But it is essential, in order to keep the body of warm-blooded animals at a constant or normal temperature, that an increase in the heat-production should be accompanied by a corresponding increase in the heat dissipation; the functions are reciprocal, and this equilibrium is regulated and maintained by a special nervous and functional mechanism. The automatic protective measures against the effects of heat are: _First._—Dilatation of the cutaneous vessels and an acceleration of the circulation through the skin and the subcutaneous tissue. By this means the dissipation of heat is increased; the sweat at the temperature of the blood, deposited upon the surface of the body, evaporates under favorable circumstances, and in this way considerable amounts of heat are abstracted from the body. In consequence of sweating and its evaporation, the blood circulating through the skin is cooled, and returning to the internal organs at a lowered temperature, prevents their overheating. _Second._—Should the action of heat be continued for a greater length of time, a large amount of blood will be retained in the skin in consequence of the loss of tonicity of the cutaneous vessels; the cutaneous circulation will be slowed, and thereby the blood, heated at the surface of the body, is prevented from returning to the internal organs and so overheating them. _Third._—In consequence of the accumulated amount of blood in the skin, a diminished amount of blood will remain in the internal organs; thus their activity, and thereby also the production of heat, will be lessened. In these processes will be found a safeguard against the excessively rapid penetration of heat to the internal organs, and against the unduly rapid elevation of the body temperature through thermic influences. As a result of the stimulating influence of cold, there first occurs contraction of the skin and its vessels. This, by restricting the dissipation of heat, brings about perfect compensation if the abstraction of heat be but slight; and but partial compensation, if the abstraction of heat is more marked. In the latter event the body temperature will continue to decline, to a greater or less degree; in the former it will remain constant. The rôle played by the skin in maintenance of the normal temperature of the body is indispensable. The normal temperature of the adult human body is 98.6° F. in the mouth, and that of the rectum and vagina is one degree higher. Fasting, sleep, and short applications of heat all decrease heat-production; during sleep the temperature of the body falls half a degree or more. Respiration by the skin varies from ½ to 1 per cent. of the total amount of oxygen taken into the body, and a somewhat lower percentage of carbonic acid is thrown off through this channel.