116 MANUAL OF PHYSIOLOGY. 



Vomiting is usually caused by irritation of the stomach itself, 

 and may be induced by either mechanical, electrical, or chemical 

 stimulation of the mucous membrane. In this way some emetics, 

 such as mustard, sulphate of copper, etc., act. It may also be 

 caused by intestinal irritation, as when a hernia is strangulated, 

 or the mucous membrane irritated by intestinal worms. 



Gentle stimulation of the fauces and neighborhood of the root 

 of the tongue commonly induces vomiting. In the early stages 

 of pregnancy the unusual condition of the uterus causes frequent 

 vomiting, which is known as "morning sickness." The irritation 

 of a calculus passing through the ureter, or a gall-stone impacted 

 in the bile duct, commonly excites vomiting. Injuries of the 

 brain, and psychical impressions, particularly those excited by the 

 sense of smell or unusual disturbance of equilibrium, may give 

 rise to vomiting. Moreover, a number of medicaments, as apo- 

 morphin, emetin, etc,, cause vomiting if introduced into the blood. 



From the foregoing facts it appears that vomiting is a complex 

 and irregular muscular act, which may be induced by the stimu- 

 lation of various parts of the internal surfaces of the body, par- 

 ticularly those which receive branches from the vagus nerve. 



One would therefore be inclined to suppose that some afferent 

 nerve channels exist in the vagus which bear impulses to a vom- 

 iting nerve centre and excite it, so as to cause it to send forth 

 peculiar and irregular impulses to the respiratory, gastric, and 

 other muscles, and give rise to their characteristic spasm. 



In short, it would seem to be a reflex act, the afferent impulses 

 of which pass to the medulla oblongata by the vagus, and the 

 efferent impulses are conveyed by the ordinary spinal nerves to 

 the respiratory muscles, by the vagus to the pharyngeal, laryn- 

 geal, and gastric muscles, and by the fifth, seventh, and ninth 

 nerves to the palatine, facial, and hyoidean muscles. This vom- 

 iting nerve centre must lie in the medulla, in very close relation- 

 ship to the respiratory centre, with which it nearly corresponds. 

 This centre may bring about the whole sequence of events known 

 as vomiting when stimulated either dfrectly by poisons, contained 

 in the blood, indirectly through the vagus, or even from the 

 higher centres by emotions or ideas. Section of the vagi renders 



