This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The precise causes of diminished vitality in old age are not known; but it is worthy of note that the corpuscular elements of the tissues have each, apparently, a life of only limited duration, and that they diminish in number as the individual becomes older, being excessively abundant before birth, and most sparingly distributed in old age. Thus the limit to the duration of vitality, which necessitates a system of reproduction for the continuance of the species, is not confined to the organism as a whole, but is found in the vital elements of which it is composed.
During health, we have seen that though the microscopic constituents of the body are continually changing, and the vital elements dying and replaced by others, yet so perfect are the arrangements for the removal of debris, that no accumulation of effete matter takes place. The baneful effects of even minute quantities of putridity in contact with Jiving textures are now well known to surgeons, and the knowledge furnishes the basis for the proper treatment of sores. But sometimes it happens that in consequence of irritation, either from without or within the body, the nutritive actions in the textural elements are deranged, the living particles attract different constituents from the blood than they are wont to do, act differently on them, and undergo rapid proliferation and disintegration. Examples of such a process are found in ulceration and suppuration, in which, although the pus thrown out consists of living corpuscles, and is the result of vital action, yet there is increased mortality of textural elements and disruption of texture. Sometimes it happens that a whole mass of texture is deprived of vitality by over-irritation, chemical alteration, withdrawal of the supplies of nourishment, or other interference with its nutrition Such a mass undergoes decomposition all the more rapidly that it is in contact with the warm body; it is called a slough or sphacelus, and when the texture which has died is bone, the death is termed necrosis and the dead part a sequestrum.
221. As it is with portions of the body, so also with the whole being; death, considered physiologically, is the permanent cessation of nutrition. The cessation of consciousness is not death; life may continue when consciousness is gone; and the relation of consciousness to the body is that it is dependent on the nutrition of the brain.
Death is sometimes spoken of as beginning either at the heart, the lungs, or the brain, which constitute the tripod of life; but the cessation of circulation, leading to the withdrawal of fit nutriment from all the textures, the nervous system included, and their saturation with debris, may claim to be the immediate cause of death in all instances.
If it be suggested that it would be better to define death as the separation of the spirit from the body, the answer is simply that the presence or absence of the spirit does not immediately affect, as far as can be seen, the vitality of the organism; and that physiology has no means to ascertain the moment of the spirit's withdrawal. Bare, fortunately exceedingly rare, cases have occurred of persons apparently dead returning to life after days of pulseless trance. In. these instances, the body remained, throughout the trance, fit to resume its functions. This it cannot do where there is decomposition. Thus, in the case of muscular fibre, we have seen that even a very slight chemical change is incompatible with vitality, as tested by electric instruments. Decomposition is the infallible evidence of death.