The Reserve air is that which usually remains in the chest after the ordinary efforts of expiration, but which can be expelled by forcible expiration. The volume of reserve air is about 100 cubic inches.

The Residual air is that portion which remains in the chest and cannot be expelled after the most forcible expiratory efforts, and which amounts, according to Dr. Hutchinson, to about 100 cubic inches.

The Vital Capacity of the chest indicates the amount of air that can be forcibly expelled from the lungs after the deepest possible inspiration, and is an index of an individual's power of breathing in disease and prolonged severe exercise. The combined amounts of the tidal, the complemental and reserve air, 230 cubic inches, represents the vital capacity of an individual 5 feet 7 inches in height. The vital capacity varies chiefly with stature. It is increased 8 cubic inches for every inch in height above this standard, and diminishes 8 cubic inches for each inch below it.

The Tidal Volume of air is carried only into the trachea and larger bronchial tubes by the inspiratory movements. It reaches the deeper portions of the lungs in obedience to the law of diffusion of gases, which is inversely proportionate to the square root of their densities.

The ciliary action of the columnar cells lining the bronchial tubes also assists in the interchange of air and carbonic acid.

The entire volume of air passing in and out of the thorax in 24 hours is subject to great variation, but can be readily estimated from the tidal volume and the number of respirations per minute. Assuming that an individual takes into the chest 20 cubic inches at each inspiration, and breathes 18 times per minute, in 24 hours there would pass in and out of the lungs 518,400 cubic inches or 300 cubic feet.

Composition of Air: Oxygen, 20.81 parts; nitrogen, 79.19, forming a mechanical mixture in which exist traces of carbonic acid and watery vapor.

The changes in the air effected by respiration are—

Loss of oxygen, to the extent of 5 cubic inches per 100 of air, or I in

20.

Gain of carbonic acid, to the extent of 4.66 cubic inches per 100 of

air or .93 inch in 20.

Increase of watery vapor and organic matter.

Elevation of temperature.

Increase and at times decrease of nitrogen.

Gain of ammonia.

The total quantity of oxygen withdrawn from the air and consumed by

the body in 24 hours amounts to 15 cubic feet, and can be readily esti mated from the amount consumed at each respiration. Assuming that one inch of oxygen remains in the lungs at each respiration, in one hour there are consumed 18 inches, and in 24 hours, 25,920 cubic inches or 15 cubic feet, weighing 18 oz. To obtain this quantity, 300 cubic feet of air are necessary.

The quantity of carbonic acid exhaled in 24 hours varies greatly. It can be estimated in the same way. Assuming that an individual exhales .93+ cubic inch at each respiration, in one hour there are eliminated 1008 cubic inches, and in 24 hours 24,192 cubic inches or 14 cubic feet, containing 7 oz. of pure carbon.

As oxygen and carbon unite to form an equal volume of carbonic acid gas, there disappears daily in the body, one cubic foot of oxygen, which in all probability unites with the surplus hydrogen of the food to form

water.

The exhalation of carbonic acid is increased by muscular exercise; nitrogenous food; tea, coffee and rice; age, and by muscular development; decreased by a lowering of temperature; repose; gin and brandy, and a dry condition of the air.

Condition of the Gases in the Blood.

Oxygen is absorbed from the lungs into the arterial blood by the coloring matter, hæmoglobin, with which it exists in a state of loose combination, and is disengaged during the process of nutrition.

Carbonic acid, arising in the tissues, is absorbed into the blood, in consequence of its alkalinity; where it exists in a state of simple solution and also in a state of feeble combination with the carbonates, soda and potassa, forming the bicarbonates; it is liberated by pneumic acid in the pulmonary tissue.

Nitrogen is simply held in solution in the plasma.

The amount of watery vapor thrown off from the lungs daily is about one pound, with which is mingled organic matter and ammonia.

Changes in the Blood during Respiration.

As the blood passes through the lungs it is changed in color, from the dark purple hue of venous blood to the bright scarlet of arterial blood.

The heterogeneous composition of venous blood is exchanged for the uniform composition of the arterial.

It gains oxygen and loses carbonic acid.

Its coagulability is increased. Temperature is diminished.

E

Asphyxia. If the supply of oxygen to the lungs be diminished and the carbonic acid retained in the blood, the normal respiratory movements cease, the condition of asphyxia ensues, which soon terminates in death.

The phenomena of asphyxia are, violent spasmodic action of the respiratory muscles, attended by convulsions of the muscles of the extremities, engorgement of the venous system, lividity of the skin, abolition of sensibility and reflex action, and death.

The cause of death is a paralysis of the heart, from over distention by blood. The passage of the blood through the capillaries is prevented by contraction of the smaller arteries, from irritation of the vaso-motor centre. The heart is enfeebled by a want of oxygen and inhibited in its action by the inhibitory centres.

ANIMAL HEAT.

