THE FROG AND VERTEBRATES IN GENERAL 203 



described above for the Frog. There is, however, a difference to be 

 noted in the way the wastes from the blood are secured for excretion. 

 In the pronephros, which is the functional kidney in the lowest 

 vertebrates and a temporary organ in the embryos of all verte- 

 brates, there is no direct connection between the uriniferous tubules 

 and the vascular system. Wastes are removed directly from the 

 coelomic fluid through the ciliated nephrostomes as in the Earth- 

 worm. In the mesonephros the vascular connection is established 

 in the glomeruli as in the Frog, but remains of the coelomic con- 

 nection persist. The tubules of the metanephros found in the 

 Reptiles, Birds, and Mammals are connected only with the vas- 

 cular system, and show no evidence of the condition found in the 

 pronephric and mesonephric types. 



The human kidney is a brown, bean-shaped body which lies in 

 the abdominal cavity, close to the dorsal body wall ; one on either 

 side of the vertebral column. The internal structure of the kidney 

 is best shown by a median longitudinal section dividing it in half. 

 The examination of the cut surface reveals three distinct regions, 

 namely, an outer cortex, a middle medullary portion, and an 

 inner pelvis. The cortex and the medullary portion are com- 

 posed of the functional tubules enmeshed in the connective tissue 

 elements for support. 



It is found that the glomeruli, together with the convoluted 

 portion of the tubules, lie in the cortex where each is in direct 

 connection through the enclosed capillaries with a tiny branch of 

 the renal artery and the renal vein. The medullary portion of the 

 kidney extends from the outer cortex to the pelvis where it ends 

 in definite projections, the pyramids. This region consists largely 

 of the lower, or collecting, portions of the tubules, all of which 

 finally open at the tips of the pyramids. The pelvis of the kidney 

 is essentially a reservoir which receives the urine from the count- 

 less uriniferous tubules opening through the tips of the pyramids. 

 The urine leaves the pelvis through the ureter which carries it 

 from the pelvis to the bladder. (W. fs. 130, 131.) 



Functional. In the Vertebrates, arterial blood containing urea 

 reaches the kidney through the renal artery. In the Frog and 

 many Vertebrates there is also a supply of venous blood received 

 through the renal portal vein. In all cases, the blood having 

 passed through the glomerular capillaries is collected into the 

 renal vein on its way back to the heart. During its passage 



