Popular Science Monthly/Volume 2/February 1873/Heat and Life

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THE full solution of the question of heat and life could only be reached by simultaneous concurrence of physics, chemistry, and biology. Ancient physiology treated of animal heat empirically, but was unable to explain its origin. That result required the discoveries of Lavoisier and the more modern researches of thermo-chemistry. After revealing the source of that heat, it was important to show how it was disposed of; and this is taught us by thermo-dynamics. And, in conclusion, only the most delicate physiological experiments could settle the modifications that take place in living beings, when subjected to the influence of a temperature either above or below that they possess normally. Medicine and hygiene already benefit by the indications yielded by pure science upon this subject. It is admitted that the study of the variations of animal heat in diseases is of the highest consequence for their comprehension, and that both diagnosis and prognosis receive unexpected light from it.

An inquiry into calorific phenomena, undertaken from various separate and independent points of view, for the solution of questions that seemed at first sight to have no mutual connection whatever, has thus obtained a body of truths which enter into combination almost of their own accord at the present time, and are found to contain the secret of a great problem in natural philosophy. A minute and extended analysis has thus resulted in an instructive synthesis, which is one of the most signal acquisitions of the experimental method.


All animals have a temperature above that of the gaseous or fluid media in which they live; that is to say, they all possess the faculty of producing heat. Warm-blooded animals maintain an almost constant temperature in all latitudes and all climates. Thus, in polar regions, man, mammals, and birds, mark only one or two degrees less than they do at the tropics. The mean temperature of birds is 41° (cent.), and that of mammals 37°. Those animals called cold-blooded produce heat also, though in a less degree; but their temperature follows the variations of that of the surrounding medium, keeping, however, a temperature a few degrees higher than it. In reptiles, this excess varies from 5° to half a degree; in fish and insects, it is still smaller; and, in the wholly inferior species, it rarely reaches half a degree. In fine, with animals that vary in temperature, the power of resistance to external causes of refrigeration increases in proportion to the perfection of the organization. It is observed, too, that in these-beings vital activity and the force of respiration have a direct relation, to the thermometric state; thus, in a medium of 7°, lizards consume eight times less oxygen than at 23°. With animals of constant temperature, the reverse is the case; the colder it is, the more active is their respiration: a man, for instance, who, in summer, consumes only a fraction over an ounce of oxygen an hour, in winter consumes, more than an ounce and a half. Apart from the state of the surrounding medium, many different circumstances exert a perceptible influence on animal heat, and produce tolerably regular variations in it. The seasons, the times of day, sleep, digestion, mode of nourishment, age, etc., are thus constant modifiers of intensity of combustion/ in breathing; but there are such order and harmony, such foresight, one may say, in the organization of the system, that its temperature continues definitively nearly the same in the physiological state.

The temperature of the human body, at the root of the tongue or under the armpit, is about 37° (cent.); this figure expresses the mean found in taking the temperatures of different points of the body, for there are certain slight variations in this respect in passing from one organ to another. The skin is the coolest part; and the more so the nearer we come to the extremities. The temperature rises, on the contrary, with increasing depth of penetration into the organism; cavities are much warmer than surfaces. The brain is-cooler than the viscera of the trunk, and the cellular tissue cooler than the muscles. Nor does the blood have the same temperature in. all parts of the body. The labors of Davy and Becquerel established the fact that the blood is warmer the nearer to the heart examinations are made. Claude Bernard measured, by methods of equal ingenuity and exactness, the temperature of deep vessels and the cavities of the heart. He showed that blood, in passing out from the kidneys, is warmer than when it enters, and the same is true of blood passing through the liver, lie ascertained, too, that the vital fluid is chilled in going through the lungs, and consequently the temperature of the left cavities of the heart is lower than that of the right, by an average of two-tenths of a degree. The last fact clearly proves that the lungs are not the furnace of animal heat, and that the blood, in the act of revivification, grows cool instead of warm.

