The experiments of Pawlow, Starling, and others show that when food is taken the stimulation of the salivary glands which it produces is passed on to the stomach, intestines and other organs, the action of each one serving to start the activity of the next. But mankind has everywhere felt the necessity for a stimulant to the central nervous system other than that of food, and in the most diverse countries and from the most diverse plants has obtained stimulants of some kind. These fall into two chief classes : (1) they either contain ethylic-alcohol or some alkaloid having an allied physiological action, or (2) they belong to the purin bodies like the constituents of beef-tea, already mentioned. The alcohol may be present in a more or less dilute form in fermented juices, or infusions, or may exist in a tolerably concentrated form in the distillate which these yield. The general action of alcohol is that of a local irritant, and after absorption it quickens the circulation and lessens the excitability of the nervous system. It thus acts as an anaesthetic both physical and mental, and renders the person who is under its influence less conscious of unpleasant conditions affecting either his body or mind. Painful stimuli such as blows, pricks, cuts or burns are felt less acutely; sensibility to the sensation of fatigue is deadened and mental pain, whatever its origin, may for the time being be abolished. It is this property of temporarily rendering people unconscious of weariness, pain or depression, and of imparting to them a feeling of well-being, hilarity and comfort that has rendered the desire for alcohol so universal. It is evident that such a power may render it a very potent agent either for good or evil. When used in moderation, and on appropriate occasions, I think it is certainly useful, but the very pleasure it gives and the temporary unconsciousness of discomfort or misery which it induces are a temptation to take it again and again in larger quantities, to the destruction of the consumer. The anaesthetic action of alcohol on body and mind was first described by Solomon, who makes the drunkard say in relation to its anaesthetic action on the body : "They have beaten me and I felt it not; I will seek it yet again." And for its effect on the mind he says : "Give wine to him that is of heavy heart, and strong drink to him that is ready to perish that he may drink and forget his misery." Healthy people, as a rule, do not need alcohol, and are better without it; but in the press of circumstances or with advancing years, when the digestive functions become less active, some form of alcohol with a meal is in some persons, I think, though not in all, advantageous. Morphine and cocaine have a similar anaesthetic action to alcohol, but they are more powerful and more liable even than it to abuse and to an even greater extent. The other class of stimulants, viz. those belonging to the purin bodies, are tea, coffee, cocoa, guarana and mate. These do not have the same anaesthetic action as alcohol, morphine or coca, but they stimulate the central nervous system, increase energy and quicken the mental processes.
Some of the purin bodies act as powerful diuretics. Urea has a distinctly diuretic action, but this power is much more marked in caffein, and its allies, theobromin, theocin, which stimulate the excreting powers of the renal tubules. They not only increase the amount of urine, but also of the products of tissue waste it contains, and thus tend to quicken the metabolic processes and stimulate the functions of all the organs of the body.