CHAP. XVII.] ELIMINATION. 207 



tiny arteries, each of which enters the dilated commencement 

 or capsule of a uriniferous tubule. These tiny arteries, enter- 

 ing the capsule, are spoken of as afferent vessels. They push 

 the thin walls of the capsule before them, break up into a knot 

 of capillary vessels, called a glomerulus, and finally issue from 

 the capsule as efferent vessels. These efferent vessels do not 

 immediately join to form veins, but break up into a close mesh- 

 work of capillaries around the tubules, before they unite to 

 form the larger vessels and pour their contents into the veins 

 forming the venous arches, between the cortex and medulla. 

 In this way the cortex of the kidney is supplied with blood. 

 The medulla also receives its blood-supply mainly from the 

 arterial arches. The blood passes down- 

 wards in straight vessels between the uri- 

 niferous tubules, to be retiirned by more 

 or less straight veins to the venous arches, 

 whence it is conveyed by large branches 

 into the renal vein, which leaves the kid- 

 ney at the hilus and pours its contents 

 into the inferior vena cava. 



The renal artery in passing into the 

 kidney is accompanied by a network of 

 nerves, called the renal plexus. They 

 are chiefly vaso-motor nerves, and regu- 

 late the contraction and relaxation of the TH E BLOOD-VESSELS CON- 



1 -i i -, i NECTED WITH THE Tu- 



renal blood-vessels. BULES. 



Secretion of urine. Urine is secreted 



from the blood in two ways. It is partly removed by a process 

 of transudation or filtration, and partly by the secretory action 

 of the cells lining the uriniferous tubules. 



(1) Into the dilated extremity or capsule of each tubule a 

 small artery enters and pushing the wall of the capsule before 

 it breaks up into a bunch of looped capillaries. The blood in 

 the loop of capillaries or glomerulus is only separated from the 

 interior of the tubule by the thin walls of the capillaries and 

 the inverted wall of the capsule, which closely covers the 

 glomerulus. The artery entering the capsule is larger than 

 the issuing vessel, and, during its passage through the glo- 

 merulus, the blood is subjected to considerable pressure. As a 

 result of this, a transudation of the watery constituents of the 



