ix EXCRETION 147 



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 network is supplied partly by 

 the renal arteries (r. ), 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 the 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 capillaries (gl), the glomerulus. 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 glorner- 

 ulus, 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 nitrogenous 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 dis- 

 solved 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 by drop into 

 the cloaca, whence it is either expelled at once, or stored for 

 a time in the bladder. 



L 2 