Ancient physiologists supposed that life has the power of producing heat; they conceived of a kind of calorific force in organized beings. Galen imagined that heat is innate in the heart—the chemic-physicians attributed it to fermentations, the mechanic-physicians to frictions. Time has dispelled these errors of supposition, and it is proved now that the heat of animals proceeds from chemical reactions taking place in the interior of the system. Lavoisier must be credited with the demonstration of this truth by experiment, As early as 1777 he discovered that air, passing through the lungs, undergoes a decomposition identical with that which takes place in the combustion of coal. Now, in the latter phenomenon, heat is thrown off; "therefore," says Lavoisier, "there must be a like release of heat in the interior of the lugs, during the interval between inspiration and expiration, and it is doubtless this caloric, diffusing itself with the blood throughout the animal economy, which keeps up a constant heat in it. There is, then, a constant relation between the heat of the living being and the quantity of air introduced into the lungs, to be there converted into carbonic acid." Such is the first capital fact brought to light by the creator of modern chemistry; but he did not rest there. He undertook to examine whether the heat theoretically produced in a given time by the formation of a certain amount of carbonic acid, that is to say. by the combustion of a certain quantity of carbon in the organism, is exactly equal to the amount of heat developed by the animal in a corresponding time. This quantity was estimated by the weight of ice melted by the animal placed in a calorimeter. Lavoisier ascertained in this way that such equality does not exist, nor was he long surprised at this, for he soon discovered that, of 100 parts of atmospheric oxygen absorbed, only 81 are thrown off by the breath in the form of carbonic acid. He concluded then, from this observation, that the phenomenon is not a simple one, that a part of the oxygen (nine per cent.) is consumed in burning hydrogen, to form the vapor of water contained in the expired air. Animal heat must be accounted for, then, by a double combustion: of carbon first, then of hydrogen; and respiration regarded as throwing off out of the animal carbonic acid and vapor of water.

Lavoisier's experiments have been repeated and varied, and his conclusions' discussed in many ways for nearly a hundred years. Several experimenters have corrected or perfected some points, but the general doctrine has not been shaken by the recognition of its secondary and very subtle difficulties, several of which still puzzle physiologists. It is, indeed, undeniable that the greater part of the reactions which occur in the system, with the production of heat, do bring out, as a result, the exhalation of watery vapor and carbonic acid from the lungs; but these two gases cannot arise from a direct combustion of hydrogen and carbon, because the system does not contain such substances in a free state. They represent really only the close of a succession of transformations, often distinct from combustions, properly so called. On the other hand, these are not the only residue of the chemical operations performed in the vital furnace. Besides the water and carbonic acid thrown off by animals in breathing, which are like the smoke of this elaboration of nutrition, they excrete by other channels certain principles which are, as it were, the scoriæ. Now, these principles of disassimilation, among which should be noted urea, uric acid, creatine, cholesterine, etc., could not be results of pure combustion, and they denote that the circulating current is the seat of extremely manifold reactions, the laws of which we are only beginning to gain a glimpse of.

The latest advances of chemistry allow us, indeed, to follow the linked sequence of the gradual transformations of nutritive substances into the cycle of vital operations. It is well, at the outset, to fix exactly the seat of these phenomena. They take place in all the points of the system traversed by the capillary vessels. The glands, the muscles, the viscera, in brief, all the organs, are in a state of constant burning—they are every instant receiving oxygen, which brings about alterations of various kinds in the depth of their substance. In a word, every organ breathes at all its points at once, and breathes in its special way. Certain physiologists of the present day are wrong in localizing the phenomena of breathing in the capillary vessels. They are merely the channels of transfer for oxygen, which, by exosmosis, penetrates their thin walls, and then effects, by direct contact with the smallest particles of the organized mass, the chemical action which keeps up the fire of life. It is easy to prove this by placing any tissue, lately detached from the body, in an oxygenated medium. We remark in this case an escape of carbonic acid, together with a development of heat, and this possibility of breathing outside the system proves clearly that such act can be accurately compared, as Lavoisier thought, to the combustion of any substance. The only difference is with regard to intensity. While a candle or a bit of wood burns rapidly, with a flame, the combustible materials of organic pulp unite with oxygen in a more slow and quiet manner, less violently and manifestly.

The blood, which flows and reflows incessantly in the most slender vessels of our bodies, and charges itself full with oxygen every time the chest heaves, is composed of very various substances. It contains mineral salts, such as chlorures, sulphates, phosphates of potassium, soda, lime, magnesia, coloring-matters, fatty particles, neutral substances of the nature of starch, and nitrogenized products, such as albumen and fibrin. The salts undergo slight changes in the torrent of circulation; they are eliminated by the chief emunctories. The neutral matters of the nature of starch are converted into glycogene and fat. The fatty particles undergo in the blood only such oxidizations as produce certain derivatives of the same order. And, last, the nitrogenized products are made over into fibrin, musculin, ossëin, pepsin, pancreatin, compounds all differing very slightly. It is the first portion of the chemical process which is effected in the principal fluid of the body. All these materials, elaborated at different points of the circulating current, and designed to be assimilated, are destroyed in the very organs in which they had been fixed. The glycogene is transformed into sugar, which is burned, yielding water and carbonic acid; the fatty acids are partly eliminated by the skin, and partly burned. As to the plastic matters which form the web of the tissues, we know little about the chemical relation which connects these with their products of destruction—urea, creatine, cholesterine, uric acid, and xanthine. Such is a rapid sketch of the principal chemical phenomena which, taking place throughout the entire system, kindle everywhere an evolution of more or less intense heat. There is no central organ, then, for feeding the vital fire—every anatomical element performs its share; and, if a nearly uniform temperature exists throughout the body, it is because the blood diffuses heat regularly into the various parts it bathes.

