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, contains less heat than the body, it absorbs heat from the body, and we call it cold. See HEAT.
The physiological action of C. 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 combus tion, 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 condi tions), 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 108° to 112°, and in mammals from 98° 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, for physiological reasons, which will be explained in the article RESPIRATION, and the loss of heat goes on, therefore, with increasing rapidity, till the fatal limit is reached, and death super venes.
.In hibernating animals (the marmot, dormouse, bat, etc.), 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. At this low temperature, the vital actions are scarcely perceptible, but when the temperature is again raised, the vital activity returns. The respirations (in marmots) fall from 500 to
14 in the hour, and are performed without any apparent movement of the walls of the chest; the pulse sinks from 150 to 15 beats in the minute; and the animals can with difficulty be aroused from their torpor.—For additional matter bearing upon this sub ject, see the articles HIBERNATION, STARVATION, and DORMANT VITALITY.
C. is one of the most powerful depressing agents, and is a fruitful cause of disease, and even of death. Thus, it is observed, that whenever the temperature of the atmos phere 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 C. 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 tempera ture 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 C. are in the production of what is commonly called frost-bite. The part so affected is deprived of circulation, and does not bleed on being wounded; it is marble-white or livid, and has lost all sensibility; and if the exposure is continued, or reaction is brought about too rapidly, it is apt to pass into gangrene. The extremities, especially the fingers and toes, and the tip of the nose, are the parts most liable to frost-bite. The remedy is exceedingly gradual restoration of the temperature, with gentle friction. In Russia, friction with snow is commonly resorted to, so as to secure against too rapid reaction. The effects of C. upon the general system are described by arctic voyagers, and a medi cal detail of them may be found in baron Larrey's interesting account of Napoleon's disastrous campaign in Russia. The circulation is much depressed; diarrhma and rheu matic pains are frequent; in the end, the general sensibility becomes impaired, and an irresistible tendency to lie down is experienced, with excessive drowsiness. If this be not resisted, death is certain. The disease commonly termed "a cold" has been already described under CATARRH.