Food may consist of vegetable or animal substances, or of such as are the production of animals. Of the latter, milk is the most important. It is the natural aliment of infancy, and enters into the diet of every people that possess the animals from which it may be advantageously obtained. The milk of the cow is most familiar to us, but that of the ass, mare, sheep, goat, and camel, is used for food in various parts of the world; and the milk of the ass and mare is employed in our own country as a medicine in certain diseases.
The milk of every animal is distinguished by certain properties from that of all others; and by no property more than the degree of richness; that is, the quantity of nourishing matter which it contains. The quality of the milk of the same animal varies with its food; thus, the milk of the cow, when she is fed in a dry mountain pasture is richer, though less abundant, than when she browses on more succulent herbage. The difference in question is still more remarkable in goat's milk. When this animal browses on the shrubs and shoots of her native mountains, her milk is much richer than when she is in a state of domestication. The richness of milk depends also on another circumstance: when the breast or udder is distended, that which is first yielded, is thin and watery: as it flows, it becomes richer; and that which flows last is the richest.
When a certain given quantity of the following kinds of milk -- that of the woman, cow, goat, and ass, is slowly evaporated, till what remains is dry, we find that the quantity of this solid matter, which determines the richness of the milk, is in the following relative proportions.
The properties of cow's milk are these; when it has stood some time it separates into cream and blue milk. Cream is composed of an oil, a little curd, and whey or serum: when churned, it separates into butter and blue milk. When to creamed or blue milk, a little rennet is added, curds and whey are produced: when carefully parted from the curd, and slowly evaporated, the whey deposits white crystals, which are the substance known by the name of "sugar of milk."
Ewe's milk much resembles that of the cow, but yields rather more cream and curd; it is therefore the richest milk we know of. Next to cow's in richness is the milk of the goat; it yields the most curd of any, but less butter than the two former. Mare's milk is thinner than that of the cow, affords less curd, and its cream by churning does not yield butter. The milk of the ass is thin, sweet, and pleasant; affording only small quantities of butter and curd, but more sugar of milk than the milk of any of the animals already mentioned. In all respects it strikingly resembles the milk of the human female.
Woman's milk varies in its qualities more than any other; a fact we might a priori expect from the endless variety in the mode of living, and circumstances of women. In those who are delicately formed, and eat sparingly, it is pale and watery, becoming whiter and thicker, however, on improving the diet. Its taste is sweet, from the large portion of the sugar of milk which it contains: in general, it is thinner and whiter than the milk of the cow, yields more cream than the milk of any animal, except the sheep, and sugar of milk next in quantity to ass's milk. It does not curdle or coagulate so as to separate its curd by any of the methods which succeed with the milk of inferior animals; a circumstance of some consequence to be remembered: neither does its cream yield butter, however long it is churned. Woman's milk is about as rich as the milk of the ass, somewhat richer than that of the mare, and considerably thinner and poorer than the milk of most other animals. It may here be observed, as it bears on the subject of nursing, that the quantity of its curd increases in proportion to the time which has elapsed since the period of parturition, -- a fact equally true as regards cow's milk, and, so far as is known, the milk of all other animals.
Woman's milk is said to have one peculiar and important quality, namely, that it will stand in an open vessel for weeks, or even months, without becoming putrid, or in any considerable degree acid: whereas cow's milk will change, first to the acetous, then to the putrefactive fermentation in a few days. "I once," says Dr. Clarke, of Dublin, "kept a few ounces of the milk of a nurse who had been delivered about 6 or 7 days, in a bottle moderately corked; it stood on the chimney piece for more than two years and was frequently opened to be examined. At the end of this period it shewed evident marks of moderate acidity, but was not putrescent." An experiment of a similar kind, which I attempted during the extreme heat of last summer, had a different result. Eight ounces of woman's, cow's, and ass's milk were put separately into three clean vials, which were afterwards lightly corked, and left on a shelf in a cool room. By the third day the acetous fermentation had begun in all of them, and in a few more days each emitted an offensive odor. The experiment, however, deserves being repeated in cool weather. 
As articles of diet, milk and its preparations hold a middle rank between animal and vegetable food. From what has been said, it follows, that the milk of the sheep, and that of the goat are less easily digested than cow's milk; and the milk of the human female, ass, and mare, the most digestible of all; and hence the best suited to the infantile stomach.
It should be noticed, that the process of boiling, by materially altering the component parts of milk, injures its qualities. In cooking this article for the nursery, therefore, the heat employed should be managed with due regard to this fact. Milk, also, which has stood for some time, differs in its sensible qualities from that which is newly drawn from the udder or breast; that something, in the mean time is lost, we may judge from the fragrant volatile particles which continue to escape until the milk is cold.