Now, how can the amount of heat to which these reactions may give rise be ascertained? Lavoisier arrived at it in a very simple manner. After comparing the oxygen absorbed by the animal with the carbonic acid and watery vapor thrown off, he deduced the weight of the carbon and hydrogen burned, by assuming that the formation of carbonic acid and of water produces in the system the same amount of heat that it would produce if taking place by means of free carbon and hydrogen. This is very nearly the result he obtained: A man weighing 132 pounds burns in 24 hours, at the average temperature of Paris, very nearly 11 ounces of carbon, and 1114 of an ounce of hydrogen, and thus develops 3,297 heat units. During the same period he loses through his lungs and skin 2 34 pounds of watery vapor, which take from him 697 heat-units. There remain, then, nearly 2,600 heat-units to account for. Other analogous estimates have been made, and physiologists have deduced from them the conclusion that a man of average weight produces in our climate 3,250 heat-units every day; that is to say, a sufficient amount of heat to raise seven gallons of water to the boiling-point. These figures, though approximations, give a sufficiently clear notion of the power of the animal economy to generate heat.

Of late years, the question has been taken up again with more exactness, thanks to the views of a new science called "heat-chemistry," which occupies itself with chemical phenomena in their relations to heat. Heat-chemistry, by the aid of very delicate apparatus for measuring heat, ascertains the number of heat-units developed or absorbed by bodies entering into combination, beginning with the noted experiments of Favre and Silbermann. Berthelot, who had made profound researches into this subject, reduces the sources of animal heat to live varieties of transformation: first, the effects resulting from the fixation of oxygen with different organic principles; then the production of carbonic acid by oxidization; then the production of water; in the fourth place, the formation of carbonic acid by decomposition; and, last, hydrations and dehydrations. The learned chemist attempted to show how the numbers obtained in the study of the heat of combustion of the different organic acids, alcohols, etc., might be applied to the compounds burned in the animal organism; but, while admitting the theoretic verity of the analogies he establishes, we cannot refrain from re marking that their practical verification is exceedingly delicate and difficult. How can we measure, at any one point of the system, the heat produced by a fleeting reaction occurring in the inmost depths of a tissue that must be lacerated to be examined?

If thermo-chemistry seems not to throw much light on physiology on this side, it reveals to it on another sources of heat that had hitherto escaped notice. Berthelot shows that carbonic acid in the system is not always formed by oxidization of carbon, but sometimes proceeds from decomposition absorbing heat. We know that alimentary substances are reducible to three fundamental types—fats, hydrates of carbon (sugars, fecula, starch), and the albuminoids. Now, the fats, in decomposing and combining with water, as it occurs under the influence of the pancreatic juice, evolve heat; and so it is with the hydrates of carbon, independent of any oxidization. And albuminous substances, too, produce very clear calorific phenomena, when their combination with water takes place with its consequent various decompositions. These facts, noted by Berthelot, must have their place in the minute and exact calculation of animal heat, which it is perhaps as yet too early to undertake. At any rate, this heat originates in the totality of those chemical transformations which are going on unceasingly in the depths of the animal organs, and are bringing about the continual renovation of the whole organized substance; in other words, nutrition; but why that nutrition—why that perpetual production of heat in the living machine?

We have now the means of answering this question, which involves the secret of one of Nature's most beautiful arrangements. The heat produced by animals is the source of all their movements; in other words, the mechanical labor they perform is a mere simple transformation of the activity of heat they develop. They do not create motive force by any voluntary operation, which would be one of the prerogatives of life; they draw it from the calorific energy stored up in the organs traversed by the blood. Besides, there is a fixed relation between the quantity of heat that disappears and the mechanical labor that appears. Yet, it is to be remarked that, if all motion by living beings is a transformation of animal heat, that heat is not wholly transformed into motion. It is partly wasted by transpiration through the skin, by touch, and especially by radiation; it is used in keeping up to a constant point the temperature of the animal, subjected to many causes of refrigeration.

The mechanical labor performed by an animal is very complex. Independently of visible muscular motions, there are all the changes of place in the interior organs, the continual passage of the blood, the contractions and dilatations of a great number of parts. Now, these actions are only possible in so far as the phenomena of breathing are taking place in the active region. Prevent arterial blood from coming to the muscle, that is to say, prevent combustion taking place, and consequent heat evolving in it, and, although the structure of the organ suffers no harm, it loses its contractile power. Mere compression of the supplying artery of the muscle, so as to check the flow of blood in it, causes the organ to grow cool, and lose its power. The labors of Hirn and Béclard have clearly established the relations between heat and muscular motion. Later experiments by Onimus have fixed, with equal precision, the efficiency of heat through the movements of circulation.

