STRUCTURE OF THE KIDNEY. 



407 



that would be injurious to it if retained, and these matters being des- 

 tined to immediate and complete removal from the system. We have 

 n that, in the Lungs, the excretion of Carbonic acid is made subser- 

 ent to the absorption of Oxygen ; and the separation of a fatty acid 

 om the blood, which is effected by the Liver, is a means of introducing 

 new supply of fatty matter into the system. There is no ulterior 

 'urpose of this kind in the secreting action of the Kidney ; the product 

 f which is invariably conveyed directly to an outlet, by which it may be 

 ischarged from the body. Some traces of Urinary organs may be de- 

 cted in most of the higher Invertebrata ; but it is in Fishes, that they 

 t present a considerable development ; and in ascending through the 

 ertebrated series, we find them rapidly increasing in the complexity 

 f their organization, and in their functional importance, although their 

 ize and extent are not so great. In Fishes, the Kidneys very corn- 

 only extend the whole length of the abdomen ; and they consist of 

 fts of uniform-sized tubules, which shoot out transversely at intervals 

 m the long ureter, and which are connected together by a loose web 



Fig. 123. 



Fig. 124. 



Kidney of foetal Boa; the urinary tubes as yet short 

 and straight. 



Embryo of Green Lizard : a, heart ; 

 6, duplex aorta; c, vena cava; d, intes- 

 tine; e, liver;/, rudiment of Wolffian 

 body ; g, g, rudiments of extremities. 



of areolar tissue, that supports the network of vessels distributed upon 

 their walls. This condition of the Urinary organs is very analogous to 

 that of the Corpus Wolffianum or temporary kidney of the embryo of 

 higher animals (Fig. 123, /). A similar condition is found in the true 

 Kidney of higher animals at an early grade of development (as shown 

 in Fig. 124) ; the tubuli uriniferi being short and straight. In their 

 more advanced condition, however, they become long and convoluted ; 

 and the ramifications of the capillary vessels come into very close rela- 

 tion with them (Fig. 125). It is in the higher Reptiles, that we first 

 meet with the distinction between the cortical and medullary substance ; 

 the former being the part in which the blood-vessels are most copiously 

 distributed, and in which the tubuli have the most convoluted arrange- 

 ment ; and the latter consisting chiefly of straight tubuli, converging 

 towards the points at which they discharge themselves into the ureter 

 (Fig. 126). The bundles of tubuli and their vascular plexuses remain 

 distinct, however, in Birds and in the lower Mammalia, so as to give to 



