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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 separate 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 said not to be 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 endless forms of disease; of violent death by starvation, or by the innumerable varieties of injury, or poison. But, in reality, the immediate cause of death is always the stoppage of the functions of one of three organs; the cerebro-spinal nervous centre, the lungs, or the heart. Thus, a man may be instantly killed by such an injury to a part of the brain which is called the medulla oblongata (see Lesson XI.) as may be produced by hanging, or breaking the neck.
Or death may be the immediate result of suffocation by strangulation, smothering, or drowning,--or, in other words, of stoppage of the respiratory functions.
Or, finally, death ensues at once when the heart ceases to propel blood. These three organs-the brain, the lungs, and the heart-have been fancifully termed the tripod of life.
In ultimate analysis, however, life has but two legs to stand upon, the lungs and the heart, for death through the brain is always the effect of the secondary action of the injury to that organ upon the lungs or the heart. The functions of the brain cease, when either respiration or circulation is at an end. But if circulation and respiration are kept up artificially, the brain may be removed without causing death. On the other hand, if the blood be not aërated, its circulation by the heart cannot preserve life; and, if the circulation be at an end, mere aëration of the blood in the lungs is equally ineffectual for the prevention of death.
30. With the cessation of life, the everyday forces of the inorganic world no longer remain the servants of the bodily frame, as they were during life, but become its masters. Oxygen, the sweeper of the living organism, becomes the lord of the dead body. Atom by atom, the
complex molecules of the tissues are taken to pieces and reduced to simpler and more oxidized substances, until the soft parts are dissipated chiefly in the form of carbonic acid, ammonia, water, and soluble salts, and the bones and teeth alone remain. But not even these dense and earthy structures are competent to offer a permanent resistance to water and air. Sooner or later the animal basis which holds together the earthy salts decomposes and dissolves-the solid structures become friable, and break down into powder. Finally, they dissolve and are diffused among the waters of the surface of the globe, just as the gaseous products of decomposition are dissipated through its atmosphere.
It is impossible to follow, with any degree of certainty, wanderings more varied and more extensive than those imagined by the ancient sages who held the doctrine of transmigration; but the chances are, that sooner or later, some, if not all, of the scattered atoms will be gathered into new forms of life.
The sun's rays, acting through the vegetable world, build up some of the wandering molecules of carbonic acid, of water, of ammonia, and of salts, into the fabric of plants. The plants are devoured by animals, animals devour one another, man devours both plants and other animals; and hence it is very possible that atoms which once formed an integral part of the busy brain of Julius Cæsar may now enter into the composition of Cæsar the negro in Alabama, and of Cæsar the house-dog in an English homestead.
And thus there is sober truth in the words which Shakespeare puts into the mouth of Hamlet
"Imperial Cæsar, dead and turned to clay,
Oh that that earth, which kept the world in awe,
THE VASCULAR SYSTEM AND THE CIRCULATION.
1. ALMOST all parts of the body are vascular; that is to say, they are traversed by minute and very close-set canals, which open into one another so as to constitute a small-meshed network, and confer upon these parts a spongy texture. The canals, or rather tubes, are provided with distinct but very delicate walls, composed of a structureless membrane (Fig. 3 A, a), in which at intervals small oval bodies (Fig. 3 A, 6), termed nuclei (see Lesson XII. § 2), are imbedded.
These tubes are the capillaries. They vary in diameter from 2000th to 10th of an inch; they are sometimes disposed in loops, sometimes in long, sometimes in wide, sometimes in narrow meshes and the diameters of these meshes, or, in other words, the interspaces between the capillaries, are sometimes hardly wider than the diameter of a capillary, sometimes many times as wide (see Figs. 16, 20, 32, 33, and 37). These interspaces are occupied by the substance of the tissue which the capillaries permeate (Fig. 3 A, c), so that the ultimate anatomical components of every part of the body are, strictly speaking, outside the vessels, or extra-vascular.
But there are certain parts which, in another and broader sense, are also said to be extra-vascular or nonvascular. These are the epidermis and epithelium, the nails and hairs, the substance of the teeth, and the cartilages; which may and do attain a very considerable thickness or length, and yet contain no vessels. However, as we have seen that all the tissues are really extra
vascular, these differ only in degree from the rest. The circumstance that all the tissues are outside the vessels by no means interferes with their being bathed by the fluid which is inside the vessels. In fact, the walls of the B
A. Diagrammatic representation of a capillary seen from above and in section: a, the wall of the capillary with 6, the nuclei ; c, nuclei belonging to the connective tissue in which the capillary is supposed to be lying; d, the canal of the capillary.
B. Diagrammatic representation of the structure of a small artery: a, epithelium; b, the so-called basement membrane; c, the circular non-striated muscular fibres, each with nucleus d; e, the coat of fibrous tissue with nuclei f.
capillaries are so exceedingly thin that their fluid contents readily exude through the delicate membrane of which they are composed, and irrigate the tissues in which they lie.
2. Of the capillary tubes thus described, one kind contains, during life, the red fluid, blood, while the others are filled with a pale, watery, or milky fluid, termed lymph, or chyle. The capillaries, which contain blood, are con