By J. MORTIMER-GRANVILLE.

Such expressions as a “cool head,” “hot-headed,” and the like, commonly relate to temperament rather than temperature; but it is essential to a full comprehension of the subject before us that the rationale of animal heat should be stated, and the laws that govern the phenomenon of temperature actual and subjective, at least cursorily, explained.

Heat and the sensation of heat are two widely different states. When, on a chilly day or after washing in cold water, a man rubs his hands until a glow of heat seems to suffuse them, there is a very slight rise of actual temperature caused by the friction; the feeling is principally due to nerve-excitement, produced mechanically by the rubbing. The blood flows more freely into, and through, the parts excited immediately afterward, as shown by the redness, but the first impression of heat is mainly one of sensation. The feeling and the fact are not even constantly related. A person may feel hot when not only the surrounding temperature but that of his body is low; or, he may feel cold when really overheated. These perverted sensations are occasionally morbid—that is to say, form part of a state of disease—or they may arise from individual peculiarities which, perhaps, render perceptions of a particular class especially acute. On the other hand, there are conditions of the body, and special sensibilities, in which the sense of heat is dulled, and even considerable elevations of temperature are not perceived. It is easy to see how impossible it must be to form a correct judgment of the actual state of heat either around or within us by simple sensation.

Throughout the world, whether man be placed in tropical heat or arctic cold, the temperature of his body must, to maintain health, be preserved at the same point—about 98.4 to .6 degrees of Fahrenheit. A very small departure from this universal mean standard constitutes or indicates disease. The external heat is comparatively unimportant, or only of secondary moment, in the economy of nature; we can not rely upon it for the compensation of differences in the heat generated within the body by the organism. Except for the production of a temporary effect, such as to give time for the reËstablishment of the normal temperature in a body chilled, as by submersion, external heat is useless for vital purposes. The only way in which it can act is by preventing the loss of more heat, and giving a slight aid to recovery by warming the surface of the body.

If when a person is cold he goes into a heated apartment, or sits before a large fire, he receives with advantage just as much heat as will bring the skin of his body up to the normal standard; as soon as that point is reached, the organism will begin to labor to get rid of the superfluous caloric, and by sweating the heat must be kept from rising above the standard. All the heat thrust upon the body above 98.6 degrees is waste and mischievous except in so far as it may promote perspiration, which probably helps to work off some of the useless and burdensome, possibly morbid and poisonous, materials that oppress the system. This is how Turkish baths, and “sweatings” generally, do good, by exciting increased activity of the skin, and, as it were, opening up new ways of egress for matters which, if retained, might offend.

So far as the heat of the body is concerned, whether in health or disease, every degree of external heat which is above the complement to form 98.6 degrees Fahrenheit with the heat of the body itself at the time, is useless and may do harm. It follows that in fever the surrounding atmosphere should be kept cool; in depressing disease, when the heat-producing powers of the organism are small, the air around should be warm. These are precisely the conclusions to which experience and observations conduct us; and the facts now briefly stated explain the reason why.

There is no warmth in clothes; the heat comes from the body itself, generated within, or the surrounding atmosphere, or from substances with which the body may be in contact. Of course clothes, like any other materials, can be charged with heat, and will take up as much thermic or heating property as their specific capacity allows. It is this capability of receiving heat which constitutes the first condition of warmth in the comparative value of different materials of dress. The second condition consists in the physical power of any fabric to hold the heat with which the article has been charged. For example, some materials will become warmer in a given time and retain their heat longer than others under the same conditions of exposure, first to heating and then to cooling influences. The principle of clothing should be to protect the body from external conditions which tend to abstract heat, when the surrounding temperature is lower than that of the body; and to strike heat into the organism, when the temperature of the outside air and of the substances with which the skin may be brought into contact is higher than that of the animal body itself.

Local temperature, that is, the heat in the several regions of the body is determined by conditions which control the circulation of the blood, and the function of nutrition or food appropriation. If the circulation is free in a part, its temperature is maintained; if, from any cause, the flow of blood is retarded, the local heat will be reduced. Any one may put this to the test by encasing the hands in somewhat tight gloves when the weather is cold. The pressure prevents the free passage of the blood through the vessels, and the temperature falls. There is no warmth of any kind in the gloves; they act simply as non-conductors of heat, and prevent the heat generated within the body from passing off. For example—if a piece of lint or rag be dipped in cold water and laid on the skin, and a sheet of impervious or non-conducting material, such as india-rubber or thick flannel, is wrapped closely round, the heat of the body will raise the cold water to a temperature at which it will be given off as steam the moment the covering is removed. When the extremities are enclosed in thick or dense coverings, their temperature will depend on the amount of heat generated within them, and if the flow of blood through the vessels is arrested or retarded, nothing is gained, but everything lost, by the measures taken to protect them from the external cold.

