and there is a gradual diminution in the stature and bulk of the body, and in its physical powers. All the functions are performed with decreased energy; and the comparative inertness of the nutritive processes is seen in the difficulty with which the effects of severe injuries are repaired, in the length of time requisite for the purpose, and frequently in the imperfection of the result.

622. During the successive periods of life, there are many remarkable changes in the relative nutrition of different organs; which we can attribute to nothing else, than to inherent differences in their own powers of development. Thus, during the early stages of foetal existence, the greatest energy of growth is seen in certain parts which are to answer but a temporary purpose, and which are afterwards completely atrophied. This is the case, for example, with the Corpora Wolffiana, which seems to answer the purpose of temporary kidneys, and in connexion with which the permanent kidneys and the genital organs are developed; and of these bodies, though of large size in the early embryo, and evidently of great importance, no trace whatever is afterwards to be discovered. So in regard to the Supra-Renal capsules, the Thymus and Thyroid glands, and other organs, we find their proportional size the greatest, and their function evidently the most active, during foetal existence and in early infancy; after which their bulk diminishes in proportion to the rest of the body, and their functional activity seems almost at an end.

623. Even in the relative development of the organs which form essential parts of the permanent structure, we find considerable variations at different periods of life. Thus the evolution of the generative system does not usually take place, until the rest of the body is approaching its maturity; but cases sometimes occur, in which this apparatus attains its full development, both in the male and the female, at a very early period of childhood, and seems capable of performing its functions. In the Human species, these organs, when once evolved, remain always in a state of preparation for the performance of their function, unless thay are atrophied through complete disuse, or have lost their vigor by age, or through excessive demands upon their activity; but in most of the lower animals, the development of these organs. is periodical through the whole of life, taking place at a certain season of the year, and being greatly influenced, it would appear, by the external temperature, and by the supply of food. Thus in the Sparrow the testes are no larger than mustard-seeds, during the greater part of the year; but in the spring, they acquire the size of large peas, and it is then only that they possess any procreative power.

624. We are not always to judge of the degree of development of organs, however, by their size alone; for the completeness of their structure, and their aptitude for the performance of their functions, must also be taken into the account. Thus in the new-born infant, the organs of Digestion and Assimilation, though of small size, are so completely formed as to be able at once to take on the duty of receiving and preparing the nutritive materials, provided these are supplied in a form adapted to their powers; the Circulating apparatus is fully adequate to transmit the products of the action of those organs to the body in general, and to bring back the results of its continual decay; and the

Respiratory organs, together with other parts of the Excretory apparatus, are so completely evolved, as to be able to separate the effete matter, and to cast it out of the system with an energy equivalent to that of the organs by which new matter is introduced and appropriated. On the other hand, the Brain, although of larger comparative size at birth, than at any subsequent period of life, is but very imperfectly developed; for its structure is not yet so far completed, as to prepare it for a state of high functional activity. In fact it would seem as if the use of the organ, as called forth by the new circumstances in which the infant is placed as soon as it comes into the world, is essential to its complete development; and the same may be said of the Muscular system.

625. During the whole period of infancy and childhood, the current of nutrition seems peculiarly directed towards the brain; for though its size does not continue to increase, in proportion with that of the remainder of the body, its structure is evidently being rendered more perfect, and its functional activity is excited with remarkable facility. Hence it is peculiarly liable to be acted on by various causes which may produce disease; and the operation of remedies, which specially affect that organ, is far more powerful than at any other period of life. Thus, whilst, a child will bear a fourth, or even a third of the dose of a purgative adequate for an adult, it is strongly affected by an eighth, or even a twelfth of the dose of a narcotic or a stimulant that would be required to produce a corresponding effect in middle life. This peculiar impressibility of the nervous system, resulting from the activity of the nutrient processes which are taking place in it, manifests itself also in other ways; thus children are peculiarly liable to have its powers depressed by any sudden shock, such as a blow, or an extensive burn or laceration; whilst, on the other hand, if the depression be not fatal, they recover from its effects much more speedily than an adult would do from a similar condition.

