32. The chief intermittently active sources of loss to the blood are found among the glands proper, all of which are, in principle, narrow pouches of the mucous membranes, or of the integument of the body, lined by a continuation of the epithelium, or of the epidermis. In the glands of Lieberkühn, which exist in immense numbers in the walls of the small intestines, each gland is nothing more than a simple blind sac of the mucous membrane, shaped like a small test tube, with its closed end outwards, and its open end on the inner surface of the intestine (Fig. 39, 1). The sweat-glands of the skin, as we have already seen, are equally simple, blind, tube-like involutions of the integument, the ends of which become coiled up. The sebaceous glands, usually connected with the hair sacs, are shorter, and their blind ends are somewhat subdivided, so that the gland is divided into a narrow neck and a more dilated and sacculated end (Fig. 39, 5). The neck by which the gland communicates with the free surface is called its duct. More complicated glands are produced by the elongation of the duct into a long tube, and the division and subdivision of the blind end into multitudes of similar tubes, each of which ends in a dilatation (Fig. 39, 6). These dilatations, attached to their branched ducts, somewhat resemble a bunch of grapes. Glands of this kind are The salivary glands and the pancreas

called racemose. are such glands.

Now, many of these glands, such as the salivary, and the pancreas (with the perspiratory, or sudoriparous glands, which it has been convenient to consider already), are only active when certain impressions on the nervous system give rise to a particular condition of the gland, or of its vessels, or of both.

Thus the sight or smell, or even the thought of food, will cause a flow of saliva into the mouth; the previously

B. The same, with only one layer of cells, a and b, the so-called basement membrane between the epithelium, a, and dermis, c.

1. A simple tubular gland.

2. A tubular gland bifid at its base. In this and succeeding figures the blood-vessels are omitted.

3. A simple saccular gland.

4. A divided saccular gland, with a duct, d. 5. A similar gland still more divided.

6. A racemose gland, part only being drawn.

quiescent gland suddenly pouring out its fluid secretion, as a result of a change in the condition of the nervous system. And, in animals, the salivary glands can be made to secrete abundantly, by irritating a nerve which supplies the gland and its vessels. How far this effect is the result of the mechanical influence of the nerve on the state of the circulation, by widening the small arteries (see p. 51) and so supplying the gland with more blood, and how far it is the result of a more direct influence of the nerve i pon the state of the tissue of the gland itself, making the cells secrete, just as a nerve when stimulated makes a muscle contract, is not at present finally determined.

The liquids poured out by the intermittent glands are always very poor in solid constituents, and consist chiefly of water. Those poured on to the surface of the body are lost, but those which are received by the alimentary canal are doubtless in a great measure re-absorbed.

33. The great intermittent sources of gain of waste products to the blood are the muscles, every contraction of which is accompanied by a pouring of certain products into the blood. That much of this waste is carbonic acid is certain from the facts (a) that the blood which leaves a contracting muscle is always highly venous, far more so than that which leaves a quiescent muscle; (b) that mus cular exertion at once immensely increases the quantity of carbonic acid expired; but whether the amount of nitrogenous waste is increased under these circumstances, or not, is a point yet under discussion.

LESSON VI.

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

matter.

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