64 GENERAL BIOLOGY 



vessels run to them shows that they are decidedly important organs. 

 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 cap- 

 sule. 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 may be collapsed 



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



(After Borradaile.) 



cap., Capillary plexus , col.t., collecting tubule ; me. and M cp., Bowman's cap- 

 sule (Malphighian capsule) ; r.a., renal artery; r.v., renal vein; r.p.v., renal portal 

 vein ; ur.t., uriniferous tubules ; w.d., Wolffian duct. 



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



(After Guyer.) 



if empty, or, if filled with the urine secreted by the kidney, may be con- 

 siderably 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 re- 

 ceive blood from the renal portal veins and to a slight extent from the 

 renal arteries. 



The functions of the kidney, as already stated, is the elimination of 

 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 (NH 2 )2CO, a white, crys- 

 talline compound, very soluble in water. 



It is interesting to remember that this was the first organic chemi- 

 cal compound actually manufactured in the laboratory. 



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



