KIDNEYS, SKIN, AND GENERAL EXCRETION. 389 



ies which surround in a very intricate way the uriniferous 

 tubules lying in the cortex. It is at this point that the real 

 secretion of the kidney takes place. After having passed 

 from the capillaries it is gathered up in veinlets, which 

 carry the blood back to the larger veins lying between the 

 cortex and the medulla, which in turn unite at the hilum 

 and leaving the kidney form the renal vein. 



The circulation through the medulla is much simpler. 

 Small arteries run inward from the main branches between 

 the cortex and medulla, and divide up into capillaries 

 which there surround the uriniferous tubules, while corre- 

 sponding veins collect this blood and carry it back to the 

 larger veins lying between the cortex and medulla, where it 

 joins the regular venous stream. The circulation of the 

 blood from the medulla is for nutritive purposes only, and 

 no active secretion takes place here. The real physiology 

 of the kidney belongs to the cortex. 



2. The Course of the Uriniferous Tubule. It was 

 just pointed out that from the main arteries lying between 

 the cortex and medulla smaller arteries ran directly out- 

 wards through the cortex towards the capsule; that these 

 arteries gave off a number of lateral branches, and that these 

 lateral branches soon ended in arterial knots called the 

 Malpighian corpuscles, but that the blood reissued from 

 these knots still arterial and then divided up into capillar- 

 ies surrounding the uriniferous tubules. It now remains to 

 trace with reference to this course for the blood, the course 

 of the secreting tubules. 



The secreting or uriniferous tubules are very long tubes 

 intricately folded and bent and form almost all of the re- 

 maining portion of the cortex. Each uriniferous tubule be- 

 gins as a sac-like dilatation surrounding a Malpighian cor- 

 puscle and forms, so to speak, the capsule around these 

 corpuscles. The blood-vessels, however, do not really lie 

 inside of this dilatation, but as in the case of the heart and 

 pericardium, folded in by a reduplication of the wall. It is 



