. The 

 effect of these movements is to compress the stomach by the descent of the 

 diaphragm, and at the same time to increase decidedly the negative pressure in 

 the thorax, and therefore in the thoracic portion of the oesophagus. During 

 one of these retching movements the act of vomiting is effected by a convulsive 

 contraction of the abdominal wall which exerts a sudden additional strong 

 pressure upon the stomach. At the same time the cardiac orifice of the 

 stomach is dilated, possibly by an inhibition of the sphincter, aided it is sup- 

 1 Archiv fur die gesammte Physiologie, 1874, Bd. viii. S. 460. 



