XIV 



\VVsTI AND I.X( KKTION 



163 



and imally end in the cortex by blind dilated extremities. The 

 blind dilated extremities are, however, in reality the beginnings 

 of the tubules, and the openings in the pelvis are their ends. 

 The closed dilated extremities arc called Malpighian cap- 

 sules. 



The wall of a tubule consists of a single layer of epithelial 

 cells. These cells vary in shape in different parts of the 

 same tubule, but arc mostly 

 cubical, and they occupy 

 so much of the tubule that 

 the bore is small. At the 

 dilated extremity the wall 

 of the tubule is very thin, 

 and at the extreme end 

 this thin wall is pushed in, 

 as it were, by a small cluster 

 of blood-vessels, called a 

 glomerulus. A Mai 

 pighian capsule therefore 

 consists of the closed 

 dilated extremity of a 

 tubule into which is pro- 

 jecting a cluster of capil- 

 laries covered by the thin 

 wall of the tubule. To 

 each glomerulus proceeds 

 a small artery, and from 



Fie;. 75. Diagram to illustrate the circulation 

 in the kidney. 



ai, Small artery giving off the branch, va, to 

 Klomerulus, ff, from which the vein, if, 

 issues. This breaks up into capillaries, which 

 surround the tubule and unite up into a small 

 vein, T, which joins the vein, vi. .M, capil- 

 laries around tubules in parts of the cortex 

 where there arc no glomeruli ; />, parts of 

 the cortex where there are glomeruli. 



each proceeds a small vein. 



Tins small vein, however, 



does not at once join 



other veins, but breaks up 



into capillaries which join 



with the capillaries lying round the tubule, and it is from these 



the veins arise which gradually unite into veins which form 



the main vein leaving the kidney at the hilus. 



Composition of Urine. -The amber - coloured fluid, 

 urine, consists of water holding certain organic and inorganic 

 substances in solution. The chief organic substances contain 

 nitrogen, and the most important of them is urea. The chief 

 inorganic substances are the chloride, sulphate, and phosphates 



