632 



OF SECRETION. 



3. The Kidneys Secretion of Urine. 



838. The Kidneys cannot be regarded as inferior in importance to the 

 Liver, when considered merely as excreting organs ; but their function only 

 consists in separating from the blood certain effete substances, which are to 

 be thrown off from it; and has no direct connection* with any of the nutritive 

 operations, concerned in the introduction of aliment into the system. Organs 

 destined to the elaboration of a Urinary secretion may be traced very low 

 down in the Animal scale. Among many of the Mollusca we find a small 

 sac, filled with a semi-fluid secretion which has been shown to contain uric 

 acid, opening into the intestine, near its anal orifice. In Insects, we often 

 meet with prolonged tubes, resembling the biliary vessels in form, but termi- 

 nating in a lower part of the intestinal tube ; in some species these are dilated 

 near their extremity into a receptacle for their secretion, or a urinary bladder. 

 Throughout the Vertebrated classes, they exist in a still more evident form. 

 They are uniformly composed of a congeries of prolonged tubes, subdividing 

 and ramifying more or less ; which spring from the ureter or efferent duct, 

 and terminate either in blind extremities, or in a plexus formed by their inos- 

 culation. There are considerable variations in the arrangement of these tubes, 

 however, in different tribes of animals. In Fishes, the Kidneys very com- 

 monly extend the whole length of the abdomen ; and they consist of tufts of 

 uniform-sized tubules, which shoot out transversely at intervals from the long 

 ureter. These tubes frequently divide* into pairs, but without any great alte- 

 ration in their diameter. They appear to terminate in crecal extremities, with- 

 out any inosculation; the number of bifurcations, and the degree of convolu- 



[Fig. 239. 



[Fig. 240. 



17 



13 



A view of the Right Kidney with its Renal 

 Capsule ; 1, anterior face of the kidney ; 2, exter- 

 nal or convex edge ; 3, its internal edge ; 4, hilum 

 renale ; 5, inferior extremity of the kidney; G, pel- 

 vis of the ureter ; 7, ureter ; 8, 9, superior and in- 

 ferior branches of the emulgent artery ; 10, 11, 12, 

 the three branches of the emulgent vein ; 13, an- 

 terior face of the renal capsule ; 14, its superior 

 edge; 15, its external edge; 16, its internal ex- 

 tremity; 17, the fissure on the anterior face of the 

 capsule.] 



A section of the Kidney, surmounted by the 

 Supra-Renal Capsule ; 1, the supra-renal cap- 

 sule ; 2, the vascular portion ; 3, 3, its tubular 

 portion, consisting- of cones; 4, 4, two of the 

 calices receiving the apex of their correspond- 

 ing cones ; 5, 5, 5, the three infundibula ; G, the 

 pelvis; 7, the ureter.] 