Serum of milk, or whey, deserves particular notice. Of all kinds of beverage it is the best for children, being of an agreeable flavour, bland, nutritive, and mildly laxative: in some constitutions, its effects are even smartly purgative. It would be an unspeakable benefit in large towns, could it be more easily procured; and no doubt, were its value generally known, the supply would soon equal the demand.
Butter, and cheese (which is a compound of curd and butter) are rather difficult of digestion, particularly the latter, which ought therefore to be given sparingly, if at all, in the early periods of infancy. They are both excellent articles of diet, nevertheless, when the 4th or 5th year has been attained. Whatever may be said against them ex cathedra, common prejudice is in their favour; and prejudice, in this instance, is right. Butter is to be given cold, or melted at a heat no higher than that of boiling water; when fried or melted at high temperature, it is altogether unfit for children.
As an article of infant's food, sugar is well worthy of notice, both on account of its use and abuse. It is very nutritive, and is said to be sufficient of itself for the maintenance of vigorous health, even in those who are laboriously employed. In the West India Islands it is used by the Negroes during crop time as their only food. It is known also to fatten poultry, pigs, and even horses.  Unless when the stomach is in order, and vigorous, sugar is apt to disagree with it, rapidly assuming the acetous fermentation, which is followed by heart-burn, sour eructations, and diarrhea. It is generally believed that sugar injures the teeth. The negroes however who eat is so freely, are remarkable for the beauty of their teeth. The truth is, it has been observed that young people who are pampered with sweet-meats have decayed teeth, and thus the inference, somewhat illogical indeed, that the sugar must be the offending cause. The sugar in such cases, has certainly some effect in the destruction of the teeth, but in no degree more than most of the other matters that enter into the confectionary, which is made of articles extremely difficult of digestion. When eaten freely and often, sweet-meats produce in children habitual disorder of the first passages, as is shewn by paleness of countenance, languor, a capricious appetite, tumid belly, flabbiness of the flesh, irregular bowels, and black or decayed teeth: the latter appearance is the effect of the continual ascent of acid from the stomach, where it is generated, into the mouth. Such is the explanation of this symptom. Let sugar and sweet-meats be given only in moderate quantities, and no mischief to the teeth will follow.
Molasses, as an ingredient of children's diet, is in some respects more valuable than sugar: for while the latter is only allowable as a condiment, molasses may be given freely as affording an agreeable and efficacious laxative. Boiling it mixed with charcoal is said to remove its peculiar luscious flavour. 
Concerning the properties of farinaceous substances, little need be said, as they are well known. Those farinacia in common use, as the flour of wheat, oatmeal, rice, the potato and its fecula or starch, arrow-root (a starch differing little in its useful qualities from that of the potato) salep, sago, tapioca, and some others, are highly nutritious, and enter more or less into the diet of the young in every country.
Flour of wheat, though less nutritive than oatmeal, is more digestible: the latter circumstance is also true of leavened, compared to unleavened, bread. The different starches, of which arrow-root is perhaps the best, are nutritive, easy of digestion, and answer well as articles of diet in early infancy.
Esculent vegetables form an agreeable and useful addition to the food in every period of life after the first dentition. When given to children, special care should be taken that they are good of their kind and in season. Of the many esculents in common use in this country, none are so digestible, and therefore so proper for children as the broccoli, cauliflower, and turnip. The latter, being mildly aperient, is perhaps the most proper of any. In cases where there exists a tendency to scrofula, or rickets, potatoes should be given sparingly, if at all; in general, however, they enter with great propriety into the diet of the young, care being taken that they are dry and mealy. When soft and watery they ought to be considered as positively unwholesome.
Fruit which is ripe is proper for children, but only under strict limitation. In a sour or unripe condition it is singularly pernicious. Of our indigenous fruits, the apple, pear, and strawberry, are unquestionably the most wholesome; and the orange, of those fruits that are imported. There are strong prejudices against stone fruits, the ground of which I have never yet been able to learn: the greatest evil connected with their use in children is the risk of the stones being swallowed, -- a circumstance to be guarded against, as they have been known to lodge in the bowels, forming the nucleus of intestinal calculi, or more commonly giving rise to severe and dangerous bowel complaints. The mucilaginous fruits, as gooseberries and currants, are more hurtful than is generally imagined: when eaten in large quantities, their hard and indigestible seeds irritate the delicate lining of the bowels, so as frequently to produce bloody stools, and other dysenteric symptoms. Their use, however, in moderation, is not to be forbidden. Preserved and dried fruits, with the exception of the prune and tamarind, should form no part of the nursery dessert. Raisins and Smyrna currants, though both are so much used, are nevertheless unfit for children. The skin of the raisin, in particular, has been found to resist even the digestive powers of a hog, passing through the animal unchanged. 
