When an extremity—for example, the hand—is immersed in cold water, the temperature of the other hand also falls. Cold water abstracts the heat of the body, at least of its superficial surface, and affects the condition of the internal organs through the nervous system. It is through an influence transmitted from the peripheral distribution of the nerves of the hand to the center, and thence reflected to corresponding anatomical nervous connections on the other side, that the fall of temperature in the one hand is due when the other hand is immersed in water. We have a right to assume, therefore, that, when cold water is applied to the whole surface of the body, changes of temperature take place within. Indeed, it has been shown experimentally by Brown-Séquard, that ice applied to the lumbar region causes a contraction of the arterioles of the kidneys, and consequent diminished blood-supply to these organs.

When a cold bath is entered, a marked sense of chilliness is experienced, the skin becomes pale and is roughened by the erection of the hair-follicles (cutis anserina), the lips are blue, the breath has a spasmodic and catching character, and the pulse is quickened. The temperature of the surface is lowered, for the blood accumulates in internal organs, and the nerves of the skin are depressed. To the change in the conditions of the blood-supply, and the impression of the cold on the peripheral expansion of the nervous system, are due the coldness of the surface, the sobbing respiration, and the feeling of discomfort and depression. If the temperature of the water be not too low, and if the bodily vigor be sufficient to withstand the shock, the condition known as "reaction" speedily ensues. The coldness and depression are succeeded by warmth and a feeling of exhilaration; the pulse quickens, and the respiration becomes easy and unembarrassed; and the muscular strength is increased. If, however, the body be immersed for too long a period, the condition of reaction is supplanted by coldness, depression, weakened pulse, and muscular debility. This result is largely due to the continuous abstraction of heat, to the accumulation of blood in the great venous trunks, and the consequent interference with the metamorphosis of tissue. If healthy reaction comes on after bathing, the effects are those to which we apply the term tonic. The circulation is invigorated, tissue-changes take place more rapidly, and the products of increased tissue-metamorphosis are found in the urine. With the increased activity of the function of assimilation, the appetite and digestive power are improved, and the body gains in weight.