We have said that the heat-producing power of aliments will be the more considerable in proportion as they contain a greater quantity of elements that need a large supply of oxygen for their combustion. Therefore, meat and fats repair the losses of the system much more speedily than vegetable substances. The latter are suitable for the inhabitants of warm countries who do not require to produce heat, which the atmosphere supplies them with abundantly. The inhabitants of cold regions, on the contrary, whose accessions of heat ought to be as continual as energetic, are urged by instinct to use meats and fats, which throw out great heat in their combustion. For instance, it is a physiological necessity that the Lapps should feed on the oil of cetacea, as it is a necessity for men of the tropics to consume only very light food. The activity of respiratory combustion and the kind of alimentation thus vary with climate, so that there is always a certain proportion maintained between the thermic state of the surrounding medium and that of the animal furnace. In like manner, in the same climate, persons who perform great mechanical labor must eat more than those who put forth but little movement. This fact, long ago observed, has received of late the clearest and surest demonstration. Yet, perhaps, it is not kept sufficiently in view in the management of public alimentation. Many examples prove the benefit that industry would derive from increasing, in all possible ways, the amount of meat used in laborers' meals. Quite recently, at a manufacturing establishment of the Tarn, M. Talabot has improved the strength and sanitary condition of his workmen by giving them meat in abundance. Under the influence of a diet almost wholly vegetable, each laborer lost on an average fifteen days work a year through fatigue or sickness. As soon as the use of meat was adopted, the average loss for each man per year was not over three days. Often enough, it must be owned, alcohol is only the workman's means of remedying the want of heat-producing elements in his food; a deceitful remedy, which buoys up the system for a time, only to sap it afterward with alarming subtlety. One of the best preventives of the abuse of alcohol would certainly be the lessening of the cost of meat.

From the point of view of the relation between heat and motion, the living being may thus be compared to an inanimate motor, as a steam-engine. In both cases, heat is engendered by combustion, and transformed into mechanical work by a system of organs more or less complex. In both cases it is at first in a state of tension, and yields motion in proportion as it is demanded for the performance of certain work. Only the living being is the far more perfect machine. While the best-made steam-engines only utilize 12100 of the disposable force, the muscular system of man, according to Hirn, accounts for 18100. On the other hand, the animated motor has this peculiarity, that its sources of heat and its mechanical arrangements are intimately commingled, that its heat is produced by organs in motion with a sort of general diffusion, and that the machine itself becomes in turn transformed within itself into heat; an incredible complication, of which science has succeeded in unravelling the simple laws only by dint of the united efforts and resources of physics, chemistry, and biology.

As some physiologists hold, heat must not only be the source of motion in the system, but must also undergo transformation into nervous activity. The functional action of the brain must be a labor, exactly like that of the biceps. Mind itself should be regarded as engendered by heat. Late experiments by Valentin, Lombard, Byasson, and especially Schiff, would seem to prove, it is thought, that there is a proportional and constant relation between the energy of nervous functions and the heat of the parts in which they are effected. Gavarret boldly concludes, from his researches, that heat has the same relations to the nervous system that it has to the muscular system; only, in the case of the muscles, the force produced exhibits itself externally by visible phenomena, while in that of the nerves it is exhausted internally in profound molecular action, which eludes any exact measurement. A given sum of heat developed in the system would thus be on one side a mechanical equivalent, and on the other a psychological equivalent. Gavarret, who is a cautious savant and true to experimental methods, doubtless does not go so far as to maintain that thought and feeling can be estimated in heat-units; he even asserts that there is no common measure between intelligence and heat; but less timid physiologists are not wanting who reduce every kind of vital manifestation to the strict laws of thermo-dynamics. A few succinct remarks may perhaps show that such physiologists err.

A comparison between the muscular and the nervous systems from the point of view of their connection with heat is a bold one for many reasons. Between nerve and muscle there exists this enormous difference—that the former is endowed with a spontaneity denied to the latter. Muscular fibre never contracts of its own accord; it needs a stimulus—its energy is borrowed. The nerve-cell, on the contrary, has in itself an ever-present, never-exhausted power of action, of which the energy is its peculiar property. Both evidently derive the principle of the activity that marks them from the same external and internal media; but, while the muscle, a mechanical organ, is limited to the obedient transformation of the force assigned to it, under the form of heat, into a measurable amount of work, the nerve, a vital organ, remains impenetrable and inaccessible to our calculations, and exerts its characteristic and sovereign powers in its own way, through a series of operations that escape all estimates of their force and heat. On the part of the muscular system, every thing can be measured; on the part of the nervous system, nothing. Impressions, sensations, affections, thoughts, desires, pleasures, and pains, make up a world withdrawn from the common conditions of determination. That superior force which, ruling all the highest animal activities, decides, suspends, checks, and governs the very transformation of heat into movement; which, asserting its independence within us, call it by what oldest name we may—soul, will, or freedom—remains the most undeniable, though the most mysterious certainty of our consciousness, this force protests against the degradation of cerebral life to mechanism. Such is the conviction, moreover, of Claude Bernard and of Helmholtz.