Some kinds of animal food enter properly into the aliment of children, as soon as they have teeth to masticate it. Beef and pork are accounted highly stimulating; that is, they increase the heat of the body, and the strength and rapidity of the circulation, more than other kinds of animal food. Mutton, veal, venison, fowls, and most kinds of fish, are, to use a popular but incorrect phrase, lighter than the former; which means that they are less stimulating, and therefore, in general, better suited to the young. The digestibility of the lean of each kind is said to be in the following order: venison, game, fowls, mutton, beef, lamb, pork, veal.  The fat of all animals, though highly nutritive, is stimulating and difficult of digestion: the same may be said (as respects indigestibility) of smoked and salted flesh. All kinds of oily fish, whether finned or shell, are stimulating and difficult of digestion; and, therefore, unfit for children: as examples, the salmon and the lobster may be mentioned. On the other hand the sparling, pike, sole, haddock, and some others, though less nourishing than the flesh of land animals, are easy of digestion, and, where variety is thought essential in the food of children, may be advantageously admitted into the bill of fare.
With respect to the methods of cooking animal food for the nursing, boiling and roasting are the best, or perhaps the only modes that are allowable. Fried, broiled, baked, and minced meats of every description; sauce, of whatever kind, except butter melted with the precaution already given, are forbidden to children under the 7th year. The gravy or dripping from roasted meat, which is little else than burnt grease, is in very general use, particularly, poured upon potatoes. This mess, however, is improper, except in the case of very robust children, and is unquestionably more suitable for the stomach of a field labourer. Gravy obtained from the insider of meat moderately roasted is not objectionable.
When animal food is not given in substance, the best of all its preparations in the infusion, or, as it is called, tea. This is preferable to every kind of broth, and is more expeditiously cooked. It is made thus, boiling water is poured on the meat (beef, mutton, veal, or fowl,) cut into small pieces, after standing for some time, it is decanted off, well skimmed, and a little salt being added, it is fit for use. It may with propriety be given to the youngest infant, and indeed should form part of the aliment of such as are reared without the breast. Oyster tea made in the same manner is highly recommended by Dr. Dewees: of its excellence and fitness there can be no question.
Pickles and spices of every kind come under the head of forbidden articles. In some cases no doubt the latter may be ordered for medicinal effect.
The kinds of beverage suitable for children may be enumerated in three words, -- water, milk, whey. Wine, in some instances, is necessary as a tonic. Spirits, in every form, are to be utterly prohibited; as also, in general, are fermented liquors.
With respect to the infusions of tea and coffee, they would be highly improper, if given alone; but when largely diluted with milk, they are perfectly harmless. In general, tea is much relished by children; perhaps not the less, from its furnishing the usual apology for those noisy migrations, which so frequently take place, from the nursery to the family table.
 On turning to Dr. Underwood's work, I find that in the many experiments on woman's milk, conducted by him, no result resembling that of Dr. Clarke's ever occurred. The truth I believe is, that woman's milk does not become acid quite so soon as the milk of the cow.
 The experiments of Magendie -- "On the nutritive qualities of substances which do not contain azote" are extremely curious. A small dog, three years old, was made to feed exclusively on sugar and distilled water. For eight days it continued in excellent health; began to fall off in the second week; in the third week became emaciated, extremely weak, and the eyes inflamed and ulcerated. On the thirty-second day of the experiment it died. Successive experiments of the same kind were performed with gum, olive oil, and butter, with similar results. Magendie's conclusion, is, that food in order to support life must be in part, at least, composed of matters containing azote. This is objected to by Dr. Paris. The latter thinks that no animal can long exist on highly concentrated nourishment, such as was each of the substances used in the above experiments, and that this is the explanation of the effects produced on the dogs. It is to be hoped Dr. Paris will put his opinion to the test of experiment. The controverted point is one of deep interest.
 I believe it is Dr. Paris who asserts this fact in his Pharmacologia. Not having the work at hand, I cannot refer more particularly to his account of the process.
 A friend, in perusing this work when in manuscript, inserted the following remark, the accuracy of which I cannot doubt -- "It has been said that the digestive powers of the hog are much weaker than those of most other animals: I believe that grains will pass through a hog nearly unchanged."
 The term "digestibility" is used here to signify facility of being digested, not the quantity of digestible matter afforded by each kind of flesh.