or lungs appears sometimes accomplished in an immeasurably small time.

Conditions for Absorption.- -1. The substance to be absorbed must, as a general rule, be in the liquid or gaseous state, or, if a solid, must be soluble in the fluids with which it is brought into contact. Hence the marks of tattooing, and the discoloration produced by silver nitrate taken internally, remain. Mercury may be absorbed even in the metallic state; and in that state may pass into and remain in the blood-vessels, or be deposited from them; and such substances as exceedingly finely-divided charcoal, when taken into the alimentary canal, have been found in the mesenteric veins; the insoluble materials of ointments may also be rubbed into the blood-vessels; but there are no facts to determine how these various substances effect their passage. Oil, minutely divided, as in an emulsion, will pass slowly into blood-vessels, as it will through a filter moistened with water; and, without doubt, fatty matters find their way into the blood-vessels as well as the lymph-vessels of the intestinal canal, although the latter seem to be specially intended for their absorption.

2. The less dense the fluid to be absorbed, the more speedy, as a general rule, is its absorption by the living blood-vessels. Hence the rapid absorption of water from the stomach; also of weak saline solutions; but with strong solutions, there appears less absorption into, than effusion from, the blood-vessels.

3. The absorption is the less rapid the fuller and tenser the bloodvessels are; and the tension may be so great as to hinder altogether the entrance of more fluid. Thus, if water is injected into a dog's veins to repletion, poison is absorbed very slowly; but when the tension of the vessels is diminished by bleeding, the poison acts quickly. So, when cupping-glasses are placed over a poisoned wound, they retard the absorption of the poison not only by diminishing the velocity of the circulation in the part, but by filling all its vessels too full to admit more.

4. On the same ground, absorption is the quicker the more rapid the circulation of the blood; not because the fluid to be absorbed is more quickly imbibed into the tissues, or mingled with the blood, but because as fast as it enters the blood, it is carried away from the part, and the blood being constantly renewed, is constantly as fit as at the first for the reception of the substance to be absorbed.

CHAPTER IX.

ANIMAL HEAT.

THE Average Temperature of the human body in those internat parts which are most easily accessible, as the mouth and rectum, is from 98.5° to 99'5° F. (36.9°-374° C.). In different parts of the external surface of the human body the temperature varies only to the extent of two or three degrees (F.), when all are alike protected from cooling influences; and the difference which under these circumstances exists, depends chiefly upon the different degrees of blood-supply. In the armpit- the most convenient situation, under ordinary circumstances, for examination by the thermometer-the average temperature is 98.6° F. (36·9° C.). In different internal parts, the variation is one or two degrees; those parts and organs being warmest which contain most blood, and in which there occurs the greatest amount of chemical change, e.g., the glands and the muscles; and the temperature is highest, of course, when they are most actively working: while those tissues which, subserving only a mechanical function, are the seat of least active circulation and chemical change, are the coolest. These differences of temperature, however, are actually but slight, on account of the provisions which exist for maintaining uniformity of temperature in different parts.

Circumstances causing Variations in Temperature.— The chief circumstances by which the temperature of the healthy body is influenced are the following:-Age; Sex; Period of the day; Exercise; Climate and Season; Food and Drink.

Age. The average temperature of the new-born child is only about 1 F. (54° C.) above that of the adult; and the difference becomes still more trifling during infancy and early childhood. The temperature falls to the extent of about 2°-5° F. from early infancy to puberty, and by about the same amount from puberty to fifty or sixty years of age. In old age the temperature again rises, and approaches that of infancy; but although this is the case, yet the power of resisting cold is less in them-exposure to a low temperature causing a greater reduction of heat than in young persons.

Sex. The average temperature of the female is very slightly higher than that of the male.

Period of the Day. The temperature undergoes a gradual alteration, to the extent of about 1° to 1.5° F. ('54—8° C.) in the course of the day and night; the minimum being at night or in the early morning, the maximum late in the afternoon.

Exercise.-Active exercise raises the temperature of the body from 1 to 2° F. ('54°—1′08° C.). This may be partly ascribed to generally increased combustion-processes, and partly to the fact, that every muscular contraction is attended by the development of one or two degrees of heat in the acting muscle; and that the heat is increased according to the number and rapidity of these contractions, and is quickly diffused by the blood circulating from the heated muscles. Possibly, also, some small amount of heat may be generated in the various movements, stretchings, and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caoutchouc alternately stretched and recoiling becomes hot.

Climate and Season. The temperature of the human body is the same in temperate and tropical climates. (Furnell.) In summer the temperature of the body is a little higher than in winter; the difference amounting to about a third of a degree F.

