« PreviousContinue »
THE FUNCTION OF ALIMENTATION.
1. THE great source of gain to the blood, and, except the lungs, the only channel by which altogether new material is introduced into that fluid, putting aside the altogether exceptional case of absorption by the skin, is the alimentary canal, the totality of the operations of which constitutes the function of alimentation. It will be useful to consider the general nature and results of the performance of this function before studying its details.
2. A man daily takes into his mouth, and thereby introduces into his alimentary canal, a certain quantity of solid and liquid food, in the shape of meat, bread, butter, water, and the like. The amount of chemically dry, solid matter, which must thus be taken into the body, if a man of average size and activity is neither to lose, nor to gain, in weight, has been found to be about 8,000 grains. In addition to this, his blood absorbs by the lungs about 10,000 grains of oxygen gas, making a grand total of 18,000 grains (or nearly two pounds and three-quarters avoirdupois) of daily gain of dry, solid, and gaseous
3. The weight of dry solid matter passed out from the alimentary canal does not, on the average, amount to more than one-tenth of that which is taken into it, or 800 grains. Now the alimentary canal is the only channel by which any appreciable amount of solid matter leaves the body in an undissolved condition. It follows, there
fore, that in addition to the 10,000 grains of oxygen, 7,200 grains of dry, solid, matter must pass out of the body by the lungs, skin, or kidneys, either in the form of gas, or dissolved in the liquid excretions of those organs. Further, as the general composition of the body remains constant, it follows either that the elementary constituents of the solids taken into the body must be identical with those of the body itself: or that, in the course of the vital processes, the food alone is destroyed, the substance of the body remaining unchanged: or, finally, that both these alternatives hold good, and that food is, partly, identical with the wasting substance of the body, and replaces it; and, partly, differs from the wasting substance, and is consumed without replacing it.
4. As a matter of fact, all the substances which are used as food come under one of four heads. They are either what may be termed Proteids, or they are Fats, or they are Amyloids, or they are Minerals.
Proteids are composed of the four elements—carbon, hydrogen, oxygen, and nitrogen, sometimes united with sulphur and phosphorus.
Under this head come the Gluten of flour; the Albumin of white of egg, and blood serum; the Fibrin of the blood; the Syntonin, which is the chief constituent of muscle and flesh, and Casein, one of the chief constituents of cheese, and many other similar but less common bodies; while Gelatin, which is obtained by boiling from connective tissue, and Chondrin, which may be produced in the same way from cartilage, may be considered to be outlying members of the same group.
Fats are composed of carbon, hydrogen, and oxygen only, and contain more hydrogen than is enough to form water if united with the oxygen which they possess.
All vegetable and animal fatty matters and oils come under this division.
Amyloids are substances which also consist of carbon, hydrogen, and oxygen only. But they contain no more hydrogen than is just sufficient to produce water with their oxygen. These are the matters known as Starch, Dextrine, Sugar, and Gum.
It is the peculiarity of the three groups of food-stuffs just mentioned that they can only be obtained (at any
rate, at present) by the activity of living beings, whether animals or plants, so that they may be conveniently termed vital food-stuffs.
Food-stuffs of the fourth class, on the other hand, or Minerals, are to be procured as well from the not-living, as the living world. They are water, and salts of sundry alkalies, earths, and metals. To these, in strictness, oxygen ought to be added, though, as it is not taken in by the alimentary canal, it hardly comes within the ordinary acceptation of the word food.
5. In ultimate analysis, then, it appears that vital foodstuffs contain either three or four of the elements: carbon, hydrogen, oxygen, and nitrogen; and that mineral foodstuffs are water and salts. But the human body, in ultimate analysis, also proves to be composed of the same four elements, plus water, and the same saline matters as are found in food.
More than this, no substance can serve permanently for food-that is to say, can prevent loss of weight and change in the general composition of the body—unless it contains a certain amount of proteid matter in the shape of albumin, fibrin, syntonin, casein, &c., while, on the other hand, any substance which contains proteid matter in a readily assimilable shape, is competent to act as a permanent vital food-stuff.
