450 DIGESTION 



esophagus, which, already somewhat distended by saliva, now becomes 

 markedly so. The abdominal muscles then contract so as to compress 

 the stomach against the diaphragm and, simultaneously, the cardiac 

 sphincter relaxes, the head is held forward and the contents of the 

 stomach are ejected through the previously distended esophagus. The 

 compression of the stomach by the contracting abdominal muscles is 

 assisted by an actual contraction of the stomach itself, as has been clearly 

 demonstrated by the x-ray method. After the contents of the stomach 

 itself have been evacuated, the pyloric sphincter may also relax and 

 permit the contents (bile, etc.) of the duodenum to be vomited. 



The act of vomiting is controlled by a center located in the medulla, 

 and the afferent fibers to this center may come from many different 

 regions of the body. Perhaps the most potent of them come from the 

 sensory nerve endings of the fauces and pharynx. This explains the 

 tendency to vomit when the mucosa of this region is mechanically stimu- 

 lated. Other afferent impulses come from the mucosa of the stomach 

 itself, and these are stimulated by emetics, important among which are 

 strong salt solution, mustard water and zinc sulphate. Certain other 

 emetics, particularly tartar emetic and apomorphine, act on the vomit- 

 ing center itself, and can therefore operate when given subcutaneously. 

 Afferent vomiting impulses also arise from the abdominal viscera, thus 

 explaining the vomiting which occurs in strangulated hernia, and in 

 other irritative lesions involving this region. X-ray observations have 

 been made on the movements of the stomach of cats after the admin- 

 istration of apomorphine (Cannon). The first change observed is an 

 inhibition of the cardiac end of the stomach, which becomes a perfectly 

 flaccid bag. About the midregion of the organ, deeper contractions then 

 start up, which sweep towards the pylorus, each contraction stopping as a 

 deep ring at the beginning of the vestibule, while a slighter wave con- 

 tinues. A very strong contraction at the incisura angularis finally 

 develops and completely divides the gastric cavity into two parts. On 

 the left of this constriction the stomach remains completely relaxed, but 

 at the right of it waves continue running over the vestibule. It is while 

 the stomach is in this condition that the sudden contraction of the dia- 

 phragm and abdominal muscles shoots the cardiac contents into the 

 relaxed esophagus. As these jerky contractions are continued, the gastric 

 walls seem to reacquire their tone. 



