444 



SECRETION. 



open into a common channel, like those of 

 the liver of the lobster or cray-fish (fig. 308.) ; 



Fig. 308. (Fig. 214. Vol. II.) 



Lobule of the liver of Astacus fluviatilis. (Mutter.) 



nor, again, whether the tubes are few and 

 of great length, lying loose in the cavity of 

 the body, and passing from one end of it to 

 the other, like the biliary vessels of insects 



Fig. 309. (Fig. 431. Vol. II.) 



Alimentary canal of Pontia brassica. 



(fig. 309.) ; or whether they are very nu- 

 merous, of less proportional length, and aggre- 

 gated in a compact mass, as in the kidney of 

 the higher animals. 



In all instances, then, the excretory organ 

 essentially consists of a limitary membrane, 

 which forms part of the integument of the 

 body, or of one of its involutions ; and of cells 

 covering the free surface of that membrane, 

 and, consequently, in direct relation with the 

 external surface. Thus we have the limitary 

 membrane of the true skin, and of the mu- 

 cous membrane of the alimentary canal which 

 is directly continuous with it, sunk into 

 follicular depressions ; and the free surfaces 



of these are lined with cells, the layers of 

 which are continuous with those of the 

 epidermis and of the gastro-intestinal epithe- 

 lium respectively. (See Mucous MEMBRANE.) 

 We trace inwards another extension of the 

 same membrane along the genito-urinary 

 passages, up to the kidneys, where it forms 

 the walls of the tubuli uriniferi ; and there, 

 too, its free surface is covered with an 

 epithelial layer of cells, which is the efficient 

 instrument of the selection of the constituents 

 of the urinary fluid, and which, when ex- 

 uviated, is conveyed along the urinary passages 

 to the exterior of the body. So, too, the 

 hepatic cells, by which the biliary matter is 

 eliminated from the blood, are brought into 

 direct continuity with those of the external 

 surface, through the hepatic ducts and gastro- 

 intestinal mucous membrane. 



The case is not different, in any essential 

 respect, with regard to the organs by which 

 the recrementitious secretions are formed. 

 Thus the lachrymal, salivary, pancreatic, and 

 mammary glands are in like manner composed 

 of a continuation of the limitary membrane of 

 the true skin, or of the mucous membrane 

 lining the alimentary canal, involuted into 

 tubes arid follicles, the free surfaces of which 

 are covered with epithelial cells. These cells, 

 drawing into themselves certain constituents 

 of the blood, are cast off when they have 

 completed their full development ; and their 

 contents, set free by the disintegration of the 

 cell-walls, are carried off by the ducts, which 

 collect them from different portions of the 

 glandular structure, and deposit them in the 

 situation where the purposes of the secreted 

 product are to be answered. 



If we attentively consider the character of 

 what is commonly designated as the absorbent 

 system, we shall see that this, too, may be re- 

 garded as a glandular apparatus; possessing, as 

 it does, the essential characters of a gland in 

 regard to its structure, and being analogous to 

 the true glands in its mode of performing its 

 function, and the difference of its purpose in 

 the general economy being accordant with the 

 difference of its anatomical relation. Putting 

 out of view for the time the absorbent glands, 

 or ganglia, we find the absorbent system to 

 consist of two series of long tubuli, one set ex- 

 tended through almost the entire body, whilst 

 the other is distributed upon the intestinal 

 canal. These tubes appear to commence 

 either in caecal origins or in loops ; they 

 coalesce with each other; and at last dis- 

 charge themselves into a common receptacle, 

 just as do the tubuli of the kidney. That 

 their origins should be widely scattered, in- 

 stead of being bound together in one compact 

 mass, is a fact of no physiological importance ; 

 having reference only to the remoteness of 

 the sources, whence are derived the materials 

 on which the particular agency of this appara- 

 tus is exerted. These materials are of two 

 kinds ; for they consist in part of the crude 

 materials selected by the lacteal division of 

 the system from the contents of the aliment- 

 ary tube, over whose walls the origins of the 



