FUNCTIONS OF VEGETATIVE LIFE. 369 



of decomposition and the regulation of its temperature. Just as, by the ac- 

 tion 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 calorific processes in check ; and this excretion is augmented, in pro- 

 portion to the elevation of the external temperature, which seems, in fact, the 

 direct stimulus to the process. In all forms of true Secretion, the selection of 

 the materials to be separated 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 Adipose tissue, we have an instance in which the secreted pro- 

 duct (separated from the blood by the cells of which this tissue essentially con- 

 sists) 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, existing in the Kidneys, which readily permits the 

 escape of aqueous fluid from the capillary vessels, into the urinary canals, by a 

 process of physical transudation, which is altogether distinct from the secretion 

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

 circulating blood. 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. 



385. 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 each glandular organ has an independent ac- 

 tion of its own, in virtue of the endowments of its component cells, can scarcely 

 now be doubted. Still, daily experience teaches that almost every secretion in 

 the body is affected by states of mind, which must operate through the nerves ; 

 and this may be fairly accounted for in part by the remarkable influence which 

 the Nervous system possesses over the Circulation, but must also be in part 

 attributed to the special agency of the Nervous force upon the chemical or vital 

 process of Secretion itself. 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 stimulated by the con- 

 tact of their contents ; but there is also evidence that this contractility may be 

 affected (as it is in those two instances) by the nervous system. Where, as 

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

 is received as feist as it is formed, for the purpose of preventing the inconve- 

 nience which its constant passage 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 Ani- 

 mal existence. 



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

 Animal system, and study those processes in which the development and main- 

 tenance of the material fabric essentially consist, we find them performed under 

 conditions essentially the same as those which obtain in Plants; and we observe 

 that the operations of the Nervous System have none but an indirect influence 

 or control over them. It is, therefore, quite philosophical to distinguish these 

 Organic Functions, or phenomena of Vegetative Life, from those concerned in 

 the Life of Relation, or Animal Life. The distinction is indeed of great practical 

 importance, and lies at the foundation of all Physiological Science; yet it is sel- 



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