Independently of the slight and usual variations that heat may present in the same species, and those it exhibits in passing from one zoological group to another, we may consider the changes it undergoes in the same individual, influenced by the various disturbances of the system. Although it remains almost insensible to modifications of the surrounding temperature, it is not the same when the complete equilibrium of the organs is affected. The concord between the different parts of the organism and the functions they discharge is so perfect that the least trouble is reflected among them, and sends disorder everywhere. The nervous system, charged with keeping up harmonious communication between all points of the living being, first takes note of the change befalling, and transmits its abnormal impression into all quarters. It is not the generator, but it is the regulator, of animal heat; that is to say, it directs and in a manner oversees its production and diffusion according to the varying needs of the system. Every lesion or affection of this system reacts on the physiological processes, and particularly on the evolution of heat. By cutting the filament of the great sympathetic nerve on only one side of a rabbit's neck, Claude Bernard produced an elevation of temperature of several degrees on that side. The blood flows toward the point where the action of the nervous system is suspended under any influence whatever, bringing with it an increase of heating force. At a point where the reverse occurs, the vessels contract, and the temperature falls.

Imperfect nutrition and fasting act on the animal heat, but not directly. The organism keeps up to its normal degree of temperature till it has exhausted its reserved store of combustible substances. Then it cools slowly down to a much lower degree. Thus, a rabbit, starved by Chassat, showed the first day a warmth of 38° 4' (cent.); two days before its death, 38° 1'; the evening before, 37° 5'; and at the moment of death, 27°. By placing it in a warm medium the moment it was about to die, the apparent activity of its functions was restored for a little while; but the renewal is of brief duration: the anatomical elements have absolutely lost their spring.

The hand of an invalid, suffering from inflammation of the chest, or from an attack of fever, is burning; that of one affected by serious asthma, or by emphysema, is as cold to the touch as marble. This is because animal heat varies greatly in different pathological states. Sometimes it rises, sometimes it falls; and the morbid influence is scarcely ever compatible with the body's degree of normal temperature. In Hippocrates's time, when examination of the pulse was not yet practised, the increase of temperature was the only element in the commonest of maladies, fever. Galen defines it quite simply as an extraordinary heat (calor præternaturalis substantia febrium). The ancients did not err. It has been admitted and proved in our days, that the elevation of the animal heat is just the specific character of the febrile condition. On the one hand, there is never any fever when the temperature continues at the normal degree; on the other, the rapidity of the pulse may reach the utmost limits, without any febrile movement, as is seen in hysteria. Whenever the bodily heat exceeds 38° (cent.), it may be affirmed that there is fever; and, whenever it falls below 36°, there is what is termed algidity. So that the normal heat varies within the narrow range of scarcely two degrees. Beyond these limits, that is, above 38° and below 36°, the temperature points out some morbid trouble. In common intermittent fever, it rises two or three hours before the chill, reaches a maximum at the close of it, and then falls. Acute and decided inflammations, such as pneumonia, pleurisy, bronchitis, erysipelas, etc., are marked by a period of thirty-six hours, or about two days, during which the heat rises slowly to 41°. Toward the third day, this heat decreases, ready to reappear in exacerbations of from half a degree to a degree, during three or seven days, at the end of which time the disorder has run its course. When the temperature gradually rises after the third day, a fatal result may he expected. Persistent heat in that case is the precursor of death. Eruptive fevers, like small-pox, scarlatina, and measles, present very important phenomena of heat. In these heat begins with the attack of the malady, and increases till the cutaneous eruption occurs. It keeps up at a maximum, which reaches 42½° (in scarlatina), till the eruption is complete, then it begins a declining course, variable with the phases of the eruption, which finishes either with scaling off as in scarlatina, or suppuration as in small-pox. And the temperature rises also in several surgical affections, bringing on a more or less inflamed and feverish condition. This is observed in wounds, and generally in every kind of traumatism, in tetanus, aneurisms, etc. In the case of strangulated hernia and of burns, and in most cases of poisoning, on the other hand, it declines in a remarkable way.

Very plainly this rising and falling of animal warmth in diseases can only be attributed to a corresponding state occurring in the energy of respiratory combustion. We do not yet exactly know the cause of these variations; that is, the mechanism by which the morbid influences stimulate or check the active production of heat. Some physicians see in it the effect of fermentations occasioned in the blood by certain microscopic beings, such as bacteria and vibriones, which may perhaps be supposed to be the fact in most febrile maladies. Others assume that, in local inflammations, it is the inflamed organ which communicates heat to the whole body, as a furnace does in a confined space. To others the disturbance seems rather to have a nervous origin, since the nerves, as we have seen, are the regulators of thermic action.

