each other not only in their inorganic origin, their crystallizability, and their definite chemical composition, but also in the part which they take in the constitution of the animal frame. They are distinguished in this respect, first, by being derived entirely from without. There are a few exceptions to this rule; as, for example, in the case of the alkaline carbonates, which partly originate in the body from the decomposition of malates, tartrates, &c. These, however, are only exceptions; and in general, the proximate principles belonging to the first class are introduced with the food, and taken up by the animal tissues in precisely the same form under which they occur in external nature. The carbonate of lime in the bones, the chloride of sodium in the blood and tissues, are the same substances which are met with in the calcareous rocks, and in solution in sea water. They do not suffer any chemical alteration in becoming constituent parts of the animal frame.

They are equally exempt, as a general rule, from any alteration. while they remain in the body, and during their passage through it. The exceptions to this rule are very few; as, for example, where a small part of the chloride of sodium suffers double decomposition with phosphate of potass, giving rise to chloride of potassium and phosphate of soda; or where the phosphate of soda itself gives up a part of its base to an organic acid (uric), and is converted in this way into a bi-phosphate of soda.

Nearly the whole of these substances, finally, are taken up unchanged from the tissues, and discharged unchanged with the excretions. Thus we find the phosphate of lime and the chloride of sodium, which were taken in with the food, discharged again under the same form in the urine. They do not, therefore, for the most part, participate directly in the chemical changes going on in the body; but only serve by their presence to enable those changes to be accomplished in the other ingredients of the animal frame, which are necessary to the process of nutrition.

CHAPTER III.

PROXIMATE PRINCIPLES OF THE SECOND CLASS.

THE proximate principles belonging to the second class are divided into three principal groups, viz.: starch, sugar, and oil. They are distinguished, in the first place, by their organic origin. Unlike the principles of the first class, they do not exist in external nature, but are only found as ingredients of organized bodies. They exist both in animals and in vegetables, though in somewhat different proportions. All the substances belonging to this class have a definite chemical composition; and are further distinguished by the fact that they are composed of oxygen, hydrogen, and carbon alone, without nitrogen, whence they are sometimes called the "non-nitrogenous" substances.

1. STARCH (C,,H10010). The first of these substances seems to form an exception to the general rule in a very important particular, viz., that it is not crystallizable. Still, since it so closely resembles the rest in all its general properties, and since it is easily convertible into sugar, which is itself crystallizable, it is naturally included in the second class of proximate principles. Though not crystallizable, furthermore, it still does assume a distinct form, by which it differs from substances that are altogether amorphous.

Starch occurs in some part or other of almost all the flowering plants. It is very abundant in corn, wheat, rye, oats, and rice, in the parenchyma of the potato, in peas and beans, and in most vegetable substances used as food. It constitutes almost entirely the different preparations known as sago, tapioca, arrowroot, &c., which are nothing more than varieties of starch, extracted from different species of plants.

The following is a list showing the percentage of starch occurring in different kinds of food:-1

Pereira on Food and Diet, p. 39. New York, 1843.

When purified from foreign substances, starch is a white, light powder, which gives rise to a peculiar crackling sensation when

Fig. 2.

GRAINS OF POTATO STARCH.

rubbed between the fingers. It is not amorphous, as we have already stated, but is composed of solid granules, which, while they have a general resemblance to each other, differ somewhat in various particulars. The starch grains of the potato (Fig. 2), vary considerably in size. The smallest have a diameter of Too, the largest of an inch. They are irregu larly pear-shaped in form, and are marked by concentric laminæ, as if the matter of which

they are composed had been deposited in successive layers. At one point on the surface of every starch grain, there is a minute pore or

Fig. 3.

STARCH GRAINS OF BERMUDA ARROWROOT.

depression, called the hilus, around which the circular markings are arranged in a concentric form.

The starch granules of arrowroot (Fig. 3) are generally smaller, and more uniform in size than those of the potato. They vary from 2000 to go of an inch in diameter. They are elongated and cylindrical in form, and the concentric markings are less distinct than in the preceding variety. The hilus

has here sometimes the form of a circular pore, and sometimes that of a transverse fissure or slit.

1

Fig. 4.

The grains of wheat starch (Fig. 4) are still smaller than those of arrowroot. They vary from robo to of an inch. in diameter. They are nearly circular in form, with a round or transverse hilus, but without any distinct appearance of lamination. Many of them are flattened or compressed laterally, so that they present a broad surface in one position, and a narrow edge when viewed in the opposite direction.

The starch grains of Indian corn (Fig. 5), are of nearly the same size with

STARCH GRAINS OF WHEAT FLOUR.

those of wheat flour. They are somewhat more irregular and angular in shape; and are often marked with crossed or radiating lines, as if from partial fracture.

Starch is also an ingre

dient of the animal body. It was first observed by Purkinje, and afterward by Kölliker,' that certain bodies are to be found in the interior of the brain, about the lateral ventricles, in the fornix, septum lucidum, and other parts which present a certain resemblance to starch grains, and which have therefore been called "corpora amylacea." Subsequently Virchow corroborated the

Fig. 5.

STARCH GRAINS OF INDIAN CORN.

above observations, and ascertained the corpora amylacea to be

really substances of a starchy nature; since they exhibit the usual chemical reactions of vegetable starch.

The starch granules of the human brain (Fig. 6), are transparent

Fig. 6.

STARCH GRAINS FROM WALL OF LATERAL VENTRICLE; from a woman aged 35.

and colorless, like those from plants. They refract the light strongly, and vary in size from 0 to 10 of an inch. Their average is 100 of an inch. They are sometimes rounded or oval, and sometimes angular in shape. They resemble considerably in appearance the starch granules of Indian corn. The largest of them present a very faint concentric lamination, but the greater number are destitute of any such appearance. They have

nearly always a distinct hilus, which is sometimes circular and sometimes slit-shaped. They are also often marked with delicate radiating lines and shadows. On the addition of iodine, they become colored, first purple, afterward of a deep blue. They are less firm in consistency than vegetable starch grains, and can be more readily disintegrated by pressing or rubbing them upon the glass.

Starch, derived from all these different sources, has, so far as known, the same chemical composition, and may be recognized by the same tests. It is insoluble in cold water, but in boiling water its granules first swell, become gelatinous and opaline, then fuse together, and finally liquefy altogether, provided a sufficient quantity of water be present. After that, they cannot be made to resume their original form, but on cooling and drying merely solidify into a homogeneous mass or paste, more or less consistent, according to the quantity of water which remains in union with it. The starch is then said to be amorphous or "hydrated." By this process it is not essentially altered in its chemical properties, but only in its physical condition. Whether in granules, or in solution, or in an amorphous and hydrated state, it strikes a deep blue color on the addition of free iodine.

Starch may be converted into sugar by three different methods. First, by boiling with a dilute acid. If starch be boiled with dilute