THE GLANDS— DEPURATION. 281 



out through an excretory duct, whose internal surface is continu- 

 ous with the mucous membrane, or skin. A simple gland is, in 

 fact, nothing more than a pouch of mucous membrane ; and a col- 

 lection of these pouches constitutes a compound one, which, if the 

 groups of which it is composed are loosely bound together like 

 grapes, as in the salivary glands, is called conglomerate ; while, if 

 they are united into a solid mass, such as the liver, the term con- 

 glohate is applied. 



By secretion is understood the process of separation of various 

 matters from the blood ; the term being also applied to the pro- 

 ducts of the process, such as saliva, bile, &c., which are commonly 

 known as secretions. These are all removed from the blood for 

 one or two purposes — first, in order to be employed for some ul- 

 terior object in the various processes going on in the body, either 

 for its own preservation, or that of others ; or, secondly, as being 

 injurious to its welfare, and therefore to be discarded. The term 

 secretion is sometimes confined to the former, while the latter 

 action receives the distinguishing term excretion : but as in many 

 cases the fluid which is removed as being injurious to the system 

 is also used for beneficial purposes the distinction is not capable 

 of being strictly maintained. The nature of the process is es- 

 sentially the same in all cases, being carried out by the development 

 of simple cells, each possessing its own independent vitality. These 

 cells select certain ingredients from the blood, and then set them 

 free by the rupture of their walls; and being situated on the free 

 surface of the lining membrane of the gland, which is continuous 

 with the mucous membrane or skin, the secreted fluid gradually 

 reaches the one or the other. It is impossible, at present, to ascer- 

 tain the precise means by which each gland is made up of cells 

 having special powers of selection ; but that the fact is so, is 

 capable of demonstration. Thus, the cells of the liver select the 

 elements of bile ; those of the salivary glands, saliva; and so on. 

 But, as we shall hereafter find, there are minute points of differ- 

 ence in the arrangement of these cells in the diff"erent glands. It 

 is now ascertained that the elements of the various secretions 

 exist in the blood ; and therefore the office of the glands is con- 

 fined to the selection and separation of their products, and they 

 have little or nothing to do with their conversion. 



DEPURATION, AND ITS OFFICE IN THE ANIMAL 

 ECONOMY. 



The whole of the various secretions which go on in the 

 body are necessary for the due preservation of its health ; but the 

 most important of the class alluded to above as excretions, must be re- 

 moved from the blood or death will speedily ensue. Thus, if saliva 

 and gastric juice. -iS well as the other secretions aiding digestion, 

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