radiation and conduction is facilitated. The influences of external temperature are even more potent in their effects than those of internal temperature, chiefly because of the much wider range of temperature to which the organism is subjected. Bodily temperature under ordinary circumstances does not vary more than 1° to 2° C. during the twenty-four hours, but external temperature may vary as much as 40° C., or more. External heat tends by exciting cutaneous nerves to reflexly diminish heat-production and thus indirectly diminish heat-dissipation; but this is to some extent antagonized by a dilatation of the blood-vessels of the skin, an excitation of respiration, and increase in the quantity of sweat, all of which tend to increase heat-dissipation, but which are unable to balance the opposite effects. Cold, on the other hand, accelerates both heat-dissipation and heat-production. The loss of heat from the body is increased because of the greater difference in the temperatures of the body and the surroundings; but, on the other hand, the cutaneous vessels are contracted, the circulation is less active, and the quantity of sweat is lessened, all of which are unfavorable to heat-dissipation. Yet while these latter alterations tend to diminish heat-loss, they are not sufficient to compensate for the increased expenditure by radiation and for the greater loss by respiration.

Circumstances which increase heat-production above the normal tend indirectly to increase heat-dissipation. Other things being equal, the greater the quantity of heat produced the greater the heat-dissipation, unless the bodily temperature be below the normal, in which case heat-production may be increased and yet heat-dissipation remain unaffected, or even be diminished, until sufficient heat has accumulated to bring the bodily temperature up to the mean standard.

The larger the surface of the body exposed to the normally cooler surroundings, the greater is the loss of heat. The larger the animal the greater the body-surface, and therefore the greater is heat-dissipation; but in proportion to body-weight smaller animals have larger body-surfaces, therefore heat-dissipation is relatively greater, although not absolutely so (see p. 430). The area of body-surface involved in heat-dissipation is affected by the position of the individual. Thus, by bringing the arms and legs in contact with the body the total surface exposed is lessened. On the other hand, animals which habitually have their legs in apposition with the trunk have their radiating surfaces increased when their legs are extended. For instance, in the rabbit extension of the legs enormously increases heat-dissipation, so that the bodily temperature is profoundly affected.

The condition of the vascular system exercises an important influence. Circumstances that excite the circulation affect heat-dissipation both directly and indirectly. Thus, heat-loss is directly increased by the excitation of the respiratory movements, by the increased secretion of sweat, and by the larger supply and increased temperature of the blood to the skin. Increased activity of the circulation also increases heat-production, and thus indirectly affects heatdissipation. Opposite conditions, of course, lessen heat-dissipation.

The larger the quantity of air respired, other things being equal, the larger

the loss of heat by this channel. The heat-loss occurs both in warming the air and in the evaporation of water from the lungs, so that the cooler and drier the air inspired the larger relatively is the heat-loss. The importance of respiration as a heat-dissipating factor is illustrated by the fact that about 10.7 per cent. of the total heat-dissipation occurs in this way (see p. 477).

Next in importance to radiation is the amount of water evaporated from the skin. Each gram of water requires 582 calories to vaporize it, and it is estimated (p. 477) that 364,120 calories are dissipated in this way, or 14.5 per cent. of the total heat-dissipation. An increase of external temperature increases the irritability of the sudoriparous glands, thus favoring secretion and heat-dissipation. The value of sweat, however, as a means of carrying off heat, is materially affected by the temperature of the air as well as by the amount of moisture present. The higher the temperature and the less the moisture the more rapidly evaporation occurs, and consequently the greater the loss of heat; when air is moist and of high temperature evaporation takes place relatively slowly, if at all. Therefore, individuals can withstand subjection to dry air of a higher temperature and for a longer period than when the atmosphere is moist. In the former case sweat is rapidly secreted and vaporized, and thus a marked rise of internal temperature may be prevented. James found that a vapor bath at 44.5° C. (112° F.) was insufferable, while dry air at 80° C. (176° F.) caused little inconvenience. When air is of high temperature and loaded with moisture we say that it is "sultry," but dry air of the same temperature is not unpleasant.

Muscular activity increases heat-production, excites the circulation and respiration, and increases the secretion of sweat, all of which directly or indirectly increase heat-dissipation.