This is a matter of the highest practical moment, and needs to be thoroughly understood. The feet can not be kept warm unless the blood circulates freely in the extremities, and that will not be the case if the boot, shoes, or stockings are tight. These last-named articles of clothing are practically the worst offenders. A stocking encircling the foot and leg closely and enveloping every part, with special pressure at the instep, around the ankle, and above or below the knees, must inevitably tend to oppose the circulation and so reduce the natural heat. The arteries which bring the blood to the extremity are set deeper than the veins that carry it back, and, as the latter are provided with valves which open toward the heart, it is too commonly supposed that the “support” afforded by the stocking will favor the return of blood more than it can impede the deeper supply-currents, and so help the circulation; but practically we know this is not the fact, for a tight stocking ensures a cold foot, and the chilliness of which many persons complain is mainly caused by the practice of gartering, and wearing stockings which constrict somewhere or everywhere.

There is a popular notion that if the feet are cold the head must be hot, and by keeping the extremities warm with wraps the “blood is drawn from the head,” and its temperature reduced. Those who have on the one hand studied the phenomenon of fever, and on the other noted the physical condition of races and individuals who habitually leave the extremities unclothed, will know that this theory of the distribution of heat is only partially true. Heat depends on the due supply of nutrient elements to the tissues. It is the expression or result of the process of local feeding. If a part is active it will be heated. When the feet are left bare the complex muscular apparatus of the extremity, which in a stiff shoe scarcely works, is called into vigorous action, the arch of the foot plays with every step, and each toe performs its share in the act of progression. This promotes growth and calls for nutrition, whereby the heat is maintained; whereas if it be simply packed away as a useless piece of organism, no amount of external heat will warm it. Work is the cause and counterpart of heat throughout the body.

The same principle applies to the head. No amount of external cooling will reduce the temperature, no drawing away of the blood by artificial expedients will permanently relieve the sense or obviate the fact of heat if the organ within the cranium is excessively or morbidly active. The brain is a peculiarly delicate and complicated organ, requiring more prompt and constant nutrition than any other part of the body, because the constituent elements of its tissue change more rapidly than those of any other in proportion to the amount of exercise. Moreover, the brain is always acting during consciousness, and even in sleep it is seldom wholly at rest, as we know from the occurrence of dreams. The faculty of nutrition is highly developed in the organ or it could not so continuously, and on the whole healthily, discharge its functions, even when other parts of the body, or the system as a whole, are suffering from disease. When the head is heated there is nearly always a local cause for it, and the remedy must be addressed to the seat of the malady. The temporary expedient of “drawing away the blood” by applying heat to the extremities is useful as far as it goes, and may suffice to enable the organ to rid itself by the contraction of its blood-vessels from a surplus charge of this fluid, but in the absence of special causes the reason of the “heat of head” is undue exercise or disturbance of nutrition in the brain itself. Perhaps the seat of the over-work and consequent heating may have been limited to a particular part of the head; for example, the apparatus of sight, or hearing, as when the head becomes heated by reading too long or in a strong light. The point to understand is that when the head is physically hot it is the seat of too much or disorderly nutrition, and either the amount of brain or sense-power exercised must be reduced or the mode of action changed, and the particular part of the apparatus of perception or thought which has been too severely taxed relieved.

The true condition of health is that in which the temperature of the body as a whole and of its several parts is not disturbed by surroundings either of heat or cold. The preservation of a natural and healthy temperature is mainly to be secured by the maintenance of a regular and well distributed circulation of blood charged with the materials of nutrition.

The first condition of a free and continuous flow of blood is a healthy heart, not hampered by irritants, mental or physical. Sudden grief or fright produces cold by arresting the circulation, and the flow may be permanently retarded by anxiety. The mind has a wondrously direct influence on the heart and blood-vessels—on the latter through the nerves, which increase or reduce the calibre of the minute arteries, as in blushing or blanching at a thought. Instead of loading the body with clothes, the “chilly” should search out the physical cause of their coldness. The blood must not only circulate freely; it must be rich in nourishing materials, and not charged with poison. An excess of any one element may destroy the value of the whole. It is too much the habit of valetudinarians and unhealthy people of all kinds, to charge the blood with substances supposed to be “heating” or “cooling” as they think the system requires them. This is a mistake. The body does not need to be pampered with cordials, or refrigerated with cunningly devised potions. If it be well nourished it will be healthy.

There is something fearful in seeing a man of high character being under an obligation to a fool.—Goethe.