it is also necessary for the perfect nutrition of the body that fat be supplied, under its own form, with the food. For the human species, also, it is natural to have them both associated in the alimentary materials. They occur together in most vegetable substances, and there is a natural desire for them both, as elements of the food.

They are not, however, when alone, or even associated with each other, sufficient for the nutrition of the animal body. Magendie found that dogs, fed exclusively on starch or sugar, perished after a short time with symptoms of profound disturbance of the nutritive functions. An exclusive diet of butter or lard had a similar effect. The animal became exceedingly debilitated, though without much emaciation; and after death, all the internal organs and tissues were found infiltrated with oil. Boussingault' performed a similar experiment, with a like result, upon a duck, which was kept upon an exclusive regimen of butter. "The duck received 1350 to 1500 grains of butter every day. At the end of three weeks it died of inanition. The butter oozed from every part of its body. The feathers looked as though they had been steeped in melted butter, and the body exhaled an unwholesome odor like that of butyric acid."

Lehmann was also led to the same result by some experiments which he performed upon himself for the purpose of ascertaining the effect produced on the urine by different kinds of food. This observer confined himself first to a purely animal diet for three weeks, and afterwards to a purely vegetable one for sixteen days, without suffering any marked inconvenience. He then put himself upon a regimen consisting entirely of non-nitrogenous substances, starch, sugar, gum, and oil, but was only able to continue this diet for two, or at most for three days, owing to the marked disturbance of the general health which rapidly supervened. The unpleasant symptoms, however, immediately disappeared on his return to an ordinary mixed diet. The same fact has been established more recently by Dr. Wm. A. Hammond, Assistant Surgeon U. S. Army, in a series of experiments which he performed upon himself. He was enabled to live for ten days on a diet composed exclusively of boiled starch and water. After the third day, how

Chimie Agricole, p. 166.

2 Journal für praktische Chemie, vol. xxvii. p. 257.

3 Experimental Researches, &c., being the Prize Essay of the American Medical Association for 1857.

ever, the general health began to deteriorate, and became very much disturbed before the termination of the experiment. The prominent symptoms were debility, headache, pyrosis, and palpitation of the heart. After the starchy diet was abandoned, it required some days to restore the health to its usual condition.

The proximate principles of the third class, or the organic substances proper, enter so largely into the constitution of the animal tissues and fluids, that their importance, as elements of the food, is easily understood. No food can be long nutritious, unless a certain proportion of these substances be present in it. Since they are so abundant as ingredients of the body, their loss or absence from the food is felt more speedily and promptly than that of any other substance except water. They have, therefore, sometimes received the name of "nutritious substances," in contradistinction to those of the second class, which contain no nitrogen, and which have been found by the experiments of Magendie and others to be insufficient for the support of life. The organic substances, however, when taken alone, are no more capable of supporting life indefinitely than the others. It was found in the experiments of the French "Gelatine Commission" that animals fed on pure fibrin and albumen, as well as those fed on gelatine, become after a short time much enfeebled, refuse the food which is offered to them, or take it with reluctance, and finally die of inanition. This result has been explained by supposing that these substances, when taken alone, excite after a time such disgust in the animal that they are either no longer taken, or if taken are not digested. But this disgust itself is simply an indication that the substances used are insufficient and finally useless as articles of food, and that the system demands instinctively other materials for its nourishment.

The instinctive desire of animals for certain substances is the surest indication that they are in reality required for the nutritive process; and on the other hand, the indifference or repugnance manifested for injurious or useless substances, is an equal evidence of their unfitness as articles of food. This repugnance is well described by Magendie, in the report of the commission above alluded to, while detailing the result of his investigations on the nutritive qualities of gelatine. "The result," he says, "of these first trials was that pure gelatine was not to the taste of the dogs experimented Some of them suffered the pangs of hunger with the gelatine

on.

'Comptes Rendus, 1841, vol. xiii. p. 267.

:

within their reach, and would not touch it; others tasted of it, but would not eat others still devoured a certain quantity of it once or twice, and then obstinately refused to make any farther use of it." In one instance, however, Magendie succeeded in inducing a dog to take a considerable quantity of pure fibrin daily throughout the whole course of the experiment; but notwithstanding this, the animal became emaciated like the others, and died at last with the same symptoms of inanition.

