Cold is the term by which we signify a relative want of sensible heat. There are therefore no determinate boundaries between cold and heat, and it is a mere arbitrary distinction to call the degrees of the thermometer below the freezing-point degrees of cold. When the atmosphere, or any substance which comes in contact with our body, is at a lower temperature than the skin, it absorbs heat from it, and we call it cold. See HEAT.
The physiological action of cold on the animal organism requires a brief notice. All animals (the warm-blooded animals to the greatest extent) have a certain power of maintaining the heat of the body, in defiance of external cold, as has been shown in the article ANIMAL HEAT. This power is mainly due to a process analogous to combustion, in which carbon and hydrogen taken into the system in food are made to unite with oxygen derived from the air by respiration. If the combustible materials are not duly furnished, or if the supply of oxygen be deficient (as in various diseased conditions), there must be a depression of temperature. Now, if the temperature of a bird or mammal (except in the case of hibernating animals) be lowered about 30° below its normal standard (which in birds ranges from 100° to 112°, and in mammals from 96° to 102°), the death of the animal is the result. The symptoms indicating that an animal or a man is suffering from a depression of the temperature of the body are—retardation of the circulation of the blood, causing lividity of the skin, which is followed by pallor, in consequence of the blood being almost entirely driven from the surface through the contraction of the vessels; a peculiar torpor of the muscular and nervous systems at the same time manifests itself in an indisposition to make any effort or exertion, and in intense sleepiness. The respiratory movements become slower (see RESPIRATION), and the loss of heat goes on, therefore, with increasing rapidity, till the fatal limit is reached, and death supervenes.
In hibernating animals (the marmot, dormouse, bat, &c.) the power of generating heat within their own bodies is very slight, their temperature following that of the external air, so that it may be brought down nearly to the freezing-point. See the articles HIBERNATION, FAST, SLEEP, LIFE, DESICCATION; also, for other phenomena connected with cold, HEAT, CLIMATE, FREEZING, ICE, THERMOMETER, TEMPERATURE.
Great or prolonged atmospheric cold is a most powerful depressing agent, and is a fruitful cause of disease and even of death. Whenever the temperature of the atmosphere is suddenly reduced, and particularly when it is reduced below the freezing-point, a considerable addition takes place to the mortality of the country at large. The effects of cold are, in ordinary circumstances, most apparent among the aged and the very young, and among those suffering from chronic disease; but when a very low temperature is long continued, even the healthy are sure to suffer, when impoverished so as not to have sufficient means of external warmth in their homes. The most direct effects of cold are in the production of what is commonly called Frost-bite (q.v.).
Cold is applied in various ways in the treatment of disease. In some forms of fever, a cold bath, or cold wet pack, is the best means of reducing a very high temperature which of itself threatens life. In many inflammations relief is best obtained by the local application of ice, or of a coiled tube through which cold water circulates. The tonic and stimulating effects of a temporary application of cold are familiar in the cold morning bath, or the use of cold water sprinkled on the face of a person who has fainted. The disease commonly termed a 'cold' has been already described under CATARRH.