Experiment 113. To show the natural temperature of the body.  Borrow a physician’s clinical thermometer, and take your own temperature, and that of several friends, by placing the instrument under the tongue, closing the mouth, and holding it there for five minutes.  It should be thoroughly cleansed after each use.

226.  The Skin as a Heat-regulator.  The great regulator of the bodily temperature is, undoubtedly, the skin, which performs this function by means of a self-regulating apparatus with a more or less double action.  First, the skin regulates the loss of heat by means of the vaso-motor mechanism.  The more blood passes through the skin, the greater will be the loss of heat by conduction, radiation, and evaporation.  Hence, any action of the vaso-motor mechanism which causes dilatation of the cutaneous capillaries, leads to a larger flow of blood through the skin, and will tend to cool the body.  On the other hand, when by the same mechanism the cutaneous vessels are constricted, there will be a smaller flow of blood through the skin, which will serve to check the loss of heat from the body (secs. 195 and 270).

Again, the special nerves of perspiration act directly as regulators of temperature.  They increase the loss of heat when they promote the secretion of the skin, and diminish the loss when they cease to promote it.

The practical working of this heat-regulating mechanism is well shown by exercise.  The bodily temperature rarely rises so much as a degree during vigorous exercise.  The respiration is increased, the cutaneous capillaries become dilated from the quickened circulation, and a larger amount of blood is circulating through the skin.  Besides this, the skin perspires freely.  A large amount of heat is thus lost to the body, sufficient to offset the addition caused by the muscular contractions.

It is owing to the wonderful elasticity of the sweat-secreting mechanism, and to the increase in respiratory activity, and the consequent increase in the amount of watery vapor given off by the lungs, that men are able to endure for days an atmosphere warmer than the blood, and even for a short time at a temperature above that of boiling water.  The temperature of a Turkish bath may be as high as 150 degrees to 175 degrees F. But an atmospheric temperature may be considerably below this, and yet if long continued becomes dangerous to life.  In August, 1896, for instance, hundreds of persons died in this country, within a few days, from the effects of the excessive heat.

A much higher temperature may be borne in dry air than in humid air, or that which is saturated with watery vapor.  Thus, a shade temperature of 100 degrees F. in the dry air of a high plain may be quite tolerable, while a temperature of 80 degrees F. in the moisture-laden atmosphere of less elevated regions, is oppressive.  The reason is that in dry air the sweat evaporates freely, and cools the skin.  In saturated air at the bodily temperature there is little loss of heat by perspiration, or by evaporation from the bodily surface.