. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Lobule of the liver of Astacus fluviatilis. (MWeh") 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
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Lobule of the liver of Astacus fluviatilis. (MWeh") 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 lachryma
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