The use of the thermometer is the only exact method of measuring the temperature in diseases. Swammerdam, in the middle of the seventeenth century, seems to have been the first to have the idea of it. De Haën and Hunter, in the last century, used it in their medical practice, but its employment at the sick-bed has really only come into importance in our own day, thanks to the labors of Bouilland, Gavarret, Roger, Hirtz, and Charcot, in France; Bärensprung, Traube, and especially Wunderlich, in Germany. These physicians were not content with proving that the temperature in illness rises several degrees; they followed the variations of the thermometer day by day, hour by hour, in the different phases of the pathologic movements. They discovered that the curves of these oscillations furnish constant types for each disease, which are modified in a regular manner, according as the disease has been left to itself or treated by one or another medicine. By the study of these pathologic curves of heat the course of diseases may be followed, and valuable indications noted in diagnosis or prognosis. In hæmorrhage of the brain, for instance, the temperature falls suddenly to 36° or even 35°, while, in the attack that takes the form of apoplexy, it continues nearly at 38°. These two disorders, quite distinct in their treatment and cure, yet often give rise to a confusion, which the thermometer will hereafter allow to be avoided. Granular meningitis is distinguished from simple meningitis by the same method; in the former the temperature does not rise, notwithstanding the extreme rapidity of the pulse, but in the latter the thermometer marks 40° or 41°.

In every case we see what advantage practical medicine may gain from the physical sciences, what precision and safety it attains by the employment of its means, in proportion to the morbid symptoms. We may add that the future of diagnosis is to be found partly here. By the banishment from medical examination of the often-uncertain judgment of the senses, by substituting as far as possible for personal and arbitrary conclusions, as well as for the feeling, always more or less confused, of the physician, the plain and impassive indications of an exact instrument, we do away with the causes that impede the methodical interpretation of the evil in question. Moreover, these instruments often reveal peculiarities that elude direct observation. They repair the omissions, correct the mistakes, guide the activity, multiply the power of our imperfect senses. From this point of view, the study, by the thermometer, of variations of animal heat in diseases, thermometric clinic, as it is called, is one of the most indisputable onward steps in medicine.


After having seen how internal heat is produced in animals, how it expends itself in them, and undergoes change into mechanical work, in fine, what spontaneous or occasional changes it passes through in them, we should study the influence of external heat on the same animals, and the various phenomena resulting from the rise or fall of temperature in the medium they live in. Quite recent researches have thrown light on these questions. Boerhaave had made some experiments, not sufficiently exact, however, on the subject. Berger and Delaroche, at the beginning of this century, undertook new ones, which gained celebrity in the schools of physiology. They placed animals in stoves containing air heated to different degrees of temperature, and noted the effects produced on life by thermic influences. The conclusion from their researches was, that all animals have the power of resisting heat for a certain length of time, and that the duration of resistance varies with the species. Small animals yield after a moderate time to a temperature of 45° to 50° (cent.). Larger ones endure heat better. Cold-blooded animals and the larva? of insects resist more energetically than warm-blooded animals; but the reverse is the case with fully-developed insects.

Delaroche and Berger studied the human subject, too, from the same point of view, and ascertained that the effect produced varies with individuals. Thus from 49° to 58° the stove grew insupportable to Delaroche himself, who became ill from the experiment, while Berger was scarcely fatigued by it. On the other hand, Berger could remain only seven minutes in a medium heated to 87°, while Blagden stayed 12 minutes in it. In tropical countries the heat often rises during the day above 40° without troubling the natives. At the Cape of Good Hope the thermometer marks 43°. Yet sometimes such a heat is murderous. It is related, among other cases, that in the month of June, 1738, in the streets of Charleston, several persons died under the influence of 41°. In Africa our soldiers are often known to be attacked with madness and to die in making a long march, under the rays of a burning sun, but here the influence of light is combined with that of heat. Duhamel mentions the account of several servant-girls of a baker, who could remain without any inconvenience at all for nearly ten minutes in an oven heated to the necessary degree for baking bread. The experiment has since been repeated. There is nothing contradictory in these facts. An animal can endure for some time a temperature much higher than its own, because the very profuse transpiration which occurs in such a case prevents the heating of the organs; yet, as we shall see, so soon as the internal heat really rises a few degrees above the normal figure, life is no longer possible.

