In all these cases, however, the products which are separated from the circulating fluids are stored up within the component cells of the fabric, instead of being cast forth from it ; and although the term secretion is commonly applied to the process, yet it would be just as correct to regard it as part of the function of nutrition. It is, in fact, exactly on the same footing with the production of fat in animals.
The absence of necessity for any other form of excretion in plants, than that w hich is carried on through the respiratory process, may be accounted for without much difficulty. A large proportion of the vegetable fabric is (from the nature of its chemical constitution) but little prone to decomposition, and pos sesses a character so permanent, that it may remain almost unchanged for an indefinite time; and those parts which are of softer texture and more actively employed in the vital processes, and which are therefore more prone to decay, are periodically thrown off and renewed. In animals, on the other hand, all the softer tissues have a strong tendency to disintegration, in virtue of their peculiar composition ; and in sonic of them a destruc tive chemical change SeCMS to be the very condition of their functional activity. For the maintenance of their vital energy, there fore, there is needed not merely a constant supply of new material, but a continual re moval of the effete particles. On this last operation, indeed, the continuance of the vital activity of animals is more closely and imme diately dependent, than it is upon the supply of aliment ; for whilst the latter may be in terrupted for a period of considerable dura tion without producing more than debility, the former cannot be checked for many hours (in the warm-blooded animals at least) with out a fatal result. Indeed, if we consider respiration as one of the excreting processes (which it undoubtedly is in a broad and philosophical acceptation of the latter term), we must say that the liberation of effete particles rnay not be suspended for more than a few minutes without death ensuing.
Turning our attention, then, in the first instance, to the excretory organs of animals, we may define them to be groups of cells, placed on the free surface of a membrane, which is directly continuous with that of the exterior of the body, whilst its attached sur face is in relation to the blood-vessels, &c. of the interior ; so that these cells, having grown and developed themselves at the expense of the materials supplied by the bloo.1, are either
cast off entire and conveyed away, or give up their contents by the rupture or deliquescence of their walls ; the products which they have selected or eliminated being thus, in either case, entirely got rid of from the interior of the fabric. The disposition of the membrane on which the cells lie, whether it be spread out on a plane surface, depressed into short rounded follicles, or extended into long and convoluted tubes, is a matter of secondary consequence ; nor is it of more importance whether the follicles be isolated, and discharge their contents by separate outlets, as those of the skin or mucous membrane (fig. 307.), or whether they are aggregated in clusters, and open into a common channel, like those of the liver of the lobster or cray-fish (fig. 308.) ; nor, again, whether the tubes are few and of great length, lying loose in the cavity of the body, and passing frotn one end of it to the other, like the biliary vessels of insects (fig. 309.); or whether they are very nu merous, of less proportional length, and agare gated in a compact mass, as in the kidney of the higher animals.
In all instances, then, the excretory organ essentially consists of a limitary membrane, which forms part of the integument of the body, or of one of its involutions ; and of cells covering the free surface of that membrane, and, consequently, in direct relation with the external surface. Thus we have the limitary membrane of the true skin, and of the mu cous membrane of the alimentary canal which is directly continuous with it, sunk into follicular depressions ; and the free _surfaces of these are lined with cells, the layers of which are continuous with those of the epidermis and of the gastro-intestinal epithe lium respectively. (See MUCOUS MEMBRANE.) We trace inwards another extension of the same membrane along the genito-urinary passages, up to the kidneys, where it forms the walls of the tubuli uriniferi ; and there, too, its free surface is covered with an epithelial layer of cells, which is the efficient instrument of the selection of the constituents of the urinary fluid, and which, when ex uviated, is conveyed along the urinary passages to the exterior of the body. So, too, the hepatic cells, by which the biliary matter is eliminated from the blood, are brought into direct continuity with those of the external surface, through the hepatic ducts and gastro intestinal mucous membrane.