94 



Anatomy of the Rabbit. 



INTERNAL STRUCTURE 

 AND FUNCTION. 



m 



to be excreted may pass into them from the blood stream. The 

 primary tubule (Fig. 5 1 A) begins in each case in the cortical 

 substance with a cup-like structure, known as a renal or Mal- 

 pighian corpuscle, containing a network or glomerulus of 



minute vessels from the branches of the 



renal artery. The blood stream is thus 



separated only by a thin membrane from 



the cavity of the tubule, the wall of which is complete. The 



terminal parts of the tube system have a characteristic course 



in the kidney substance, which accounts for the difference in 



appearance as between the cortex and 

 medulla, and are ultimately connected with 

 common collecting tubules opening on 

 the surface of the papilla. The excreted 

 fluid, urine, contains characteristic nitro- 

 genous waste materials, usually urea, but 

 in some cases hippuric acid. These mater- 

 ials are formed in the liver and perhaps 

 elsewhere in the body. 



Like all other parts of the urinogenital 

 system, the structure and embryonic 

 development of the mammalian kidney 

 affords a remarkable illustration of the 

 extent to which the adult form of an organ 

 is dependent upon ancestry. In the verte- 

 brate phylum, three pairs of kidneys have 

 been recognized. They occur in antero- 

 posterior order in the body, either in embryo 

 or adult, they are of increasing special- 

 ization, and their order of appearance and 

 functional value are directly associated with the degree of general 

 specialization of the groups in which they occur. These organs 



homologies of have 1)een desi 8 nated as pronephros, 



vertebrate kidneys, mesonephros, and metanephros. The 



metanephric kidney is characteristic of 

 mammals, while the mesonephros is embryonic. The latter is, 

 however, the adult kidney in the frog and allied animals, its duct 

 in the male serving both as reproductive duct and ureter. The 



Fig. 51. Kidney tubule.-;. A, 

 plan of arrangement in adult 

 mammal: c, cortex; m, medulla; 

 gl, vascular glomerulus; tc, 

 proximal convoluted portion of 

 tubule; tr. proximal straight por- 

 tion of tubule (Henle's loop); 

 tp, collecting tubule to renal 

 papilla. From Weber, after v. 

 Ebner. B, plan of a single 

 primitive kidney tubule in 

 lower vertebrates, glomerulus; 

 np, nephrostome; cl, coelomic 

 epithelium; t, body of tubule; 

 d, longitudinal duct. 