The surface of the body as a radiating surface cannot be regarded in the same light as an indifferent, inanimate surface, such as metal or wood. The coefficient of radiation (the quantity of heat emitted during a unit of time at a standard temperature from a given area) in an inanimate body remains fixed, because the surface itself is virtually unchangeable; but the coefficient for the living organism is subject to material alterations. These alterations depend chiefly (1) upon the actions of the pilo-motor mechanism whereby the relation of the natural covering (hair or feathers in the lower animals) of the body to the skin is effected; (2) upon changes in the conductivity of the skin owing to variations of the blood-supply; (3) upon the varying thickness of the skin in different species, in different individuals, and in different parts of the body; (4) upon the temperature of the surroundings; (5) upon the extent of the body-surface exposed; (6) upon the character of the clothing. When the arrector pili muscles contract the skin is made tense and the cutaneous bloodvessels are pressed upon and rendered anæmic, thus lessening the quantity of fluid in the skin and as a consequence lowering the coefficient of dissipation; moreover, in animals whose natural covering is fur or feathers, these fibres cause an erection of one or the other, as the case may be, and in this way affect the radiating coefficient. The coefficient is enormously increased by

removing the natural covering, such as the fur of the rabbit, under which circumstances, even though the animal be subjected to a relatively high external temperature, heat-dissipation is so enormously increased that death ensues within two or three days. When one side of the body of a horse was shaved and the animal subjected to an atmosphere having a temperature of 0° C., the temperature of the skin of the shaven side fell 8° in forty minutes, while the temperature of the unshaven side fell only 0.5°.

The coefficient is diminished where there is excessive sebaceous secretion, and where grease is artificially applied, and by an accumulation of subcutaneous fat; it is increased by wetting the skin, as by sweat or bathing; and it is affected by many other circumstances.

Through the operations of the nervous system heat-dissipation may be affected directly or indirectly by action upon the heat-dissipating and heatproducing processes-circulation, respiration, sudorific and sebaceous glands, and arrector pili muscles.

There are many drugs which directly or indirectly affect heat-dissipation. Drugs which cause dilatation of the cutaneous vessels tend to increase heatdissipation; conversely, those which cause contraction of the blood-vessels hinder dissipation. Diaphoretics increase heat-loss essentially by increasing the amount of sweat. Respiratory excitants increase the loss of heat by means of the increased volume of air respired. Drugs which increase heat-production tend to indirectly increase heat-dissipation.

All pathological states which affect heat-production tend to similarly disturb heat-dissipation. Conditions of malnutrition favor heat-dissipation by causing a loss of subcutaneous fat, but this is to a greater or less extent compensated for by the enfeeblement of the circulation, respiration, and metabolic processes in general. In fever, both heat-production and heat-dissipation are generally increased, the former being affected more than the latter, so that the bodily temperature rises. In some forms of fever the rise of temperature is essentially due to diminished heat-dissipation.

D. THE HEAT-MECHANISM.

The heat-mechanism consists of two fundamental parts, one being concerned in heat-production, and the other in heat-dissipation. Heat-production is briefly expressed as thermogenesis; and heat-dissipation, as thermolysis. The operations of these mechanisms are so intimately related that fluctuations in the activity of one are rapidly compensated for by reciprocal changes in the other, so that under normal conditions heat-production and heat-dissipation so nearly balance that the mean bodily temperature is maintained within narrow limits.

The regulation of the relations between heat-production and heat-dissipation is termed thermotaxis, which regulation may be effected by alterations in either thermogenesis or thermolysis.

The Mechanism concerned in Thermogenesis.-The portion of the heatmechanism concerned in heat-production consists of (1) thermogenic tissues, (2) thermogenic nerves, and (3) thermogenic centres.

The Thermogenic Tissues.-Almost if not every tissue of the body may be regarded as being a heat-producing structure. The very fact that oxidative processes lie at the bottom of all forms of vital activity, and that heat-production is a concomitant of oxidation, leads inevitably to the conclusion that as long as cells possess life they must produce heat. There are, however, certain of the bodily structures, especially the skeletal muscles and the glands, which are exceptionally active as heat-producers. Indeed, in the case of the skeletal muscles the heat-producing processes are of such a character as to justify the belief that with them thermogenesis is a specific function, because heat is produced not merely as an incidental product of activity but as a specific product. When a muscle contracts, heat is evolved as an incident of the performance of work, and when it is at rest heat is produced not only as an incident of growth and repair but as the result of a specific act. This latter is proved by the fact that when the muscles have been in a state of prolonged rest, when the chemical changes concerned in growth and in repair of waste are practically inactive, heat-production continues to a marked degree.. Moreover, the quantity which is produced varies with the immediate needs of the economy and bears a reciprocal relationship to the quantity of heat formed in other structures,' and is regulated apparently by specific nerve-centres.

