

THE KIDNEY 185 



in sections of the kidney. Small non-medullated nerve -trunks 

 may be demonstrated in transverse sections of the cortex, espe- 

 cially near the bases of the medullary pyramids, where they will 

 be seen in company with the blood-vessels of the arcades. Lymph- 

 channels are also to be seen in the vicinity of the vessels of the~ 

 hilum, and in the connective tissue of the capsule. 



The histology of the kidney will be comprehended better by a 

 reference to its function. The separation from the blood of a 

 quantity of water, together with certain excremeiititious matters, 

 is effected, partly in the Malpighian bodies, and partly in the 

 tubules. The vascular tuft the glomerulus is covered with a 

 close -fitting membrane composed of flat cells. The blood in this 

 plexus parts with a certain amount of its water, which passes 

 through the walls of the capillaries and through the cells m covering 

 them. Whether this be due to osmosis or to some selective power 

 of the cells we have no concern suffice it to say that certain salts 

 afterward appearing in the urine do not leave the blood at this 

 point. The efferent glomerular arteriole, it will be remembered, 

 breaks into a capillary plexus, which brings the blood close to the 

 walls of the convoluted tubules. It is believed that the cells lining 

 these tubules select from the blood circulating in the contiguous 

 capillaries such effete materials as escaped elimination from the 

 glomeruli. 



Moreover, it seems that some of the water which escaped 

 in the first instance and entered the proximal convoluted tubules, 

 is here returned to the blood by the intervention of the same 

 tubular lining cells which excrete the salts. Without referring 

 to any further work done by the kidney, it is important to 

 understand this part of the structural scheme : That the first part 

 of the uriniferous tubule is the prominent excreting part. That 

 the latter portion of the tubule the portion in the Malpighian 

 pyramids, the straight tubule is for the collection and drainage of 

 the urine already excreted. And that between the excreting first part 

 and the draining second part, there exists a narrow looped tubule 

 the loop of Henle. The effect of this narrowing and tortuosity of 

 the tubule will be to present a resistance to the outflow of urine 

 from the proximal portion of the tubule. The diluted urine, 

 excreted in the Malpighian bodies, is held back for a while in 

 the proximal convoluted tubule, and time given for the comple- 

 tion and perfection of the excretory processes by the tubular 

 parenchyma. 



