The proximate principles, which are the most important in nourishing the body, are albumen and fibrin. These constitute the greater part of all the softer animal tissues, and are also found in certain classes of vegetables, such as peas, beans, lentils, and many seeds. Hence, in many cases, a vegetable diet, especially if embracing any of those articles, would be sufficient to sustain life, even if no animal food should be eaten. But no animal can exist for a long time if permitted only to eat substances destitute of nitrogen, as in the case of a dog fed entirely on sugar, which lived but thirty days. And owing to this fact, Baron Liebig proposes to call substances used for food, containing nitrogen, “elements of nutrition,” and those containing an excess of carbon, “elements of respiration;” since, according to his view, the food is necessary to support the growth of the body by replacing the effete and worn-out particles with new matter, and also to keep up the supply of fuel, in order to promote a sufficient degree of heat in the system. Accordingly, under the first division would be included all lean meats and vegetables, such as peas, &c.; while the fat of animals, vegetable oils, sugars, tubers, (as the potato,) and all other substances containing starch, would be included under the latter division.
I.
This definition of exhalants is from the theory of Haller and others. It is now believed that the fluids exude through the thin coats of the blood vessels. This process is called exosmose, and is the exhalation of old physiologists.
J.
It is a well-established fact, in animal and vegetable physiology, that membranes possess the property of allowing fluids and gases to pass through them in either direction, and also to permit two fluids to pass in opposite directions at the same time. This property is designated endosmose when a fluid passes from without a body inward; and exosmose when the reverse takes place. The first is called imbibition. One of the most striking instances of this, in the human system, is shown in the lungs, where carbonic acid and water pass out through the mucous membrane of the bronchial tubes and air-cells; and the oxygen of the air enters the blood through the same membrane. By this process of imbibition, the oxygenation of the blood is much more readily and faithfully accomplished; inasmuch, as by the immense number of bronchial tubes and air-cells a larger quantity of blood is exposed to a greater portion of air, than if the blood were directly laid open to the atmosphere in a mass, or the air were immediately transmitted through it.
Since the function of respiration is to free the system of superfluous carbon and hydrogen, by union with the oxygen of the air, it follows that the greater the amount of the products to be expelled, the larger the quantity of oxygen will be required to effect this purpose, as we find to be the case with those who consume large quantities of food.
The quantity of oxygen daily consumed through the lungs by an adult is about 32.5 oz., and the carbon in the food 13.9 oz. But in order to convert this whole amount of carbon into carbonic acid, which passes off through the lungs and skin, 37 oz. of oxygen are required; the remaining 4.5 oz. being absorbed by the skin. If the supply of food remain the same, while the amount of oxygen in the inspired air is diminished, the superfluous carbon will induce disease in the system, as is the case of those persons who are limited in their supply of air of a proper quality or quantity, and, consequently, have less appetite for food than those who are abundantly supplied with air of the proper standard of health; and in children, who proportionally consume more food than adults, and who are more active, thereby causing a more rapid circulation of blood, and, consequently, the removal of more superfluous particles of matter.
In children we notice the need of air, by their disposition to be much in the open air, and often inspiring more deeply than is common in older persons. 449 Also, if the carbon of the food does not have a requisite supply of oxygen from the air, or other sources, the body becomes emaciated, although nourishing food may be used. And on the other hand, if there be a diminished supply of food, but an abundance of atmospheric air, leanness and emaciation are sure to follow; owing to the fact that if the oxygen has no waste carbon from the body to unite with, it combines with the fat, and some other soft portions of the body, which the Author of nature seems to have provided for this very purpose; as is seen in the case of hibernating animals, who enter their places of winter abode sleek and fat, but crawl out in the spring not merely deprived of their fatty matter, but also with great diminution of all the softer parts, which have given up their share of carbon to supply animal heat. One important cause of emaciation in febrile diseases is the greater rapidity of the pulse and respiration, which consume more carbon than is afforded by the scanty supply of food that is taken, although profuse perspiration, which almost always occurs in some stages of fevers, greatly diminishes the full state of the body.