When the muscles are contracting less than one-fifth of the energy appears as work, and more than four-fifths as heat. The contractions of the heart also furnish an appreciable percentage of heat as an accompaniment of contraction; and considerable heat is formed indirectly by the resistance offered by the the blood-vessel walls to the blood current. Indeed, the entire work of the heart becomes converted into heat, representing approximately 5 to 10 per cent. of the total heat-production. The quantity formed as by-products of the activity of various structures during a state of muscular quiet is doubtless small compared with the quantity produced by the muscles.

The Thermogenic Nerves and Centres.—Heat-production may occur independently of, but under normal circumstances it is regulated by, the nervous system. A muscle separated from all nervous influences continues to produce heat, but considerably less than before, and it ceases to respond to the demands of the system for more or less heat as do muscles with their nerves intact. Injuries to certain parts of the cerebro-spinal axis affect heat-production in muscles, in some instances causing an increase and in others a decrease; but these changes do not occur if the nervous communication between the centres and muscles is destroyed.

Thermogenic Nerves.-Specific thermogenic nerve-fibres have not as yet been isolated, although the researches of Kemp, Reichert,3 Schultz,1 and others indicate that such fibres exist. In the skeletal muscles probably three independent kinds of processes go on which produce heat, one subservient to the contractions of the muscles, as observed in locomotion, etc.;

1 Rübner: Sitzungsberichte d. königl. Bayer. Akad. der Wissenschaft, 1885, Heft 4.

4 Schultze: Archiv für experimentelle Pathologie und Pharmakologie, 1899, Bd. 43, S. 193.

another in the form of contraction known as shivering; and a third, giving rise to heat as the only important phenomenon. The heat produced by muscles in ordinary or general muscular acts and in repair and growth is a mere incident to activity; but the heat arising during shivering is undoubtedly a specific product―i. e., the object of the shivering is a production of heat (see p. 433). If the nerve-fibres which convey the impulses that cause shivering be ordinary motor fibres, then these fibres are not only motor fibres, but specific thermogenic fibres in so far as they are connected with heatproduction by this act. There are also, apparently, fibres which are entirely distinct from the motor fibres, and which convey impulses that give rise to heat-production as a specific product, and even in the entire absence of motor phenomena. Thus, in a curarized animal in which all motor activity of the skeletal muscles is abolished, an enormous increase of heat-production may occur (Reichert) which cannot satisfactorily be explained in any other way than by assuming the existence of such specific thermogenic fibres. Our information at present is, however, so limited that we can do scarcely more than speculate.

Our knowledge of the character of the afferent fibres which carry impulses that reflexly affect thermogenesis is very unsatisfactory. There can be no doubt that sensory impulses arise in various parts of the organism, especially in the skin, which exercise important influences upon the heat-producing processes. Thus, cooling the skin reflexly excites heat-production, which cannot be attributed to indirect influences upon other functions, but whether or not there exist specific afferent thermogenic fibres is not known. It is possible that the temperature nerves of the skin, the cold and the heat nerves, may be responsible for reflex excitation or depression of heat-production.

The Thermogenic Centres.-The existence of specific thermogenic centres has for many years been conceded, but it has only been recently that hypothesis has given place to fact. The most important results of recent research may be generalized as follows: (1) That the irritation of the skin by heat or cold is followed by marked changes in thermogenesis, which effects are to a certain extent entirely independent of vasomotor and other incidental changes, and which, therefore, are due in part to an increase of heat-production dependent directly upon efferent thermogenic impulses. (2) That injury or excitation of certain parts of the brain is followed by an increase of heat-production. (3) That injury or excitation of certain other parts of the brain is followed by diminished heat-production. (4) That injury of the spinal cord may be followed by an increase or decrease of heat-production which cannot be entirely accounted for by vaso-motor and other attendant alterations. (5) That after operations upon certain parts of the cerebro-spinal axis there follows an increase or decrease in the quantity of CO2 formed, indicating a corresponding effect on the heat-producing processes.

The results of recent calorimetric work show that there are definite regions of the cerebro-spinal axis which are apparently specifically concerned in thermogenesis; that the effects of excitation or destruction of each region are more