The alimentary substances of the second class, however, viz., the sugars and the oils, have been sometimes thought less important than the albuminous matters, because they do not enter so largely or so permanently into the composition of the solid tissues. The saccharine matters, when taken as food, cannot be traced farther than the blood. They undergo already, in the circulating fluid, some change by which their essential character is lost, and they cannot be any longer recognized. The appearance of sugar in the mammary gland and the milk is only exceptional, and does not occur at all in the male subject. The fats are, it is true, very generally distributed throughout the body, but it is only in the brain and nervous matter that they exist intimately united with the remaining ingredients of the tissues. Elsewhere, as already mentioned, it is deposited in distinct drops and granules, and so long as it remains in this condition must of course remain inactive, so far as regards any chemical nutritive process. In this condition it seems to be held in reserve, ready to be absorbed by the blood, whenever it may be required for the purposes of nutrition. On being reabsorbed, however, as soon as it again enters the blood or unites intimately with the substance of the tissues, it at once changes its condition and loses its former chemical constitution and properties.

It is for these reasons that the albuminoid matters have been sometimes considered as the only "nutritious" substances, because they alone constitute under their own form a great part of the ingredients of the tissues, while the sugars and the oils rapidly disappear by decomposition. It has even been assumed that the process by which the sugar and the oils disappear is one of direct combustion or oxidation, and that they are destined solely to be consumed in this way, not to enter at all into the composition of the tissues, but only to maintain the heat of the body by an incesThey have been therefore

sant process of combustion in the blood.

termed the "combustible" or "heat-producing" elements, while the

albuminoid substances were known as the nutritious or "plastic" elements.

This distinction, however, has no real foundation. In the first place, it is not at all certain that the sugars and the oils which disappear in the body are destroyed by combustion. This is merely an inference which has been made without any direct proof. All we know positively in regard to the matter is that these substances soon become so altered in the blood that they can no longer be recognized by their ordinary chemical properties; but we are still ignorant of the exact nature of the transformations which they undergo. Furthermore, the difference between the sugars and the oils on the one hand, and the albuminoid substances on the other, so far as regards their decomposition and disappearance in the body, is only a difference of time. The albuminoid substances become transformed more slowly, the sugars and the oils more rapidly. Even if it should be ascertained hereafter that the sugars and the oils really do not unite at all with the solid tissues, but are entirely decomposed in the blood, this would not make them any less important as alimentary substances, since the blood is as essential a part of the body as the solid tissues, and its nutrition must be provided for equally with theirs.

It is evident, therefore, that no single proximate principle, nor even any one class of them alone, can be sufficient for the nutrition of the body; but that the food, to be nourishing, must contain substances belonging to all the different groups of proximate principles. The albuminoid substances are first in importance because they constitute the largest part of the entire mass of the body; and exhaustion therefore follows more rapidly when they are withheld than when the animal is deprived of other kinds of alimentary matter. But starchy and oleaginous substances are also requisite; and the body feels the want of them sooner or later, though it may be plentifully supplied with albumen and fibrin. Finally, the inorganic saline matters, though in smaller quantity, are also necessary to the continuous maintenance of life. In order that the animal tissues and fluids remain in a healthy condition and take their proper part in the functions of life, they must be supplied with all the ingredients necessary to their constitution; and a man. may be starved to death at last by depriving him of chloride of sodium or phosphate of lime just as surely, though not so rapidly, as if he were deprived of albumen or oil.

In the different kinds of food, accordingly, which have been

adopted by the universal and instinctive choice of man, the three different classes of proximate principles are all more or less abundantly represented. In all of them there exists naturally a certain. proportion of saline substances; and water and chloride of sodium are generally taken with them in addition. In milk, the first food supplied to the infant, we have casein which is an albuminoid substance, butter which represents the oily matters, and sugar of milk belonging to the saccharine group, together with water and saline matters, in the following proportions:-'

In wheat flour, gluten is the albuminoid matter, sugar and starch the non-nitrogenous principles.

The other cereal grains mostly contain oil in addition to the

Eggs contain albumen and salts in the white, with the addition of oily matter in the yolk.

The accompanying analyses of various kinds of food are taken from Pereira on Food and Diet, New York, 1843.