CHAPTER XIII.

ANIMAL HEAT.

General considerations-Limits of variation in the normal temperature in man —Variations with external temperature—Variations in different parts of the body-Variations at different periods of life-Diurnal variations-Relations of animal heat to digestion-Influence of defective nutrition and inanition-Influence of exercise, mental exertion, and the nervous system, upon the heat of the body.

THE process of nutrition in animals is always attended with the development of heat, and produces a temperature more or less independent of external conditions. This is true in the lowest as well as the highest animal organizations; and analogous phenomena have even been observed in plants. In cold-blooded animals, nutrition may be suspended by a diminished external temperature, and certain of the functions become temporarily arrested, to be resumed when the animal is exposed to a greater heat. This is true, to some extent, in certain warm-blooded animals that periodically pass into a condition of stupor, called hibernation ; but in man, and nearly all the warm-blooded animals, the general temperature of the body can undergo but slight variations. The animal heat is essentially the same in the intense cold of the frigid zones and under the burning sun of the tropics; and if, from any cause, the body become incapable of keeping up its temperature when exposed to cold, or moderating it when exposed to heat, death is the invariable result.

The production of animal heat is so closely connected with nutrition, that in serious pathological modifications of

this process, as in the essential fevers or extensive inflammations, the temperature of the body becomes an important guide, particularly in prognosis. The clinical value of a recognition of the temperature in disease has only been fully appreciated within a few years, especially since the very elaborate observations of Wunderlich, and other German observers.'

The study of the temperature in different classes of animals presents very great interest, but the limits of a work on pure human physiology restrict us to the phenomena as observed in man, and in animals in which the processes of nutrition are similar, if not identical. We shall therefore treat of the subject from one point of view, and consider it as follows:

1. The normal temperature in the human subject, with its variations in different parts of the body and at different periods of life.

2. The diurnal variations in the animal temperature, and the relations of alimentation, digestion, respiration, nutrition, exercise, and the nervous system.

3. The means by which the temperature of the body is kept within the limits necessary to the preservation of life and health.

Limits of Variation in the Normal Temperature in Man.-A great number of observations have been made upon the normal temperature in the human subject under different conditions; but we shall cite those only in which all sources of error in thermometry seem to have been avoided, and in which the results present noticeable peculiarities. One of the most common methods of taking the general temperature has been to introduce a delicate thermometer, carefully protected from all disturbing conditions, into the axilla, reading off the degrees after the mercury has become abso

1 HIRTZ, Chaleur dans l'état de maladie.—Nouveau dictionnaire de médecine, Paris, 1867, tome vi., p. 772, et seq.

lutely stationary. Nearly all observations made in this way agree with the results obtained by Gavarret,' who estimates that the temperature in the axilla, in a perfectly healthy adult man, in a temperate climate, ranges between 97.7° and 99.5°. Dr. Davy, from a large number of observations on the temperature under the tongue, estimates the standard, in a temperate climate, at 98°." When we examine the temperature of the blood in the deeper vessels and the variations in different parts, we shall see that the axilla and the tongue, being more or less exposed to external influences, do not exactly represent the general heat of the organism; but these are the situations, particularly the axilla, in which the temperature is most frequently taken, both in physiological and pathological examinations. As a standard for comparison, we may assume that the most common temperature in these situations is 98°, subject to variation within the limits of health of about 0.5° below and 1.5° above.

Variations with External Temperature.-There can be no doubt that the general temperature of the body varies, though within very restricted limits, with extreme changes in climate. The results obtained by Davy, in a large number of observations in temperate and hot climates, show an elevation in the tropics of from 0.5° to 3°. It is well known, also, that the human body, the surface being properly protected, is capable of enduring for some minutes a heat much greater than that of boiling water. Under these conditions, the general temperature is raised but very slightly, as compared with the intense heat of the surrounding atmosphere According to the observations of Dr. Dobson, the temperature was only raised to 99.5° in one instance, 101.5° in an

1 GAVARRET, De la chaleur produite par les êtres vivants, Paris, 1855, p. 100. All the temperatures, unless it be otherwise stated, are given according to the Fahrenheit scale.