The Functional Activity of all the organs and tissues of the body is attended by the evolution of heat, which is independent, for the most part, of external conditions. Heat is a necessary condition for the due performance of all vital actions; though the body constantly loses heat by radiation and evaporation, it possesses the capability of renewing it and maintaining it at a fixed standard. The normal temperature of the body in the adult, as shown by means of a delicate thermometer placed in the axilla, ranges from 97.25° Fahr. to 99.5° Fahr., though the mean normal temperature is estimated by Wunderlich at 98.6° Fahr.

The temperature varies in different portions of the body, according to the degree in which oxidation takes place; being the highest in the muscles during exercise, in the brain, blood, liver, etc.

The conditions which produce variations in the normal temperature of the body are: age, period of the day, exercise, food and drink, climate, season and disease.

Age. At birth the temperature of the infant is about 1° F. above that of the adult, but in a few hours falls to 95.5° F., to be followed in the course of 24 hours by a rise to the normal or a degree beyond. During childhood the temperature approaches that of the adult; in aged persons the temperature remains about the same, though they are not as capable of resisting the depressing effects of external cold as adults. A diurnal variation of the temperature occurs from 1.8° F. to 3.6° F. (Jürgensen); the maximum occurring late in the afternoon, from 4 to 9 P.M., the minimum, early in the morning, from 1 to 7 A.M.

Exercise. The temperature is raised from 1° to 2° F. during active

contractions of the muscular masses, and is probably due to the increased activity of chemical changes; a rise beyond this point being prevented by its diffusion to the surface, consequent on a more rapid circulation, radiation, more rapid breathing, etc.

Food and drink. The ingestion of a hearty meal increases the temperature but slightly; an absence of food, as in starvation, produces a marked decrease. Alcoholic drinks, in large amounts, in persons unaccustomed to their use, cause a depression of the temperature, amounting from 1° to 2° F. Tea causes a slight elevation.

External temperature. Long continued exposure to cold, especially if the body is at rest, diminishes the temperature from 1° to 2o F., while exposure to a great heat slightly increases it.

Disease frequently causes a marked variation in the normal temperature of the body, rising as high as 107° F. in typhoid fever, and 105° F. in pneumonia; in cholera it falls as low as 80° F. Death usually occurs when the heat remains high and persistent, from 106° to 110°F.; the increase of heat in disease is due to excessive production rather than to diminished elimination.

The source of heat is to be sought for in the chemical combinations taking place during the general process of nutrition, and the amount of its production is in proportion to the activity of the internal changes.

Every contraction of a muscle, every act of secretion, each exhibition of nerve force, is accompanied by a change in the chemical composition of the tissues and an evolution of heat. The reduction of the disintegrated tissues to their simplest form by oxidation; the combination of the oxygen of the inspired air with the carbon and hydrogen of the blood and tissues, results in the formation of carbonic acid and water and the generation of a large amount of heat.

Certain elements of the food, particularly the non-nitrogenized substances, undergo oxidation without taking part in the formation of the tissues, being transformed into carbonic acid and water, and thus increase the sum of heat in the body.

Heat-producing Tissues. All the tissues of the body add to the general amount of heat, according to the degree of their activity. But special structures, on account of their mass and the large amount of blood they receive, are particularly to be regarded as heat producers; e. g :—

1. During mental activity the brain receives nearly one-fifth of the entire volume of blood, and the venous blood returning from it is charged with waste matters, and its temperature is increased.

2. The muscular tissue, on account of the many chemical changes occurring during active contractions, must be regarded as the chief heatproducing tissue.

3. The secreting glands, during their functional activity, add largely to the amount of heat.

Of the entire quantity of heat generated in the body, it is estimated that only a small proportion is utilized, as five-sixths escape by radiation and evaporation, the remaining one-sixth being utilized in keeping the body at the normal temperature standard, 98.6° F., and in the production of muscular force.

The body loses heat by radiation and evaporation from the general cutaneous surface, the respiratory passages and by the urine and fæces. About 75 per cent. of all the heat lost escapes from the skin. In passing through the lungs the temperature of the blood is lowered by about 1° Fahr.

The nervous system influences the production of heat in a part, by increasing the amount of blood going through it by its action upon the vaso-motor nerves. Whether there exists a special heat centre has not been satisfactorily determined, though this is probable.

SECRETION.

The Process of Secretion consists in the separation of materials from the blood, which are either to be again utilized to fulfill some special purpose in the economy, or are to be removed from the body as excrementitious matter; in the former case they constitute the secretions, in the latter, the excretions.

The materials which enter into the composition of the secretions are derived from the nutritive principles of the blood, and require special organs, e. g., gastric glands, mammary glands, etc., for their proper elaboration.

The materials which compose the excretions pre-exist in the blood, and are the results of the activities of the nutritive process; if retained within the body they exert a deleterious influence upon the composition of the blood.

Destruction of a secreting gland abolishes the secretion peculiar to it, and it cannot be formed by any other gland; but among the excreting organs there exists a complementary relation, so that if the function of one organ be interfered with, another performs it, to a certain extent.