626. During the periods of youth and adolescence, the chief energy of development (except in regard to the generative system, already noticed), appears to be directed towards the Muscular apparatus; which then increases in vigor, in a degree which surpasses its increase of size; and the circulating and respiratory organs, upon whose energetic action there is then a corresponding demand, are peculiarly liable to disturbance of function, inducing disease in themselves or in other parts. The maladies of this period are for the most part of a sthenic or inflammatory character; resulting, as we shall presently see, from the excessive activity of the assimilating processes, which are disposed to produce more fibrine than the wants of the body require. Or if, on the other hand, there be an imperfect elaboration of the nutrient materials, as happens in the tubercular diathesis, its effects are peculiarly liable to manifest themselves at this period, when the demand for nutritive matter is greatly augmented by the activity of the muscular system.

627. In adult age, there should be such a balance of all the functions, arising from the due development and proper use of each organ, as may preserve the body in the state of health and vigor, without any marked change in the relative dimensions of its different parts, through a long

series of years. The digestive, assimilating, and excreting organs, as they were the first to come to maturity, are commonly the first to fail in their activity; but this is very generally the result of over-exertion of their powers, the amount of food introduced into the stomach being rarely (among the higher and middle classes of society at least) kept down to the real wants of the system. The muscular apparatus usually experiences the effects of this diminished nutrition, sooner than the nervous system; the vigor of the latter being often sustained in a remarkable degree (as shown by the energy of the mental operations) through a protracted life, when it has not been overtasked at an earlier period. The very slight impairment of the nutrition of the nervous system, during the general emaciation which results from a wasting disease, or during that more gradual decline of the bodily vigor which is consequent upon advancing age, is a phenomenon which strongly marks it out as the part of the body, to the maintenance of whose integrity everything else is subservient; and this is still more remarkably shown. in the phenomena of starvation, in which state, notwithstanding the disappearance of the whole of the fat, and the reduction of the weight of the body in general by about 40 per cent., the nervous system appears to lose little or none of its substance (§ 117).

3. Of Death, or Cessation of Nutrition.

628. The general cessation of the Nutritive operations, in Death, usually depends, as formerly explained (§ 65), upon the cessation of the supply of Nutriment, in consequence of the stagnation of the Circulating current; and this stagnation may result from the direct operation of three causes; namely,-failure in the propulsive power of the Heart, or Syncope,-obstruction to the flow of blood through the pulmonary capillaries, consequent upon a deficient supply of air, or Asphyxia,— and a disordered state of the blood itself (§ 534), which at the same time weakens the power of the heart, and prevents the performance of those changes in the systemic capillaries, which afford a powerful auxiliary to the circulation; a mode of death, for which the term Necræmia has been proposed. Each of these conditions may be dependent upon a variety of remote causes, which cannot be here particularized. But it is evident that, when either one of them has been established, the nutritive processes must speedily cease, although they may continue for a short time at the expense of the blood in the capillaries of the part. The cooling of the body is another cause of their cessation; and this is one reason why molecular death (or the death of the individual organs and tissues) follows so much more closely on somatic death (or the cessation of the circulating and respiratory functions), in warm-blooded than in cold-blooded animals. In either case, however, the solid tissues may preserve for a time their independent vitality; and changes may take place in them, which indicate the continuance of their nutritive actions to a certain extent, even when they have been disconnected from the body. There are undoubtedly cases, however, in which the loss of vital power is as complete and immediate in the solids as in the fluids; the want of ability to avail themselves of nutriment being as decided in

the former, as the deficiency of supply is in the latter. This is seen, for example, when death results from a sudden and violent shock, which destroys the vitality of the whole system alike (§ 604); molecular death being here consentaneous with somatic.