The study of these phenomena had scarcely been carried further, when in 1842 Claude Bernard devoted to it certain researches, which he resumed and finished last year, and of which he has just published the results. This physiologist used a pine box, divided into two parts by a grating, on which the animal subjected to the experiment is placed. The box rests on a cast-iron plate, and the whole is arranged on a furnace which warms the air of the apparatus more or less. A window, placed in the side of the box, allows the head of the animal to be fixed outside of it at will. Examining animals, subjected under these conditions to the influence of air more or less warm, Bernard verified the first observations of Berger and Delaroche, and made new and more important ones. Boerhaave had given as the cause of death the application of hot air to the lungs, preventing the cooling of the blood. Bernard showed by experiments that hot air, acting on the skin, creates a rise of temperature more rapidly fatal than when this fluid is merely introduced into the pulmonary vessels. He proved also that, when the hot air is damp, the phenomena take a more rapid course, and death occurs much more quickly and at a lower temperature than in dry air. This difference must result from the fact that dampness promotes a rise in temperature.

When an animal is subjected to the poisoning effects of heat, it presents a series of uniform and characteristic phenomena. It is at first a little disturbed, then panting, its movements of respiration and circulation accelerate, it grows slowly hotter through the circulation, which, carrying the blood continually from the surface to the centre, bears heat also along with it, then at a given moment it falls into convulsions, the beating of its heart ceases, and it dies uttering a cry. By means of the thermometer it is noted that the temperature of the animal, in every case, is higher by four or five degrees (cent.) than the figure which represents the normal warmth. Thus at first the animal is excited, its functions seem to be performed with fresh vigor, very much as, in the first rays of April sunshine, the pulsations of life in all beings become more rapid; but this stimulus is only fleeting, and soon, when it reaches a certain degree, this heat gives place to the cold of death. Bernard carefully examined animals dying under these conditions, and the first phenomenon that struck him was the rapidity with which corpse-like rigidity came on. The heart grew suddenly insensible to any stimulus; effused spots appeared at several points on the skin. The heat fixed in coagulation the soft matter that composes the muscular fibres. These had the look of being struck with lightning. On the other hand, the arterial blood of the animal grew black, ill-supplied with oxygen, overloaded with carbonic acid, and assumed the look of venous blood. Yet in this state the blood has not lost its physiological properties, and under the influence of a new supply of oxygen can regain its normal state, and grow ruddy again. The heat, provided the degree be not too elevated, only promotes activity in sanguine combustion, without changing the blood. Nor does the nervous system either appear to suffer much. The element most deeply affected is muscle; heat is a poison of the muscular system, like sulpho-cyanuret of potassium, and the upas-antiar. It is the loss of the vital properties of this system, which, by bringing about rigidity of the muscles, then the stoppage of circulation, and consequently of respiration, is a necessary cause of death. This destruction of the contractile muscular fibre occurs toward 37° or 39° in cold-blooded animals, toward 43° or 44° in mammals, toward 46° or 48° in birds, that is, speaking generally, at a temperature five or six degrees higher than the natural temperature of the animal. Bernard calls attention to the fact that in no case is it allowable to suppose that life opposes a kind of resistance to the excessive heating; on the contrary, vital movement tends to quicken it, and that may be readily understood. The internal heat produced by the animal unites with the acquired heat, and the renewal of the blood, which is the condition of the heating, then occurs with much greater activity. Let us add that quite lately Demarquay applied this toxic action of heat on the muscles in the happiest manner, and without suspecting it. He cured patients suffering from those frightful muscular contractions which characterize tetanus, by subjecting them to the influence of caloric, and making them take very hot air-baths. The rise of temperature in the tetanized muscles was sufficient to modify them, and restore them to a healthy state. Here the poison worked a cure.

Such are the effects on animals of the elevation of temperature. Let us now see what becomes of them when immersed in cold media. Some curious facts with respect to the freezing of certain animals have long been known. During his voyage to Iceland, in 1828 or 1829, Gaimard, having exposed in the open air a box filled with earth in which toads were put, opening it after a certain time, found the reptiles frozen, hard and brittle; but they could be restored to life when put in warm water. Many ancient authors cite similar cases, and we can almost bring ourselves to understand how a great English physiologist might for a moment have given them the whimsical interpretation that he did. John Hunter fancied it might be possible to prolong life indefinitely by placing a man in a very cold climate, and there subjecting him to periodical freezing. The man, he said, would perhaps live a thousand years, if, at the end of every ten years, he were frozen for a hundred, then thawed out at the end of the term for ten years more, and so continuously. "Like all inventors," Hunter adds, "I expected to make my fortune by this scheme, but an experiment completely undeceived me." Putting carp into a freezing mixture, he observed, in fact, that, after being entirely frozen, they were dead, past recovery. The case is the same with all other animals, as the late and very remarkable experiments of F. A. Pouchet have proved.

