The stomach has nerve connections with the cerebro-spinal axis through the vagi, and the splanchnic branches of the sympathetic, and in the walls of the organ itself are numerous ganglion cells. The sympathetic connections do not seem to have any influence on the muscular coats, for neither their stimulation nor section has any marked effect on their movements. If the vagi be severed, stomach contractions still occur, but no form of local stimulation produces the normal gastric motions, even if the* organ be quite full of food, therefore it would appear that the local nerve centres are not sufficient to excite the normal rhythmical muscular action. Moreover, stimulation of the cut vagi leading to the stomach causes active movements when the stomach is full. It is not merely the presence of food that produces the movements, as is shown by the fact that the motions increase as the contents of the stomach diminish, but conditions incidental to digestion (hyperemia, etc.), probably also act as a stimulus.
Vomiting is the ejection of the contents of the stomach by means of a convulsive action of the respiratory and abdominal muscles associated with an abnormal contraction of the stomach wall, which aids in opening the cardiac orifice while it keeps the pylorus firmly closed.
The act of vomiting is commonly preceded by (i) a feeling of sickness or nausea, (2) a great secretion of saliva, (3) retching. The latter consists in a violent inspiratory effort, in the midst of which the root of the tongue and the larynx are raised and the rima glottidis suddenly closed so as to prevent air entering the windpipe. The inspiratory muscles still acting, and the pharynx and upper part of the oesophagus being held open, air is drawn into the gullet and dilates this tube nearly as far as the opening into the stomach. A contraction of the muscle fibres radiating from the oesophagus over the stomach then opens the cardiac orifice and allows some gas to escape. Now the act of 11 vomiting is completed if at this moment - the mouth and pharynx being open, the larynx closed, the oesophagus on the stretch, the cardiac orifice relaxed, and the pylorus firmly closed - the expiratory muscles forcibly contract, and, pressing upon the abdominal cavity, give a sudden stroke to its contents so as to empty the stomach. The wall of the stomach also contracts evenly throughout, but not with any forcible anti-peristaltic action such' as would greatly aid in the operation of rapidly ejecting the vomit. The chief object attained in the adult by the action of the muscular coat of the stomach seems to be the relaxation of the cardiac orifice. In children, when the fundus is little developed, and the fibres radiating over the stomach from the oesophagus are numerous and strong, the act of vomiting requires less effort on the part of the respiratory muscles; the frequent puking of suckling infants being accomplished by the gastric muscle alone. When the vomit is emitted, the hyoidean, laryngeal, and neck muscles relax, and the air is forcibly driven out of the partially distended lungs so as to clear away any remaining particles from the upper part of the air passages.
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, usually excite 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 apomorphin, 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 stimulation of various parts of the internal surfaces of the body, particularly 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 vomiting 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, laryngeal and gastric muscles, and by the fifth, seventh and ninth nerves to the palatine, facial and hyoidean muscles. This vomiting nerve centre must lie in the medulla, in very close relationship 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 directly 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 vomiting impossible, as it cuts off both the commonest source of stimulus going to the centre, and also the important efferent impulses which cause the muscle coat of the stomach to contract and to open the cardiac orifice.