The human body, as we have seen, contains a large quantity of proteid matter in one or other of the forms which have been enumerated; and, therefore, it turns out to be an indispensable condition, that every substance which is to serve permanently as food, must contain a sufficient quantity of the most important and complex component of the body ready made. It must also contain a sufficient quantity of the mineral ingredients which are required. Whether it contains either fats or amyloids, or both, its essential power of supporting the life and maintaining the weight and composition of the body remains unchanged.
6. The necessity of constantly renewing the supply of proteid matter arises from the circumstance that the secretion of urea from the body (and consequently the loss of nitrogen) goes on continually, whether the body is fed or not: while there is only one form in which
nitrogen (at any rate, in any considerable quantity) can be taken into the blood, and that is in the form of a solution of proteid matter. If proteid matter be not supplied, therefore, the body must needs waste, because there is nothing in the food competent to make good the loss of nitrogen.
On the other hand, if proteid matter be supplied, there can be no absolute necessity for any other but the mineral food-stuffs, because proteid matter contains carbon and hydrogen in abundance, and hence is competent to give origin to the other great products of waste, carbonic acid and water.
In fact, the final results of the oxidation of proteid matters are carbonic acid, water, and ammonia; and these, as we have seen, are the final shapes of the waste products of the human economy.
7. From what has been said, it becomes readily intelligible that, whether an animal be herbivorous or carnivorous, it begins to starve from the moment its vital food-stuffs consist of pure amyloids, or fats, or any mixture of them. It suffers from what may be called nitrogen starvation, and, sooner or later, will die.
In this case, and still more in that of an animal deprived of vital food altogether, the organism, so long as it continues to live, feeds upon itself. In the former case, those excretions which contain nitrogen, in the latter, all its waste products, are necessarily formed at the expense of its own body; whence it has been rightly enough observed that a starving sheep is as much a carnivore as a lion.
8. But though proteid matter is the essential element of food, and under certain circumstances may suffice, by itself, to maintain the body, it is a very disadvantageous and uneconomical food.
Albumen, which may be taken as the type of the proteids, contains about 53 parts of carbon and 15 of nitrogen in 100 parts. If a man were to be fed on white of egg, therefore, he would take in, speaking roughly, 3 parts of carbon for every part of nitrogen.
But it is proved experimentally, that a healthy, fullgrown man, keeping up his weight and heat, and taking a fair amount of exercise, eliminates 4,000 grains of
carbon to only 300 grains of nitrogen, or, roughly, only needs one-thirteenth as much nitrogen as carbon. However, if he is to get his 4,000 grains of carbon out of albumen, he must eat 7,547 grains of that substance. But 7,547 grains of albumen contain 1,132 grains of nitrogen, or nearly four times as much as he wants.
To put the case in another way, it takes about four pounds of fatless meat (which generally contains about one-fourth its weight of dry solid proteids) to yield 4,000 grains of carbon, whereas one pound will furnish 300 grains of nitrogen.
Thus a man confined to a purely proteid diet, must eat a prodigious quantity of it. This not only involves a great amount of physiological labour in comminuting the food, and a great expenditure of power and time in dissolving and absorbing it; but throws a great quantity of wholly profitless labour upon those excretory organs, which have to get rid of the nitrogenous matter, three-fourths of which, as we have seen, is superfluous.
Unproductive labour is as much to be avoided in physiological, as in political, economy; and it is quite possible that an animal fed with perfectly nutritious, proteid matter should die of starvation: the loss of power in various operations required for its assimilation overbalancing the gain; or the time occupied in their performance being too great to check waste with sufficient rapidity. The body, under these circumstances, falls into the condition of a merchant who has abundant assets, but who cannot get in his debts in time to meet his creditors.
9. These considerations lead us to the physiological justification of the universal practice of mankind in adopting a mixed diet, in which proteids are mixed either with fats, or with amyloids, or with both.
Fats may be taken to contain about 80 per cent. of carbon, and amyloids about 40 per cent. Now it has been seen that there is enough nitrogen to supply the waste of that substance per diem, in a healthy man, in a pound of fatless meat; which also contains 1,000 grains of carbon, leaving a deficit of 3,000 grains of carbon. Rather more than half a pound of fat, or a pound of sugar, will supply this quantity of carbon. The former, if properly subdivided, the latter, by reason of its