230 GENERAL VIEW OF THE FUNCTIONS. 



taken up by a distinct system of vessels, termed Lymphatics; which may be 

 considered as an extension of the Lacteal system through the fabric at large. 

 There is good reason to believe, that the special function of the Lymphatics 

 is, like that of the Lacteals, to minister to Nutritive absorption (although 

 other substances may find their way into them, by the mere physical process 

 of imbibition); the latter being especially destined to take up assimilable 

 matter from the digestive cavity, whilst the former absorb the products of the 

 secondary digestion, which seems to be continually going on in every part of 

 the body. (See Chap. XL, Sects. 1 and 2.) Of these, however, a portion 

 may still be destined to immediate excretion. 



278. The various Secretions which have not already been adverted to, 

 appear for the most part to have for their object the performance of some 

 special function in the system, rather than the conveyance out of it of any 

 substances which it would be injurious to retain. This is the case, for 

 example, in regard to the secretion of the Lachrymal, Salivary, and Mam- 

 mary Glands, as well as with that of the Mucous and Serous Membranes. 

 The Excretion of fluid from the cutaneous surface, however, appears to 

 answer two important purposes, the removal from the body of a portion of 

 its superfluous fluid, and the regulation 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 pre- 

 vented from rising too high; for by the continual excretion from its surface, 

 of fluid which has to be carried oft' by evaporation, a degree of cold is gene- 

 rated, which keeps the calorific 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 process. In all forms of true Secre- 

 tion, the selection of the materials to be separated from the blood, is accom- 

 plished, like selective Absorption, by the agency of cells. These are de- 

 veloped 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 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 Kidneys. This readily permits 

 the escape of aqueous jftwid 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 accumu- 

 lation 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 ( 271). 



279. 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 slates 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 e fleets, whether they be local or general. The flow of the 

 secreted fluids through their efferent ducts, seems to be principally caused by 



