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The amount exhaled under ordinary circumstances, is, probably, nine or ten ounces.

Respiration occasions great changes in the blood itself. It loses its dark crimson venous hue, and assumes the bright scarlet colour of arterial blood. Its temperature is raised by 1° or 2° F.; the quantity of oxygen which it contains is increased; that of carbonic acid and nitrogen is diminished, and the fibrin becomes more abundant, whence the greater coagulability of arterial blood.

It was formerly supposed that the oxygen of the atmosphere united directly with the carbon of the venous blood in the lungs to form carbonic acid, and that heat was thus generated. It has, however, more recently been proved that venous blood contains twenty-five per cent. of carbonic acid for twenty contained in arterial blood, whilst arterial blood contains ten per cent. of oxygen as compared with five per cent. contained in venous blood. Moreover, the heat of the blood in the lungs is scarcely, if at all, greater than that of other parts of the body, which would not have been the case had the carbonic acid evolved in respiration been directly formed in the lungs, and had the latter organs been thus the focus and distributors of heat to the economy. From these well-established facts. has been drawn the conclusion that the greater part of the carbonic acid evolved during respiration is not formed in the lungs, but merely thrown off by the venous blood during respiration.

According to this view, the one now generally adopted, the carbonic acid is formed in the ultimate or elementary tissues of the economy by the union of the oxygen absorbed during respiration, and carried by the arterial blood to the capillaries, with the carbon resulting from the disintegration of these tissues, and also with the chyle-wafted carbon subplied by food and not converted into tissue. Simultaneously with this generation of carbonic acid, water is also formed by the union of part of the same oxygen with the hydrogen of the tissues and of the food elements. These chemical and organic changes are attended with the evolution of heat, which is thus constantly emitted in every part of the body, and probably with the development of force.

Whilst the oxygen absorbed by the arterial blood is thus combining with the carbon and hydrogen furnished by the disintegration of tissue, and by recently digested food, to form carbonic acid and water, the still more obscure and mysterious function of molecular nutrition is also taking place. The minute divisions of the systemic capillaries bring the arterial blood in connection with the various structures which compose the economy, penetrating them in every sense, or spreading a network on their surface. These tissues have the vital power of selecting from the arterial blood, chyle renovated, the elements they require for their growth and for the repair of the waste that is constantly taking place in them.

For the nutritive process to take place in a normal manner, the blood must be healthy, and there must be a due supply of nervous power. The process itself appears to be essentially a vital one, each tissue forming itself and repairing its waste from the same blood element. Each particular tissue has, no doubt, its individual duration of life, after which it is broken up and is superseded by new particles, new tissue. Thus every component part of the animal organization has its average duration of life, and then degenerates and dies, to be superseded and replaced.

The entire series of nutritive processes above described take place under the direct control of the nervous power of the individual, an important physiological fact which explains the influence of nervous exhaustion and depression in producing diseases of nutrition.

Change, constant change, is, therefore, the law of organic life. The molecular elements of the tissues of which the animal economy is composed are constantly dying, being resolved into their chemical elements, and are as constantly renewed. This renovation, this repair, takes place principally, as well as the original formation, out of the nitrogenous elements of the blood, the albumen and fibrin. All influences that arrest, in an appreciable manner, these organic changes must, if prolonged, be prejudicial to nutrition.



We have traced solid food through its various phases of elaboration up to its final destination. It was stated at first that articles of food may be divided into two great classes, the nitrogenous and the non-nitrogenous; that nitrogenous food is principally represented by animal substances, in which nitrogen is the prominent element, whereas nonnitrogenous food is represented by vegetable substances, principally composed of carbon-nitrogen being present only to a much more limited extent.

We have now arrived at the explanation of these facts, and find that nitrogenous food is principally used in repairing the wear and tear of our tissues, in forming nitrogenous flesh compounds; whereas the carbonaceous and vegetable food furnishes nitrogen in small quantity, and is principally employed in supplying materials for animal heat and force.

The organic changes that take place during nutrition during the formation and consolidation of tissues out of the blood, and during the disintegration of these same tissues-are attended with the evolution of heat; but the heat thus produced is not alone sufficient to keep up the temperature of warm-blooded animals to their natural standard, surrounded as they usually are by a much lower temperature. To compensate for the constant


radiation and loss of heat, part of the chemical elements of the food consumed is burnt.

The union of oxygen with carbon and hydrogen which constantly takes place in the animal economy is an example of combustion as perfect as that of the oil in a lamp, or as that of coal in a fire, and is attended with the same evolution of heat. The only difference is, that with the lamp or fire the combustion is rapid, and attended with the evolution of light and flame; whereas, in the animal economy, it is slow, and unattended with these merely accessory phenomena; the heat is gradually and imperceptibly emitted. The slow combustion of the chyle-transformed carbonaceous elements of food probably commences as soon as the blood has become loaded with oxygen in passing through the lungs, although the principal change no doubt takes place in the systemic capillaries.

Owing to the ever-varying food requirements for the production of heat, in accordance with temperature, warm-blooded animals are exposed to a double diet difficulty or error. They may not take enough heat-producing food, in which case they are cold, chilly; or they may take too much, and the excess has to be eliminated. The same difficulty occurs with the tissue-producing or nitrogenous element of food: too little or too much may be taken. In the former case the economy is under-fed, and waste of tissue, emaciation, occurs; in the latter, the oversupply has to be eliminated.

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