FUNCTIONS OF VEGETATIVE LIFE. 83 



the removal from the body of a portion of its superfluous fluid, and the regu- 

 lation of its temperature. Just as, by the action of the Lungs, the conditions 

 are supplied, by which the temperature of the body is kept up to a certain 

 standard, so, by that of the Skin, it is prevented from rising too high; for by 

 the continual excretion from its surface, of fluid which has to be carried off by 

 evaporation, a degree of cold is generated, which keeps the calorifip processes 

 in check; and this excretion is augmented, in proportion to the elevation of 

 the external temperature, which seems, in fact, the direct stimulus to the pro- 

 cess. In all forms of true Secretion, the selection of the materials to be sepa- 

 rated from the blood, is accomplished, like selective Absorption, by the agency 

 of cells. These are developed in the interior of the secreting organ ; and when 

 they are distended with the fluid they have imbibed, their term of life, appears 

 to have expired, so that they burst or liquefy, yielding their contents to the 

 ducts, by which the secreted product is conveyed away. In the case of Adi- 

 pose tissue, we have an instance in which the secreted product (separated 

 from the blood by the cells of which this tissue essentially consists) is not 

 carried out of the body, but remains to form a constituent part of it. The 

 regulation of the amount of fluid in the vessels, is provided in a kind of safety- 

 valve structure, which has been lately shown to exist in the Ipdneys. This 

 readily permits the escape of aqueous fluid from the capillary vessels, into the 

 urinary canals, by a process altogether distinct from the secretion of the solid 

 matter, which it is the office of the kidneys to separate from the circulating 

 fluid. Hence, if the excretion of fluid from the skin be checked by cold, so 

 that an accumulation would take place in the vessels, the increased pressure 

 within them causes an increased escape of water through the kidneys. The 

 relation between the true process of Secretion, which is performed by the 

 selective power of cells, and that of simple Transudation, is the same as that 

 which has been already pointed out between Selective Absorption, and simple 

 Imbibition ( 86). 



96. There is no sufficient reason to believe, that the Nervous System has 

 any more direct influence on the process of Secretion than it has been stated 

 to have on that of Nutrition. That almost every secretion in the body is affected 

 by states of mind, which must operate through the nerves, daily experience 

 teaches ; but the very remarkable degree of control which the nervous system 

 possesses over the circulation, appears sufficient to explain any of these effects, 

 whether they be local or general. The flow of the secreted fluids through 

 their efferent ducts seems to be principally caused by the proper contractility 

 of these, which (like that of the heart and alimentary canal) is directly stimu- 

 lated by the contact of their contents ; but there is also evidence that this con- 

 tractility may be affected (as it is in those two instances) by the nervous system; 

 and thus we have an additional means of influence, by which the nervous 

 system can operate on these processes, since its power is probably not confined 

 to the large ducts, but extends to their ultimate ramifications. Where, as 

 happens in the case of the urinary excretion, there is a reservoir -into which it 

 is received as fast as it is formed, for the purpose of preventing the incon- 

 venience which its constant passages from the body would otherwise occasion, 

 the power of emptying this reservoir is usually placed in some degree under 

 the dominion of the will, although chiefly governed by reflex action. It is 

 obvious that such a provision is by no means essential to the function ; and that 

 it has for its object the adaptation, merely, of that function, to the conditions of 

 .Animal existence. 



97. Thus we see that, when we enter, as it were, into the penetralia of the 

 Animal system, and study those processes of which the Life of the material 

 fabric essentially consists, we find them performed under conditions essentially 

 the same as those which obtain in Plants ; and we observe that the operations 



