3. Effects resulting from the Modification of Temperature.—Perhaps the most important, certainly the most common, effects of water upon the living organism are those which result from its modifications of the temperature of the body in its various modes of application. These effects vary greatly according to the temperature, and the duration of the application. General and local applications also differ in their results.

It should be remarked that all of the effects of water are really the results of the vital resistance of the system in its attempts to remove abnormal or unusual conditions, or to accommodate itself to new circumstances.

Baths are divided into six classes, according to their temperature, as follows:—

1. Cold,33°to60°F.
2. Cool,60°„70°
3. Temperate,70°„85°
4. Tepid,85°„92°
5. Warm,92°„98°
6. Hot,98°„112°

For the sake of simplicity, we will consider the effects of water applications under three heads; viz., cold, warm, and hot.

The Cold Bath.—Under this head we will consider applications of all temperatures below 85° F. Cold or cool water, applied to any portion of the body, causes instant contraction of the small arteries of the part, through its influence upon the sympathetic or vasomotor system of nerves. So long as the application of the unusual temperature is continued, the vascular contraction is maintained, and the part seems nearly bloodless. If the cold is below 33° F., and is long continued, destruction of the tissues, by freezing, will result.

If a moderately cool or cold temperature is maintained for some time, the blood-vessels of the part are more or less permanently contracted, and the blood supply thus lessened. If, on the other hand, the application is very brief, the contraction of the vessels is only momentary, and is followed by a proportionate degree of relaxation, and a corresponding increase in the supply of blood to the part.

A very cold bath applied to any considerable portion of the body, and continued more than a very brief time, produces headache, dullness, sometimes nausea and vomiting, loss of sensibility, and other unpleasant and painful symptoms.

It is thus seen that the effects of cold are quite different—exactly opposite, in fact—as the application is a prolonged, or a brief one. The long application produces effects in some degree permanently sedative, while the brief application is followed by a momentary condition which may be termed shock, and which is usually followed very quickly by a reaction analogous to stimulation when produced in any other manner.

Effect of Cold upon the Pulse.—The experiments of Drs. Currie, Bell, and others, show conclusively that the cold bath has the uniform effect of diminishing the frequency of the heart’s action from ten to twenty beats in a minute below the usual standard. Upon the first application of cold, there is a slight increase in the rate of pulsation; but this soon subsides, and is succeeded by a marked diminution. The ultimate effect is the same, whether the application is made at its maximum degree of severity or not; but if the application is first warm, being gradually reduced in temperature, the result is reached without the occurrence of the unpleasant shock, or feeling of chilliness, which attends the sudden application of cold, especially in persons of delicate nervous sensibilities. The amount and after duration of the diminished rate of pulsation depends upon the temperature and duration of the bath. In health, it does not commonly extend beyond a few hours at most.