ix EXCRETION 147 



glandular epithelium, which, in some parts (ur. tu, ur. tu"), 

 are ciliated. The Malpighian capsules are lined with 

 flattened cells of pavement epithelium. 



The arrangement of the blood-vessels is peculiar. Like 

 other organs, the kidney is permeated by a network of 

 capillaries (cp) which form a close mesh between the 

 urinary tubules, so that the cavity of the tubule is 

 separated from the blood only by the thickness of the 

 gland-cells and of the capillary wall. The capillary net- 

 work is supplied partly by the renal arteries (r. a), partly 

 by the renal portal vein (r. pt. v), and is drained by the 

 renal veins (r. v). It is in the behaviour of the renal 

 arteries that tbe chief peculiarity of the kidney-circulation 

 lies. On entering the kidneys they break up into smaller 

 and smaller arteries, but each of the ultimate branches 

 (af. v), instead of discharging into the general capillary 

 network, passes to a Malpighian capsule, in the interior 

 of which it breaks up into a little bunch of coiled capil- 

 laries, the glomerulus (gl) . From this the blood is carried 

 off by a minute vessel (ef. v) by which it is poured into the 

 general capillary network and finally discharged into 

 the renal vein (r. v). 



Renal Excretion. While circulating through the 

 glomerulus, water and certain soluble matters are 

 separated from the blood and make their way into the 

 Malpighian capsule and thence into the urinary tubule. 

 As the blood circulates through the general capillary 

 network, the gland-cells of the tubules excrete, out of 

 the materials brought to them by the blood, the nitro- 

 genous waste matter urea, in the formation of which 

 the liver plays an important part ; it is discharged 

 from the cells into the cavity of the tubule, where it is 

 dissolved in the water separated out of the glomerulus. 

 In this way the urine is formed. Accumulating in the 

 tubules, it makes its way into the ureter and thence drop 



L 2 



