These views are thrown out as hints, rather than as settled ideas. It would be premature, in the present state of our knowledge, to at tempt to decide questions of •such importance without much further examination; and we can only attain a balance of probabilities by interpreting the insufficient results of observa tion by the aid of the best analocies we can find. The whole subject has made immense progress during the few years which have elapsed since the commencement of the pre sent work ; but here, as elsewhere, retardations have occurred through hasty generalization and dogmatic assumptions ; and much patient, well directed, sagacious observation will be needed to unmvel the many intricate questions that yet remain to be solved.
Varying activity of the nutritive processes. —Without any change in the character of the nutritive processes which we have been de scribing, there may be considerable variations in their degree if activity ; and this, either as regards the entire organism or individual parts, • though most commonly the latter. These va riations may be so considerable as to constitute disease ; though there are some which take place as part of the regular series of physiolo , gical phenomena. Thus the nutritive processes "Ulloold have a degree of activity more than suffi cient to supply the waste of the body during the whole period of infancy, childhood, and adolescence, until, in fact, its\ full dimensions are attained ; whilst, on the other_hand, they are usually less rapid than the disintegrating processes in old age, so that the bulk of the body diminishes. Now as the waste of the body, so far from being more rapid in old age than in childhood, is much less so, it follows that the difference in the activity of the nutri tive processes in these two states must be very considerable; and this is manifested, not only in the greater demand for food which exists in the child (relatively to the bulk of its body), but also in the greater quickness and facility with which injuries are repaired. Local va riations may also occur as part of the regular train of vital actions in the adult ; thus we perceive an enormous increase in the amount of tissue contained in the uterus and mammary glands during pregnancy, and a decrease in the bulk of the thymus gland after the first year of infancy.' Now in these cases we see that increased nutrition is invariably connected with increased functional activity, and dimi nished nutrition with diminished functional activity : and this we shall find to be the con stant rule in regard also to those variations which must be considered as abnormal.
Increased nutrition, or hypertrophy, is never known to affect the whole body to a de gree sufficient to constitute disease. It cannot be produced as a consequence of the ingestion of an undue supply of food, for this does not increase the formative activity of the tissues, but merely renders the blood richer in nutritive materials, a part of which the excreting organs are called on to be continually removing, with out its being rendered subservient to the wants of the body ; whilst another part may be em ployed in the nutrition of one particular tissue, the adipose, which has a tendency to increase with the superfluity of non-azotized food, pro vided that the requisite amount of cellular tissue be generated to hold the Fatty matter. But
examples of hypertrophy of particular tissues or organs are very common. Thus any parti cular set of muscles which is subjected to- fre quent and energetic use acquires a great in crease in bulk, as we see in the arms of a black smith or waterman, the legs of an opera-dancer, &c. The hypertrophy of' these muscles is a consequence of their increased functional acti vity, which being produced by an exertion of the will, and unaccompanied with any inju rious effects on the system, can scarcely be re garded as morbid. But there are many in stances in which the involuntary.muscles ac quire a greatly-increased strength, in conse quence of an obstruction to their action which results from disease. Thus we see the right ventricle of the heart become hypertrophied (and dilated at the same time) where chronic pulmonary disease produces a difficulty in the propulsion of the blood through the vessels of the lungs ; the muscular fibres of the bladder become enormously hypertrophied, when stric ture, diseased prostate, or other causes pro duce a demand for increased expulsive force on the part of the bladder ; and those of the stomach also become so in cases of stricture of the pylorus. As an instance of hypertrophy of a secreting organ in consequence of an undue excitement of its function, we may notice the enlargement which usually takes place in the kidney, when its fellow is incapacitated by disease. And the nervous system presents us with a very lemarkable case of hypertrophy of a part resulting from over-excitement of its function ; for if young persons who naturally show precocity of intellect are encouraged rather than checked in the use of their brain, the increased nutrition of the organ (which grows faster than its bony case) occasions pressure upon its vessels, it becomes indurated and inactive, and fatuity and coma are the result. Local hypertrophy may be induced also by local congestions ; but in such cases it will usually be found that the form of tissue produced is of the lowest kind, unless the functional activity of the part be increased by the congestion. Thus when disease of the heart produces long-continued congestion of the lungs, liver, spleen, &c., the.bulk of these organs increases ; but chiefly by the produc tion of an additional amount of interstitial areolar tissue, which may result (as we have seen) from the simple consolidation of fibrin ; and partly also (in the case of the spleen espe cially) by the gorging, of their distensible veins with blood.—One of the least explicable cases of hypertrophy is that which takes place in the thyroid gland, causing bronchocele. So little is known of the normal office of this org,an, that it cannot be determined whether its in creased size be due to an increased activity of its functional operations, or to an unusual formative activity in its tissue, depending on some hidden cause. The connection of this disorder with causes which affect the whole constitution, rather than individual parts, would seem to indicate the former.