The influence of cold on organized beings varies, according as we regard superior animals or the inferior species. In general, it may be said that it requires a very low surrounding temperature to chill many animals, because the vital heat they develop resists the process with energy. Yet the mammals of arctic regions, in spite of their thick coat of fur, can only brave the temperature of the pole (sometimes equal to 40° (cent.) below zero, the freezing point of mercury) by living under the snow where they make their lair. The Esquimaux, too, dig huts in it, where they pass their wretched days. When the organism can neither react nor protect itself against temperatures so low, death by freezing quickly overtakes it. The body is stiffened, and retains afterward a state of remarkable incorruptibility. Every one knows the story of the antediluvian mammoths, discovered in the polar ice, where they had been buried, as fresh as animals just dead. While heat destroys the tissues, cold preserves them.

Through what mechanical means does cold become mortal? It seems to act on the nervous system. Travellers relate that in polar regions an unconquerable disposition to sleep overcomes men attacked by very low temperatures. On the icy shores of Terra del Fuego, Solander said to his companions, "Whoever sits down falls asleep, and whoever falls asleep never wakes again." This inclination is so overpowering that many of his attendants gave up to it, and he himself sank down for a moment on the snow. It is said that, during the winter of 1700, two thousand soldiers of Charles XII.'s army perished in the sleep to which they surrendered, under the influence of cold. Its action on the nervous centres, however, is only secondary and consequent on another phenomenon, studied by Pouchet, which reveals this as the secret of death. When the temperature of the interior of the body sinks to 10° or 12° below zero (cent.) the cold freezes the blood more or less, thoroughly disorganizing its globules, and it is this alteration which, either at once or when the blood becomes fluid again, destroys all the vital functions. Larrey relates the case of Sureau, chief apothecary of the French army in Russia, who, when chilled to freezing by a painful march in the snow, did not die until the moment they began to restore warmth. Experiments on animals show that they keep themselves alive as long as they are maintained in a state of half congelation, and die whenever their temperature and circulation are so far restored as to permit the blood-globules, disorganized by cold, to be diffused throughout the vessels. Death occurs, therefore, whenever the quantity of these globules is sufficient to produce a considerable disturbance in the system, that is, whenever the frozen part is at all extensive. An animal entirely frozen, and consequently containing in its congealed blood no globules but those unfit for life, is dead, without possibility of resurrection. Thawing it only restores a soft flaccid, discolored body, with opaque eyes. If freezing only attacks a limb, it becomes gangrenous, and is destroyed. Pouchet deduced from these examinations a judicious, practical conclusion. If it is true that, in cases of partial freezing, the death of the individual is due to the disorganized globules reentering the circulation and corrupting the blood, it is plain that, the more sudden the invasion of these globules is, the more rapidly death will supervene. It follows, that, by resisting this invasion, by means of ligatures, or extremely slow thawing, we might succeed in preventing the poisoning. The diseased globules which, pouring in a flood into the heart and lungs, would imperil life by the sudden alteration of the blood, will apparently disturb it merely in an unimportant way, if they are dropped into the blood by slow degrees.

Thus the late researches of experimental physiology explain for us the effects of heat and cold, regarded as toxic agents. The former is a poison of the muscular fibre, the latter a poison of the blood-globules. The case is the same with heat as with the other elements of the cosmic medium, in which the animated being lives. It enfolds the most contradictory powers, like the tender flower, spoken of by Friar Lawrence, in "Romeo and Juliet," from which may be distilled both safety and danger. It can by turns support health, heal disease, or inflict death.

Man is, then, the weak plaything of all those silent forces that surround and press upon him. In vain he enslaves them; he cannot escape the inflexible laws that subject the equilibrium of life to that of the lowest physico-chemical conditions. He has at least the consolation of knowing these laws, and guiding his existence so as to soften their severity as far as possible. When Nature crushes him, she is unconscious of it, unconscious of herself: man, so small, is greater than these blind greatnesses, because his peculiar greatness is consciousness. The subject we have been studying is a grand proof of this; but its full, imposing interest would not be understood were we to end without giving the answer to the last question it suggests. Whence comes this heat developed by chemical phenomena in the living system? It comes from aliments which, in the last resort, are all drawn from plants, and they have borrowed it from the sun. When the vegetables, whose combustion takes place within the animal, there throw off a certain amount of potential energy, as heat, they do but transmit to it the force which the sun has supplied them with. It is, then, a portion of solar radiation, stored up at first by the plant, which the animal makes disposable and converts to use, whether for resisting cold or for securing the regular play of his motive functions. Thus we may say, with exact truth, the sun is the inexhaustible source, as it is the perpetual spring of life. From this point of view, science confirms the intuitions of oldest date, and man's poetic dreams in the childhood of the race. Reason completes the instructions of its long experience by harmonious agreement with the simple and natural sentiment felt by the first of men, when for the first time they looked on the splendor of day.—Revue des Deux Mondes.