I. Physiological Relations

Vomiting is a complex act, in which the respiratory muscles, the abdominal walls, the walls of the stomach, the sphincter of the cardiac orifice, and the oesophagus and pharynx participate. Occasionally it is to he regarded as a strictly physiological process for removing excess of food from the stomach, as in the regular sickness of infants after a full meal of milk. It is determined and directed by an elaborate nervous mechanism, consisting of a special centre, the vomiting centre, in the medulla; of afferent nerves from the fauces, stomach, abdominal viscera, and peritoneum, the chief of which are the glosso-pharyngeal, vagus, and sympathetic, and, indeed, from other parts of the body-the sensory nerves generally; and of efferent nerves (the vagus, phrenic, and intercostals) to the muscles, cardiac orifice, and certain associated parts to be presently mentioned. Vomiting may be induced by impressions originating in the areae supplied by any of the afferent nerves; by stimulation of the centre by certain substances which reach it through the blood; or by the downward flow to it of certain mental impressions, such as nauseous tastes, foul smells, disgusting or terrifying sights, and depressing ideas.

With the evacuation of the stomach there occur certain associated acts which are of great importance to the therapeutist. A flow of saliva may precede vomiting, as is well seen in some reflex cases. The gall bladder may be forcibly emptied of bile, which regurgitates into the stomach and is vomited. Expiratory movements, such as sneezing and coughing, frequently occur at the beginning of sickness, indicating the spread of the stimulant impressions to the associated respiratory centre in the medulla; and it must be carefully observed that an expiratory effect is also produced by compression of the chest during the evacuation of the contents of the stomach, as well as at the end of the act, when the air is forcibly expelled through the larynx to prevent the entrance of solid particles. Thus vomiting tends to empty the respiratory passages, as well as the upper part of the alimentary canal. The stimulant effect of emetics on the salivary flow is frequently accompanied by a secretion of bronchial mucus; and this being expelled by the upward current of air, tends further to clear the passages.

Whilst the respiratory and gastric centres are thus powerfully stimulated in vomiting, the cardiac and vascular centres are greatly depressed, the action of the heart and the pulse being reduced in force-at least, between the acts of sickness, and a sense of faintness and giddiness overspreading the patient from further cerebral anaemia. At the same time, the motor centres in the brain, and probably in the cord, are lowered, leading to prostration and inability to support the weight of the body, and compelling recumbency. Lastly, the centres of perspiration are stimulated, causing the profuse sweating familiar in many cases of sickness. Altogether, the student will appreciate how extensive is the physiological disturbance produced by vomiting, and how great is the influence which it furnishes us over several of the most important functions of the body.

II. Pharmacodynamics

Vomiting may be excited by certain substances and measures, which are called emetics. Emetics are said to be either (1) direct, when they act upon the stomach itself; or (2) indirect, when they act upon the vomiting centre or some other part of the nervous mechanism. Direct emetics are the larger of the two classes. They include warm water, Infusion of Chamomile, Salt and Water, Mustard, Carbonate of Ammonia, Sulphate of Zinc, Alum, and Sulphate of Copper. They are necessarily given by the mouth. Indirect emetics are a small group of drugs, including only Ipecacuanha, Antimony, and Apomorphia. These excite vomiting by whatever channel they may be admitted into the blood-subcutaneously, by the mouth, or by the rectum. For the same reason they produce greater general depression, that is, depress the other vital centres in the medulla more than moderate doses of the direct emetics. Physical irritation of the fauces is a ready emetic measure of the indirect class; and nauseous drugs, such as castor oil and rhubarb, frequently act on the nerves of the same part, but are not given with this intention. Ipecacuanha and Antimony act on the stomach as well as on the centre, and are really, therefore, direct and indirect emetics.

The means at our disposal for averting or arresting vomiting are as various as the parts of the extensive mechanism upon which they act. They may be called anti-emetics. First of these may be mentioned the measures which reduce the irritability of the vomiting centre, such as the recumbent posture, nourishing food, Amyl-Nitrite, Nitro-Glycerine, Alcohol, Opium, Chloral, the Bromides, and Diluted Hydrocyanic Acid. A second class, more readily available, comprise the sedatives of the afferent nerves from the stomach, such as Hot Water, Ice, Diluted Hydrocyanic Acid, Carbonic Acid, Bismuth, Dilute Alkalies, Opium, Ipecacuanha and Calomel in small doses; measures which act indirectly upon the stomach and reduce the irritability of its nerves, such as poultices or blisters to the epigastrium; and sedatives of the afferent nerves to the vomiting centre from other organs, for instance, demulcents to the throat, poultices to the abdomen, or applications to the os uteri.

III. Pathological Relations

Vomiting being regarded for our present purpose as a physiological act, it may be considered to be disordered, (1) if excessive; and (2) if defective, insufficient, or absent when it would be salutary or desirable. We will illustrate each of these conditions.

1. Excessive vomiting occurs as the result of disorder or disease of the stomach; morbid conditions of other parts of the abdomen, such as hernia, cough, severe pain, injury or disease of the brain, or disturbance of the circulation and senses, including sea-sickness. The cause of vomiting may be in the centre itself, especially as a consequence of previous violent vomiting, or of urea and certain extrinsic poisons, such as antimony.