88 PHYSIOLOGY. 



tractions and slightly assisted by gastric movements also, some of the 

 contents of the stomach is forced into the opening of the oesophagus, 

 where its movement toward the pharynx and mouth is aided by con- 

 tractions of the oesophageal circular fibers : the reverse of what occurs 

 when a bolus of food is swallowed. 



Thus there are two separate and distinct acts occurring during 

 vomiting: (a) the dilating of the cardiac sphincter and (6) the 

 expulsive movements of the abdominal muscles. The absence of 

 either act is detrimental to the accomplishment of vomiting. The 

 pyloric gate is usually closed during vomiting; so that little or no 

 substances find their way into the duodenum. However, when the 

 gall-bladder is very full, the movements of the surrounding organs 

 force its contents into the duodenum and very frequently some of the 

 bile finds its way into the stomach, from whence it passes out through 

 the oesophagus, pharynx, and mouth in bilious vomiting, 



That the expulsive impetus is mainly given by the contractions 

 of the abdominal walls and not the gastric movements alone has been 

 proved by experiment. The stomach of an animal was excised and 

 replaced with a bladder filled with water and attached to the esoph- 

 agus by means of a rubber tube. When the wound was closed and 

 an emetic injected, the contents of the bladder were immediately 

 expelled through the mouth. 



Vomiting is normally considered to be a reflex action, although 

 in some instances vomiting may proceed at will or be acquired after 

 some practice. The afferent nerves are principally the fifth, the 

 glosso-pharyngeal, and the vagus. The center of vomiting is located 

 in the medulla oblongata. The efferent impulses are conveyed by the 

 vagi to the stomach, phrenics to the diaphragm, and various spinal 

 nerves to the abdominal muscles. Thus vomiting may arise : 



1. From irritation of the stomach, as when this organ is too full. 



2. From tickling the vault of the palate. 



3. From intestinal irritation by worms. 



4. From irritation of the uterine mucous membrane during the 

 first three months of pregnancy. 



5. The remembrance or sight of disgusting sights, or pathological 

 disorders of the brain may cause it, which proves that the brain is 

 united to a vomiting center. 



6. The use of emetics, which do not all act alike. 



Thus, some emetics, as copper sulphate, mustard, etc., produce 

 emesis because of their irritating effects upon the peripheral nerves 