629. But as each component part of the Animal fabric has an individual life of its own, so must it have a limited duration of its own; the period of termination of its vital activity, or its death, being quite independent of that of the body at large, excepting in so far as the operations of the latter are requisite to afford it a constant supply of appropriate nutriment, and to maintain its temperature at the proper elevation. It is perfectly compatible, on the other hand, with the Life of the entire organism, that certain parts of it should be continually in course of decay and renewal; and, in fact, we find that the most important parts in the vital functions are performed by tissues whose individual duration is comparatively brief, but which are renewed as fast as they degenerate. We have a well-marked example of this in the case of the leaves of trees, which are the chief agents in the preparation of the nutritious fluid, at whose expense the permanent tissues of the trunk and branches are generated; and although there is nothing in the Animal body at all comparable to the complete exuviation which commonly takes place in the Plant at the close of the season of vegetative activity, yet there is a continual death and separation of parts that have performed their function, which in the end makes up a much larger aggregate. Thus there is scarcely a less complete renewal of the epidermis in Man, in the course of twelve months, than there is in Serpents, Frogs, &c., which throw it off periodically; the only difference being, that in the one case the whole is exuviated and renewed at once, whilst in the other there is a continual interchange. In the exuviation of the antlers of the Deer, and of the milk-teeth of all Mammalia, we have very marked examples of this limitation of the life of individual parts, even in the highest Animals; and as a general proposition it may be stated, that every part must degenerate, when it has gone through the whole series of changes in which its Life consists, and that it must then either die and decay, or must be so altered in its constitution, as to be able to remain inactive without further change.

630. Hence we see that the duration of vital activity must be cæteris paribus, in the inverse ratio of its energy; that is, the life of any part, or of the entire organism, must be shortened by any excess of functional activity; whilst it may be prolonged by such a degree of repose, as does not involve an impairment of its nutrition. We see this most remarkably exemplified in the case of cold-blooded animals; the duration of whose lives, after they have sustained some fatal injury (such as the removal of the heart or of the lungs), or are placed in any other circumstances incompatible with its continuance, is in the inverse proportion to the elevation of the temperature to which they are exposed, and therefore to the degree of their vital activity (§ 128). Now although this variation is comparatively little observable in the rate of life of that portion of the fabric of warm-blooded animals which is concerned in their organic functions (the temperature to which it is subjected being nearly constant), it is clearly seen in those organs whose functional

activity is more under the control of the individual, and is therefore less constant. Thus, in Man, we continually notice that the duration of the powers of the Brain and the Generative system is the longest, when these organs have been moderately exercised; and that it is much curtailed by the excessive use of either. The duration of their activity, however, is not increased by partial or entire disuse of the organs; for this induces a state of atrophy, on the principles already mentioned. Now we have every reason to believe, that what is true of individual parts and organs, is true also of the whole structure; and that the existence of the entire bodily fabric may thus come to an end, without any special disease, in consequence of the limit originally set to its powers of self-renovation. It is but rarely, however, that this occurs; the various accidents of life, the neglect of ordinary precautions for the preservation of health, and hereditary tendencies to various kinds of morbid action, being too frequently the means of cutting off the term of Human existence, long before its natural expiration.

4. Disordered Conditions of the Nutritive Processes.

631. Having thus passed in review the general conditions, under which the ordinary Nutritive processes take place, it may be well to add a few words in relation to two of their abnormal states; one or other of which is concerned in a very large proportion of the diseases that afflict the human race. In one of these, there is a tendency to the excessive production of fibrine in the blood; whilst in the other, there is a want of the proper nutritive power in the tissues, which is apparently due to an imperfect elaboration of that important material. The one of these conditions is termed Inflammation; whilst the other, which . is less active, but more insidious, is known as the Tubercular Diathesis. 632. The extraordinary tendency to the production of Fibrine in the blood, which has been already noticed (§ 531) as one of the most important characters of Inflammation, seems to be always conjoined with a depressed vitality of the tissues of some part of the body, which indisposes them to the performance of their regular nutritive operations; and this part may undergo a variety of changes, according to the degree in which it is affected. The depressed condition of its nutritive operations involves, on the principles explained in the preceding chapter, a languor in the movement of blood through it, together with a distensible state of the capillaries, which causes them to contain a far greater amount of that fluid than under ordinary circumstances. On the other hand, there is a tendency to the production of an increased amount of plastic material in the blood, as if for the reparation of the part whose vitality is lowered. What is the immediate cause of this production is still doubtful; but we see the consequences of the deficiency of it in those asthenic or unhealthy Inflammations, which so frequently involve the destruction of a large amount of tissue; the degeneration of the part first affected soon extending itself to others, if there be no limit set up by the reparative powers of the blood.-In ordinary or sthenic Inflammation, of which the increase of fibrine in the blood, and a diminution in the power of appropriating it on the part of the tissues, are the most characteristic