196.

3 DAVY, Researches, Physiological and Anatomical, London, 1839, vol. i., p.

• DAVY, loc. cit.

other, and 102° in a third, when the body was exposed to a heat of more than 212°. MM. Delaroche and Berger, however, found that the temperature in the mouth could be increased by from 3° to 9°, after sixteen minutes' exposure to intense heat. This was for the external parts only; but it is not at all probable that the temperature of the internal organs ever undergoes such extensive variations.

It is very difficult to estimate the temperature in persons exposed to intense cold, as in Arctic explorations, because the greatest care is always taken to protect the surface of the body as fully as possible; but experiments have shown that the animal heat may be considerably reduced, as a temporary condition, without producing death. In the latter part of the last century, Dr. Currie caused the temperature in a man to fall 15° by immersion in a cold bath; but he could not bring it below 83°. This extreme depression, however, lasted only two or three minutes, and the temperature afterward returned to within a few degrees of the normal standard.' Nearly the same results were obtained by Hunter, in a series of experiments on a mouse. With an external temperature of 60°, he found the temperature in the upper part of the abdomen 99°, and in the pelvis 96°. The animal was then exposed for an hour to a cold 1 DOBSON, Experiments in an Heated Room.-Philosophical Transactions, London, 1775, p. 463, et seq.

DELAROCHE, Expériences sur les effets qu'une forte chaleur produit dans l'économie animale.—Thèses de Paris, 1806, tome i., No. xi. M. Delaroche, in connection with M. Berger, made a number of very interesting experiments upon the influence of high temperatures upon the general heat of the body. Delaroche remained for eight minutes exposed to a temperature of 176°, and the temperature under the tongue was raised from a little over 98° to nearly 107°. In an experiment of the same kind by Berger, the temperature was raised, in sixteen minutes, from 98° to nearly 105°. Enclosed in a hot steam-bath of from 100° to 120°, the temperature, in one instance, was raised, in thirteen minutes, to over 103°, and in another, in fifteen minutes, to 101° (Loc. cit., pp. 43, 44).

3 CURRIE, An Account of the remarkable Effects of a Shipwreck on the Mariners; with Experiments and Observations on the Influence of Immersion in fresh and salt Water, hot and cold, on the Powers of the living Body.-Philosophical Transactions, London, 1792, p. 204, et seq.

atmosphere of 13°, and there was a diminution of the temperature at the diaphragm of 16°, and at the pelvis of 18°.

These results show that while the normal variations in temperature in the human subject, even when exposed to great climatic changes, are very slight, generally not ranging beyond two degrees, the body may be exposed for a time to excessive heat or cold, and the extreme limits, consistent with the preservation of life, may be reached. As far as has been ascertained by direct experiment, these limits are 83° and 107°; giving a range of about 15° below and 9° above the average standard under normal conditions."

Variations in different Parts of the Body.—It is to be expected that the temperature of the internal organs should be higher and more constant than that of parts, like the axilla or mouth, more or less exposed to loss of heat by evaporation and contact with the cool air; and the differences observed in the blood in certain parts, as in the two sides of the heart, have important bearings, as we shall hereafter show, upon the various theories of animal heat. We shall here simply note the variations observed in the blood in different situations, and confine ourselves chiefly to late observations, which have generally been made with apparatus much more reliable and delicate than was formerly employed.

A great number of experiments have been made upon modifications in temperature accompanying the general change of the blood from arterial to venous; but perhaps the most exact and elaborate are those by M. Claude Bernard. For measuring the temperature in different parts of the vascular system, he used the exceedingly delicate "me

1 HUNTER, Observations on certain Parts of the Animal Economy, London, 1792, p. 114.

We have referred only to observations upon the influence of the surrounding temperature in man and mammals generally. Certain important peculiarities in this regard have been observed in hibernating animals, and in reptiles, fishes, and insects, the consideration of which belongs to comparative physi. ology.