dissipation is increased. Bathing the skin with cold water increases heat-loss by the vaporization of water as well as by conduction.

The excitation of the cutaneous nerves by cold reflexly increases thermogenesis, and to such an extent that heat-production may even exceed the quantity dissipated, thus causing an increase of bodily temperature. This rise, which is transient, may amount to 0.2° C. or more, and is followed by a reaction in which the temperature may fall 0.2° C. or more below the normal, and continue subnormal for some hours; this fall in turn is succeeded by a supplementary reaction in which the temperature may rise slightly above the normal.

The chief reactions brought about by moderate external cold are constriction of the cutaneous blood-vessels, a diminution of the quantity of sweat secreted, increased tonicity of the pilo-motor muscles, and increased tonicity of the skeletal muscles. The action upon the latter muscles may be so marked as to cause shivering, which increases respiratory activity (see p. 432) and presumably similarly increases heat-production.

Moderate external heat causes dilatation of the cutaneous vessels, excites the general circulation and thus increases the blood-supply to the skin, excites respiratory movements and the sweat-glands, but decreases thermogenesis. Owing to the dilatation of the blood-vessels of the skin and the excitation of the circulation the temperature and the quantity of the blood supplied to the skin are increased, so that conditions are caused which are favorable to an increased loss of heat by radiation. Increased activity of the respiratory movements means a larger volume of air respired, and consequently a greater loss of heat in warming the air and in the evaporation of the larger quantity of water from the lungs. The increase in the quantity of sweat formed also favors heat-dissipation by means of the larger amount of water evaporated from the skin. External heat also causes diminished tonicity of the muscles, and consequent diminished thermogenesis which is probably due to a lessening of the activity of the chemical changes in the muscles.

When external temperature is excessive and continued, heat-regulation is rendered impossible: if extreme cold, heat-dissipation takes place more rapidly than heat-production, so that bodily temperature falls until death results; if extreme heat, heat-dissipation is so interfered with that heat accumulates within the organism, causing a continuous rise of temperature which finally causes death.

Abnormal Thermotaxis.-By this term is meant the regulation of the heatprocesses under conditions in which the mean bodily temperature is maintained at a standard above or below the normal, as in fever and in animals from which the hair has been shaved. It is assumed that under normal conditions the heat-centres are "set," as it were, for a given temperature of the blood, and that when the temperature of the blood goes above or below this standard a compensatory reaction occurs, so that thermogenesis and thermolysis are properly affected to bring about an adjustment. In fever it may be considered that the centres are set for a higher temperature than the normal; the higher the fever, the higher the adjustment. The centres may be set for subnormal

temperatures, as in the case of a rabbit shaved, whose temperature may remain 2° or 3° below the normal for a week or more. When the cause of the abnormal condition disappears, the centres are readjusted to the normal standard.

E. POST-MORTEM RISE OF TEMPERATURE.

A rise of temperature after death is not uncommon; indeed, in case of violent death of healthy individuals, and after death following convulsions, a rise in temperature is almost invariable. This increase is due to continued heat-production and to diminished heat-dissipation. Heat-production after death may be due to continued chemical activity in the muscles and other structures which are not dead but simply in a moribund state. There is, as it were, a residual metabolic activity which remains in the cells until their temperature has been reduced to such a standard that the molecular transformations cease-in other words, until the death of the cells occurs. Consequently, the higher the temperature of the individual at the time of somatic death (the cessation of the circulation and respiration), the longer heat-production continues, because the longer the time required to cool the cells to such a degree that their chemical processes no longer go on. Heat is also produced during the development of rigor mortis. The more quickly rigor sets in, and the more intense it is, the greater is the abundance of heat produced.

The tendency to an increase of bodily temperature is favored by the marked diminution of heat-dissipation which occurs immediately upon the cessation of the circulation and respiration. Therefore, while both heat-production and heat-dissipation fall at once and enormously at the time of death, heat-dissipation may be decreased to a more marked degree than heat-production, so that heat may accumulate and the bodily temperature rise.

Temperature Sense.-(See Cutaneous Sensibility, in the section on Special Senses.)

