NATURE OF THE SECRETING PROCESS. 393 



CHAPTER IX. 



OF SECRETION. 



1. Of the secreting process in general ; and of the instruments by which 



it is ef 



710. WE have seen that, in the process of Nutrition, the circulating 

 current not only deposits the materials, which are required for the 

 renovation of the solid tissues ; but also takes back the substances, 

 which are produced by the decay of these, and which are destined to 

 be thrown off from the body. We have also seen, that it supplies the 

 materials of certain fluids, which are separated from it to effect various 

 purposes in the economy; such as the Salivary and Gastric fluids, which 

 have for their office to assist in the reduction of the food. Now the 

 process b^L which the fluids of the latter kind are separated from the 

 Blood, is precisely the same in character as that by which the products 

 of decay are eliminated from it ; and the structure of the organs con- 

 cerned in the two is essentially the same. Hence both processes are 

 commonly included under the general term Secretion, which simply 

 denotes separation. Considered in its most general point of view, this 

 designation may be applied to every act, by which substances of any 

 kind are separated from the blood. Thus the function of the cells, 

 which are concerned in the elaboration of the organizable plasma, may 

 be termed one of Secretion, because they draw from the blood a supply 

 of Albumen, upon which their converting action is exercised ; but as 

 the product of their operation is returned to the blood again, and is 

 employed for higher purposes in the economy, the process is usually 

 termed Assimilation. In the same manner, the elaborating action of 

 the Lymphatic Glands, with the Spleen, Thymus Gland, &c., is not 

 usually termed Secretion ; since, although it is exercised upon matter 

 drawn from the blood, the product appears to be delivered back into 

 the circulating current, through the medium of the Lymphatic System. 

 (CHAP, v.) With much more justice, however, the process of Respira- 

 tion may be regarded as one of Secretion ; for it consists, as we have 

 seen, in the constant elimination of a substance from the blood, which 

 cannot be retained in it without the most injurious consequences. 



711. There is an important difference in the characters of the prin- 

 cipal products of the Secreting process, which is connected with the 

 purposes that are to be answered by their separation. Some of these 

 products are altogether different from the ordinary constituents of the 

 animal fabric, and from the materials which the blood supplies for the 

 nutrition of these. So different are they, that their presence in the 

 circulating current has an injurious effect ; and the great object of their 

 separation is the maintenance of the purity of the blood. These poi- 

 sons, for such they may be considered, are generated in the system by 

 the decay and decomposition to which its several parts are liable ; and 

 they are just as noxious to it, as if they were absorbed from without. 



