When the temperature of the air rises much above the average, the escape of heat is correspondingly hindered; and when the general body temperature begins to rise by this retention of caloric, we have the sequence of events detailed in the last paragraph. But before the blood can become warmer by the influence of the increased external temperature, the warm air, by stimulating the skin, brings about certain changes, independent of the body temperature, which satisfactorily check the tendency to an abnormal rise. This can be shown by the local application of external heat, by means of which (a) a rush of blood to the skin, and (b) copious sweat secretion may be induced in a part. This is brought about by impulses sent directly from the skin to the centres regulating the vasomotor and secretory mechanisms, and thus causing vascular dilatation and secretive activity. If a part only be warmed, a local effort is made to cool that part, and this has but little influence on the general body temperature.

When, however, the atmosphere becomes very warm, all the cutaneous vessels dilate simultaneously, and the escape of heat is greatly increased; while, at the same time, so much blood being occupied in circulating through the skin, the deeper - heat producing - tissues are supplied with less blood, and therefore generate a less quantity of heat. Thus a marked rise in the external temperature, which at first sight would seem to impede the escape of heat from the body, really facilitates it, by causing, through the vascular and glandular nerve mechanisms of the skin, a greater exposure of the blood to the cooler air, and a greater quantity of moisture to be evaporated from the warm skin. When the temperature of the air reaches that of the body, the only way of disposing of the heat generated in the body is by evaporation, for radiation and conduction become impossible. In animals like man, whose cutaneous moisture is great, external heat seldom causes marked change in the rate of breathing, but in animals whose cutaneous secretion is limited, external heat distinctly affects their respiratory movements, as may be seen by the panting of a dog on a very warm day, even when the animal is at rest.

Almost more important than facilitating the escape of heat in very warm weather, are the arrangements for preventing its loss when the surroundings are unusually cold. In this case, the cold, acting as a stimulus to the vaso-constrictor nerve agencies of the skin, causes the blood to retire from surface and fill the deeper organs, where more heat is produced. This bloodless skin and the underlying fat then act as a non-conducting layer or boundary protecting the warm blood from the cooling exposure. At the same time the secretion of the sweat is controlled by a special nerve mechanism, which lessens evaporation and soon checks the secretion, thereby enabling the body to remain at the normal standard temperature.

It would then appear that the chief factors regulating the body temperature belong to the expenditure department, and may be said to be - (a) variation in the quantity of blood exposed to be cooled, and (b) variation in the quantity of moisture produced for evaporation.

These regulators have to compensate not only for differences of external temperature, but also for great fluctuations in the amount of heat produced in the tissues.

The regulating power of the skin, etc., appears to be adequate for the perfect maintenance of uniform temperature only within certain limits. When these limits are passed by the rise or fall in the surrounding medium, the preservation of a uniform ternperature soon becomes impossible. These limits vary much in different animals, many of which have special coverings protecting them from external influences, and retain their warmth in a temperature seldom above o° C. In man the limits vary according to many circumstances, e.g., both extremes of age are more sensitive to changes of temperature. It would appear that for about 1o° C. above and below the body temperature our skin-regulating mechanisms are adequate, but beyond these limits external changes affect our general temperature, and if continued become injurious. Of course, by imitating with clothing the natural protection with which some animals are endowed, we can aid the normal regulating factors, and bear much greater extremes of temperature with safety or even comfort.

It is somewhat surprising that our bodies are always at the same temperature, no matter how hot or cold we feel. This is quite true, and our sensations of being hot or cold are explained as follows: When we feel hot our cutaneous vessels are full of warm blood, and this communicates to the cutaneous nerve terminals - the sensory nerves - the sensation of general warmth. On the other hand, when the cutaneous vessels are empty, the sensory nerves are directly affected by the cold of the external air. Since the full or empty state of the vessels of the skin depends generally on the heat or cold of the air, we use the expressions "it is hot or cold" and "we are hot or cold," as synonymous, because both ideas arise from the state of the skin. But we can make ourselves feel warm by violent exercise even on a frosty day, because we generate so much heat by muscular action that the cutaneous vessels have to be dilated in order to get rid of the surplus, and our skin vessels being full we feel warm. Our feelings, when we say we are warm or cold, simply depend upon our cutaneous vessels being full or empty of warm blood.

The local appreciation of differences of temperature will be discussed in the chapter dealing with the sense of Touch.