64 



General Biology 



blood vessels run to the kidneys shows that these organs are decidedly 

 important. Each kidney contains a great number of coiled tubes, called 

 uriniferous tubules, each one of which begins in a Malpighian body near 

 the ventral surface (Fig. 16). This body consists of a knot of blood 

 vessels, called the glomerulus, and a surrounding membrane, known as 

 Bowman's capsule. This capsule is really the thinned out and expanded 

 end of a uriniferous tubule which has become pushed in by the 

 glomerulus. All excretions are carried by the uriniferous tubules to a 

 collecting tubule, and thence to the ureter. The ureter of each kidney 

 passes caudad ( ) toward the cloaca, emptying 



therein, thence into the bladder, a large two-lobed sac. This latter organ 



7 ffr fe vein ^ .Capillary phxus 



^lL^W??s>^7\ £olfockng iubule 



Mrinif&rvus 

 tuoidz 



Renal, 

 artery* 



"Jlenal 



Sow man's min 

 (MalpigMan) 

 '" Capsule, 



- -Qlorn&rulus 



A 



Fig. 16. A. Diagram Showing Formation of Renal Tubules and Bowman's Capsule. 



(After Borradaile.) 

 B. Diagram Showing Relation of Glomerulus and Renal Tubules to the Blood Vessels. 



(After Guyer.) 



may be collapsed if empty ; or, if filled with the urine secreted by the 

 kidney it may be considerably distended. The ventral surface of the 

 kidney has a great many ciliated ( ) funnels, called 



nephrostomes (Fig. 168), whose expanded ends open into the coelom. 

 In the young frog these are connected with the renal tubules, while in 

 the adult they open into branches of the renal vein. The renal arteries 

 and the renal-portal vein carry the blood to the kidney, leaving again 

 by the renal veins. The glomeruli are supplied only with arterial blood, 

 but the renal tubules receive, blood from the renal veins and to a slight 

 extent from the renal arteries. 



The function of the kidney is to eliminate the waste-matter from 

 the blood. The excretion itself, known as urine, is composed of a large 

 number of compounds in solution. Most of the nitrogen leaves the body 

 in the form of urea CO(NH 2 ) 2 , a white, crystalline compound, very 

 soluble in water. 



It is interesting to note that urea was the first organic chemical 

 compound actually manufactured in the laboratory. 



Urea represents the final product of the breaking down of the nitro- 