VOL. I.-32

IX. THE CHEMISTRY OF THE ANIMAL BODY.

Introduction.-Living matter contains hydrogen, oxygen, sulphur, chlorine, iodine, fluorine, nitrogen, phosphorus, carbon, silicon, potassium, sodium, calcium, magnesium, and iron. Abstraction of one of these elements means death to the organism. The compounds occurring in living matter may for the most part be isolated in the laboratory, but they do not then exhibit the properties of animate matter. In the living cell the smallest particles of matter are arranged in such a manner that the phenomena of life are possible. Such an arrangement of materials is called protoplasm, and anything which disturbs this arrangement results in sickness or in death. Somatic death may result from physical shock to the cell; or it may be due to the inability of the cell or the organism to remove from itself poisonous products which are retained in the body so affecting the smallest particles that functional activity is impossible. Pure chemistry adds much to our knowledge of physiology, but it must always be remembered that the conditions present in the beaker glass are not the conditions present in the living cell, for physical and chemical results are dependent on surrounding conditions; hence the necessity and value of animal experimentation. From chemical changes, the physical activities, i. e. the motions characteristic of life, result. Hence the chemistry of protoplasm is the corner-stone of biology. The plan of this section is designed to consider the substances concerned in life in the order usually followed by chemical text-books, and to compare as far as possible the results obtained in pure chemistry with the chemical changes in the organism.

THE NON-METALLIC ELEMENTS.

HYDROGEN, H=1.

This gas is found as a constant product of the putrefaction of animal matter, and is therefore present in the intestinal tract. It is found in the expired air of the rabbit and other herbivorous animals, and in traces in the expired air of carnivorous animals, having first been absorbed by the blood from the intestinal tract. By far the greater amount of hydrogen in the animal and vegetable worlds, as well as in the world at large, occurs combined in the form of water, and it will be shown that the proteids, carbohydrates, and fats, characteristic of the organism, all contain hydrogen originally derived from water. In the atmosphere is found ammonia in traces, which holds hydrogen in combination, and this is a second source of hydrogen, especially for the construction of the proteid molecule.

Preparation.—(1) Through the electrolysis of water, by which one volume

of oxygen is evolved on the positive pole and two volumes of hydrogen on the negative.

(2) Through the action of zinc on sulphuric acid,1

Zn + H2SO1 = ZnSO4 + H2.

(3) Through putrefaction (by which is understood the change effected in organic matter through certain lower organisms, bacteria) hydrogen is liberated in the intestinal canal from proteid matter, and especially from the fermentation of carbohydrates:

In putrefaction in the presence of oxygen the hydrogen formed immediately unites with oxygen, producing water; hence, notwithstanding the enormous amount of putrefaction in the world, there is no accumulation of hydrogen in the atmosphere.

Both bacteria and an enzyme can liberate hydrogen by acting on calcium formate,

Ca (CHO2)2 + H2,O=CaCO3 + CO2 + 2H2,

and this same reaction may be brought about by the action of metallic iridium, rhodium, or ruthenium on formic acid. An enzyme is a substance probably of proteid nature capable of producing change in other substances without itself undergoing apparent change (example, pepsin). Bunge' calls attention to the fact that the above reaction may be brought about by living cells (bacteria), by an organic substance (enzyme), and by an inorganic metal. This similarity of action between organized and unorganized material, between living and dead substances, is shown more and more conspicuously as science advances.

Properties.-Hydrogen burns in the air, forming water, and if two volumes of hydrogen and one of oxygen be ignited, they unite with a loud explosion. Hydrogen will not support respiration, but, mixed with oxygen, may be respired, probably being dissolved in the fluids of the body as an inert gas, without effect upon the organism. Hydrogen may pass through the intestinal tissues into the blood-vessels, according to the laws of diffusion, in exchange for some other gas, and may then be given off in the lungs. Nascent hydrogen-that is to say, hydrogen at the moment of generation—is a powerful reducing agent, uniting readily with oxygen (see p. 505).

OXYGEN, O=16.

Oxygen is found free in the atmosphere to the amount of about 21 per cent. by volume, and is found dissolved in water and chemically combined in arterial blood. It is swallowed with the food and may be present in the stomach, but it entirely disappears in the intestinal canal, being absorbed by respiratory exchange through the mucous membrane. It occurs chemically combined with metals so that it forms one-half the weight of the earth's crust; it likewise occurs combined in water and in most of the materials forming animal and vegetable organisms. It is found in the blood in loose chemical

1 It is not within the scope of this work to give more than typical methods of laboratory preparation. For greater detail the reader is referred to works on general chemistry. Physiologische Chemie, 2d ed., 1889, p. 167.