The organs which convert food into nutriment are the organs of alimentation; those which distribute nutriment all over the body are organs of circulation; those which get rid of the waste products are organs of excretion.

20. The organs of alimentation are the mouth, pharynx, gullet, stomach, and intestines, with their appendages. What they do is, first to receive and grind the food. They then act upon it with chemical agents, of which they possess a store which is renewed as fast as it is wasted; and in this way separate it into a fluid containing nutritious matters in solution or suspension, and innutritious dregs or faces.

21. A system of minute tubes, with very thin walls, termed capillaries, is distributed through the whole organism except the epidermis and its products, the epithelium, the cartilages, and the substance of the teeth. On all sides, these tubes pass into others, which are called arteries and veins; while these, becoming larger and larger, at length open into the heart, an organ which, as we have seen, is placed in the thorax. During life, these tubes, and the chambers of the heart, with which they are connected, are all full of liquid, which is, for the most part, that red fluid with which we are all familiar as blood.

The walls of the heart are muscular, and contract rhythmically, or at regular intervals. By means of these contractions the blood which its cavities contain is driven in jets out of these cavities into the arteries, and thence into the capillaries, whence it returns by the veins back into the heart.

This is the circulation of the blood.

22. Now the fluid containing the dissolved or suspended nutritive matters which are the result of the

process of digestion, traverses the very thin layer of soft and permeable tissue which separates the cavity of the alimentary canal from the cavities of the innumerable capillary vessels which lie in the walls of that canal, and so enters the blood, with which those capillaries are filled. Whirled away by the torrent of the circulation, the blood, thus charged with nutritive matter, enters the heart, and is thence propelled into the organs of the body. To these organs it supplies the nutriment with which it is charged; from them it takes their waste products, and, finally, returns by the veins, loaded with useless and injurious excretions, which sooner or later take the form of water, carbonic acid, and urea.

23. These excretionary matters are separated from the blood by the excretory organs, of which there are three-the skin, the lungs, and the kidneys.

Different as these organs may be in appearance, they are constructed upon one and the same principle. Each, in ultimate analysis, consists of a very thin sheet of tissue, like so much delicate blottingpaper, the one face of which is free, or lines a cavity in communication with the exterior of the body, while the other is in contact with the blood which has to be purified.

The excreted matters are, as it were, strained from the blood, through this delicate layer of filtering tissue, and on to its free surface, whence they make their escape.

Every one of these organs eliminates the same products, viz. water, carbonic acid, and urea, or some nitrogenous compound of like import. But they eliminate them in various proportions-the skin giving off much water, little carbonic acid, and

still less urea; the lungs giving off much water, much carbonic acid, and a minimum of urea, or ammonia (which is one of the products of the decomposition of urea); the kidneys separating much water, much urea, and a minimum of carbonic acid.

24. Finally, the lungs play a double part, being not merely eliminators of waste, or excretionary, products, but importers into the economy of a substance which is not exactly either food or drink, but something as important as either, to wit, oxygen. It is oxygen which is the great sweeper of the economy. Introduced by the blood, into which it is absorbed, into all corners of the organism, it seizes upon those organic molecules which are disposable, lays hold of their elements, and combines with them into the new and simpler forms, carbonic acid, water, and urea.

The oxidation, or in other words, the burning, of these effete matters, gives rise to an amount of heat which is as efficient as a fire to raise the blood to a temperature of about 100°; and this hot fluid, incessantly renewed in all parts of the economy by the torrent of the circulation, warms the body, as a house is warmed by a hot-water apparatus.

25. But these alimentary, distributive or circulatory, excretory, and combustive processes would be worse than useless if they were not kept in strict proportion one to another. If the state of physiological balance is to be maintained, not only must the quantity of aliment taken be at least equivalent to the quantity of matter excreted; but that aliment must be distributed with due rapidity to the seat of each local waste. The circulatory system is the commissariat of the physiological army.

Again, if the body is to be maintained at a tolerably even temperature, while that of the air is constantly varying, the condition of the hot-water apparatus must be most carefully regulated.

In other words, a combining organ must be added to the organs already mentioned, and this is found in the nervous system, which not only possesses the function already described of enabling us to move our bodies and to know what is going on in the external world; but makes us aware of the need of food, enables us to discriminate nutritious from innutritious matters, and to exert the muscular actions needful for seizing, killing, and cooking; guides the hand to the mouth, and governs all the movements of the jaws and of the alimentary canal. By it, the working of the heart is properly adjusted and the calibres of the distributing pipes are regulated, so as indirectly to govern the excretory and combustive processes. And these are more directly affected by other actions of the nervous system.

26. The various functions which have been thus briefly indicated constitute the greater part of what are called the vital actions of the human body, and, so long as they are performed, the body is said to possess life. The cessation of the performance of these functions is what is ordinarily called death.

But there are really several kinds of death, which may, in the first place, be distinguished from one another under the two heads of local and of general death.

27. Local death is going on at every moment, and in most, if not in all, parts of the living body. Individual cells of the epidermis and of the epithelium are incessantly dying and being cast off, to be replaced by others which are, as constantly, coming into sepa

rate existence. The like is true of blood-corpuscles, and probably of many other elements of the tissues.

This form of local death is insensible to ourselves, and is essential to the due maintenance of life. But, occasionally, local death occurs on a larger scale, as the result of injury, or as the consequence of disease. A burn, for example, may suddenly kill more or less of the skin; or part of the tissues of the skin may die, as in the case of the slough which lies in the midst of a boil; or a whole limb may die, and exhibit the strange phenomena of mortification.

The local death of some tissues is followed by their regeneration. Not only all the forms of epidermis and epithelium, but nerve, connective tissue, bone, and, at any rate, some muscles, may be thus reproduced, even on a large scale. Cartilage once destroyed is not restored.

28. General death is of two kinds, death of the body as a whole, and death of the tissues. By the former term is implied the absolute cessation of the functions of the brain, of the circulatory, and of the respiratory organs; by the latter, the entire disappearance of the vital actions of the ultimate structural constituents of the body. When death takes place, the body, as a whole, dies first, the death of the tissues sometimes not occurring until after a considerable interval.

Hence it is that, for some little time after what is ordinarily called death, the muscles of an executed criminal may be made to contract by the application of proper stimuli. The muscles are not dead, though the man is.

29. The modes in which death is brought about appear at first sight to be extremely varied. We speak of natural death by old age, or by some of the