Now, this interstitial change must take place constantly, during the whole life of the entire structure ; but its activity varies ac cording to certain conditions to which the fabric is subjected. One of the most im portant of these conditions is heat. It is well known that the tendency to decomposition, which is charactetistic of organic compounds, is dependent upon the heat to which they are subjected : thus a compound which passes rapidly into decomposition at 100°, shall be much less prone to decay at 60°, and shall be permanent at 32°. And again, the vital ac tivity of the several parts of the organised fabric is so dependent upon the same stimulus, that a very moderate depression of tempera ture serves to reduce it, or even to suspend it altogether. Now, when the activity' of a part is thus reduced, so that it lives more slowly, its duration is proportionally increased, and interstitial change and renewal are scarcely required. We have obvious examples of this in the activity of all the functions in warm blooded as compared with cold-blooded ani mals ; in the superior entrgy of all the vital operations of birds, whose temperature is 10° or 12° above that of the Mammalia ; and, on the other hand, in the torpor of cold-blooded animals, and of hyhernating Mamrnalia, when the temperature of their bodies is depressed nearly' to the freezing point. In the state of greatest activity, all parts of the body live fast ; their duration is proportionally dimi nished ; interstitial death and decomposition are continually. taking place ; the results of this decomposition have to be got rid of from the body ; and a corresponding demand is set up for nutrient materials, to be applied to the renovation of the structure. 0,n the other hand, a reduction of temperature, which di ,ninishes the vital activity of the living tissues, tends also to increase their duration ; and this not merely by causing them to live more slowly, but by obstructing the spontaneous decom position of their organic constituents. This reduction may be carried to such an extent as, on the one hand, to suspend all vital action, whilst, on the other, it prevents decomposi tion ; so that the body remains in a state of dormant vitality, undergoing no change what ever for an indefinite period, but ready for a renewal of its vital activity whenever an in crease of temperature shall awaken its slum bering energies. (See LIFE.) The more nearly a living structure is reduced to this con dition, the less interstitial change does it un dergo ; the less nutriment, therefore, does it require ; and the less effete matter is there to be thrown off.
The activ:ty of that spontaneous interstitial change, w hich takes place as a part of the mere vegetative life of the animal organism, further varies in accordance with the period of life of the fabric taken as a whole. Thus all the tissues, even those most consolidated, are undergoing continual changes in the young animal, in which the processes of decay and renewal go on much faster than in the adult ; and in the adult, than in the aged person. Thus we have seen that the duration of the deciduous teeth is very limited; whilst that of the permanent teeth may be coeval with the life of the entire animal, little or no inter stitial change taking place in them during the whole of that period. So also the component parts of the bony structure, which in the adult are almost permanent, and in the aged become so remarkably solidified that little or no in terstitial change can take place in them, are liable in the growing child to continual de composition ; no part of the substance of a long bone having any permanence, but the in terior layers of the shaft being removed (by absorption,it is commonly said, but the absorp tion being probably in reality preceded by de generation), so as to enlarge the medullary ca vity, in proportion as new layers are formed on the external surface, This may be partly ac counted for by the imperfect degree in which, so long as the entire organism is undergoing rapid increase, the normal structure is de veloped in any one portion of it ; for as the degree of consolidation is less, the tendency to decay will be greater. But this explanation
is not in itself sufficient, and we must be con tent, for the present, to regard it as a general law, that, with the advance of life, the duration of the individual components of the organism increases, whilst their functional activity diminishes. (See AGE.) 3. But, in the third place, the exercise of the Animal functions seems to be essentially destructive of the structures which are their instruments ; every operation of the muscular and nervous systems appearing to require, as its necessary condition, a disintegration of a certain portion of their tissues, probably by the union of their elements with the oxygen supplied by arterial blood. The duration of the existence of these tissues may be clearly shown to vary inversely with the use that is made of them, being less as their functional activity is greater. Hence, when an animal is very inactive, it requires but very little nutri tion ; if in moderate activity, there is a mode rate demand for food ; but if its nervous and muscular energy be frequently and powerfully called into exercise, the supply of aliment must be increased, in order to maintain the vigour of the system. In like manner, the amount of the effete matters, which result from the disintegration and decay of those tissues, must increase with their activity, and diminish in proportion to their freedom from exertion.
4. A necessity for the secreting process may further arise within the system froni the ingestion of superfluous aliment. This would not be the case, if the amount of food prepared by the digestive process, and taken up by absorption into the current of the cir culation, were always strictly proportional to the demand for nutriment created by the wants of the system. There can be no doubt that almost every individual who is not re strained by considerations of economy, or by fear of unpleasant consequences, from indulg ing his natural appetite, really takes in more food than the wants of his system absolutely require ; and all that is not appropriated to the reparation of the waste, or to the increase in the weight of the body, must be thrown off by the excreting organs, without having ever been converted into organised tissue. The superfluous portion of the non-azotised con stituents of the food may be deposited as fat in those individuals vvho have a disposition to the production of adipose tissue ; but the azotised constituents cannot be applied in like manner to the unlimited increase of the mus cular and other tissues ; and that which is not speedily converted into organisable material, and drawn off' from the blood by conversion into organised tissue, would accumulate in juriously in the circulating current, and would taint it by decomposition, if it were not con tinually removed by the excreting processes.