I. Physiological Relations

Heat is produced in every act, of vital energy; is distributed throughout the body; and is finally lost in the surrounding medium. In so-called "cold-blooded" animals, the vital heat is lost as rapidly as it is produced; in "warm-blooded" animals the heat produced does not escape until a certain amount has accumulated within the system. Thereupon loss sets in, and exactly balances the production, whilst the accumulated store remains constant, and is known as the "body heat," amounting, in man, to 98.4 degrees.

So wide is the range, so sudden are the changes, of the external temperature to which man is exposed, and so variable the amount of heat produced in the system at different moments, that in the course of its evolution the body has come to possess a complex and sensitive nervous mechanism, by which its temperature is controlled. This mechanism consists of governing centres, afferent nerves from impressionable parts, and efferent nerves to active organs. The afferent thermal nerves, originating in the skin, and possibly in other parts of the body, such as the mucous membranes and viscera, carry impressions of temperature (heat and cold) to the brain and cord. There these impressions are specially received by three of the great centres, viz. the cerebrum, where they become sensations of temperature; the sweat centres in the cord and medulla; and the metabolic or trophic centres, the centres of nutrition, in the brain (?pons) and cord. They also fall into the vasomotor, cardiac, respiratory, and possibly the renal and other visceral centres. Efferent impulses from the sweat centres proceed to the sudoriparous glands, which they stimulate or depress, as the case may be; from the metabolic centres they are directed to the various sources of heat production-the muscles, glands, etc., which they depress or stimulate. At the same time, the circulation through the skin is modified, as well as the blood pressure generally, the respiration, renal secretion, and probably every other bodily function in some degree.

Thus, when the temperature of the air rises, the regulative mechanism comes into action, and two great effects are produced: (1) there is increased loss of heat by the perspiration, by cooling of the blood in the dilated cutaneous vessels, and by cooling of the blood in the lungs; and (2) there is diminished production of heat in the muscles, glands, etc. The same effect follows a rise of the internal temperature due to increased metabolic activity, such as muscular exercise: a "warm glow" is felt, the skin flushes and perspires, the circulation and respiration are increased, and the activity of other metabolic organs, such as the liver, is for the time lowered. The skin is the principal channel of loss of heat in man; but during and after exertion a large amount of heat must be carried off by respiration, which is familiarly known to be the chief means of refrigeration in the dog.

Conversely, if the temperature of the surface be lowered by cooling of the atmosphere, two reflex effects are at once produced through the nervous system, viz.: (1) diminished loss of heat, by contraction of the vessels of the skin, by arrest of perspiration, and by reduced activity of the circulation and lungs; and (2) increased production of heat in the metabolic organs, especially the muscular, digestive, and circulatory. A similar result follows lowering of the internal temperature by diminished metabolism in some of the organs. Thus Quinia and Salicylic Acid, whilst they diminish the amount of the urea and therefore probably of the heat produced in the system, make little or no impression on the temperature of a healthy man, doubtless because the channels of loss are partially closed, and the metabolism of certain organs increased, by the regulating mechanism.

II. Pharmacodynamics

1. Temperature Of The External Media

Temperature Of The External Media. This is completely under our control. The atmosphere is the ordinary external medium of loss or gain of the bodily temperature, and the air of every well-constructed room or ward can be warmed or cooled at pleasure. We may select the climate in various ways, according to its temperature; the sub-tropics, such as Madeira, Egypt, and the Riviera, being especially valuable as affording warm climates. When a more rapid and extreme influence of the external temperature is desired, water may be substituted for air, in the form of baths, wet-packs, and sponging. The varieties, action, and uses of water applied in these several ways are described in the next chapter. By means of the prolonged cold bath, at a temperature varying between 32° and 60° Fahr., heat may be readily abstracted from the body; and the cold wet pack, cold affusion, or sponging a part or the whole of the exposed skin with cold or even tepid water, has a similar effect. These measures are known as external refrigerants. Heat may be locally abstracted by similar means, which will also have a general effect in reducing the temperature of the body. Thus, cold water may be injected into the rectum or vagina; ice or wet compresses applied to the skin; ice or cold water swallowed; or irrigation with cold water may be used over a part. The cooling that attends evaporation is a powerful means of reducing the local temperature; and a variety of saline, spirituous, and acid solutions, such as Carbonate or Chloride of Ammonium, Spirit and Water, Brandy and Water, Vinegar and Water, or various combinations of salts, acids, and spirits, may be employed for this purpose.

2. The Cutaneous Circulation

The Cutaneous Circulation. This affords us a powerful means of abstracting the body heat, inasmuch as we can modify the fulness of the vessels and the rate of flow through them. Thus we may cool the blood by dilating the cutaneous vessels by the warm bath, by Alcohol, Spirit of Nitrous Ether, or warm draughts, or by these measures combined. Opium and Chloral have the same effect. If the blood-flow be accelerated through the dilated vessels, the refrigeration is increased, and in this way cardiac stimulants of every kind, such as Alcohol and Digitalis, reduce the body temperature. Draughts of water, whether cold or hot, temporarily distend the vessels, and produce a similar effect. The opposite methods for preserving the heat of the body, by contracting the superficial vessels and reducing the activity of the cutaneous circulation, are of no therapeutical interest.