SECRETION. 



457 



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, which 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 from 

 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 who 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. 



5. Again, it cannot be deemed improbable 

 that the changes which the crude aliment un- 

 dergoes, from the time of its first reception 

 into the absorbents and blood-vessels, to that 

 of its conversion into organised tissues and 

 into the materials of secretions eliminated for 

 some special purpose in the economy, involve 

 the liberation of many products, of which the 

 elements are superfluous, and therefore injuri- 

 ous to the system if retained within it. The 

 condition of organic chemistry, however, is not 

 at present such as to admit of anything being 

 advanced with certainty under this head. 



From these various sources, then, a lanje 

 amount of effete matter is being continually 

 received back from the tissues into the current 

 of the circulation, or is generated in the blood 

 by the changes to which it is itself subject ; 

 and it is one great object of the secreting ap- 

 paratus, to free that fluid of the products which 

 would rapidly accumulate in it, but for the 