Food and Drink.-The effect of a meal upon the temperature of a body is but small. A very slight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat (・5° to 1° F.). Warm alcoholic drinks, as well as warm tea and coffee, raise the temperature (about 5° F.).

In disease the temperature of the body deviates from the normal standard to a greater extent than would be anticipated from the slight effect of external conditions during health. Thus, in some diseases, as pneumonia and typhus, it occasionally rises as high as 106 or 107° F. (41°—41′6° C.); and considerably higher temperatures have been noted. In Asiatic cholera, on the other hand, a thermometer placed in the mouth may sometimes rise only to 77° or 79° F. (25°—26·2 C.).

The temperature maintained by Mammalia in an active state of life, according to the tables of Tiedemann and Rudolphi, averages 101° (38′3° C.). The extremes recorded by them were 96° and 106°, the former in the narwhal, the latter in a bat (Vespertilio pipistrella). In Birds, the average is as high as 107° (41.2° C.); the highest temperature, 11125° (46·2° C.) :

being in the small species, the linnets, &c. Among Reptiles, while the medium they were in was 75° (23.9°C) their average temperature, was 82.5° (31.2° C). As a general rule, their temperature, though it falls with that of the surrounding medium, is, in temperate media, two or more degrees higher; and though it rises also with that of the medium, yet at very high degrees it ceases to do so, and remains even lower than that of the medium. Fish and invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average of fish have, are generally 7° (3·8° C.) warmer than the water around them.

The difference, therefore, between what are commonly called the warm and the cold-blooded animals, is not one of absolutely higher or lower temperature; for the animals which to us in a temperate climate, feel cold (being like the air or water, colder than the surface of our bodies), would in an external temperature of 100° (37.8° C.) have nearly the same temperature and feel hot to us. The real difference is that what we call warmblooded animals (Birds and Mammalia), have a certain "permanent heat in all atmospheres," while the temperature of the others, which we call cold-blooded, is "variable with every atmosphere." (Hunter.)

The power of maintaining a uniform temperature, which Mammalia and Birds possess, is combined with the want of power to endure such changes of body temperature as are harmless to the other classes; and when their power of resisting change of temperature ceases, they suffer serious disturbance or die.

Sources and Mode of Production of Heat in the Body. The heat which is produced in the body arises from combustion, and is due to the fact that the oxygen of the atmosphere taken into the system is ultimately combined with carbon and hydrogen, and discharged from the body as carbonic acid and water. Any changes, indeed, which occur in the protoplasm of the tissues, resulting in an exhibition of their function, are attended by the evolution of heat and the formation of carbonic acid and water. The more active the changes, the greater is the heat produced and the greater is the amount of the carbonic acid and water formed. But in order that the protoplasm may perform its function, the waste of its own tissue (destructive metabolism), must be repaired by the due supply of food material and therefore for the production of heat food is necessary. In the tissues, therefore, two processes are continually going on: the building up of the protoplasm from the food (constructive metabolism), which is not accompanied by the evolution of heat but possibly by the reverse, and the oxidation of the protoplastic materials, resulting in the production of energy, by which heat is produced and carbonic acid and water are evolved. Some heat also is generated in the combination of sulphur and

phosphorus with oxygen, but the amount thus produced is but small.

It is not necessary to assume that the combustion processes, which ultimately issue in the production of carbonic acid and water, are as simple as the bare statement of the fact might seem to indicate. But complicated as the various stages may be, the ultimate result is as simple as in ordinary combustion outside the body, and the products are the same. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combination be rapid and direct, as in ordinary combustion, or slow and almost imperceptible, as in the changes which occur in the living body. And since the heat thus arising will be distributed wherever the blood is carried, every part of the body will be heated equally, or nearly so.

This theory, that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxidation of combustible materials existing in the tissues, has long been established by the demonstration that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat generated in the animal within the same period: an amount capable of maintaining the temperature of the body at from 98-100° F. (36.8°-37.8° C.), notwithstanding a large loss by radiation and evaporation.

It should be remembered that some heat may be introduced into the body by means of warm drinks and foods, and, again, that it is possible for the preliminary digestive changes to be accompanied by the evolution of heat.

Chief Heat-producing Tissues.-The chemical changes which produce the body-heat appear to be especially active in certain tissues :-(1), In the Muscles, which form so large a part of the organism. The fact that the manifestation of muscular energy is always attended by the evolution of heat and the production of carbonic acid has been demonstrated by actual experiment; and when not actually in a condition of active contraction, a metabolism, not so active but still actual, goes on, which is accompanied by the manifestation of heat. The total amount set free by the muscles, therefore, must be very great; and it has been calculated in a way which will be referred to later on, that even neglecting the heat produced by the quiet