EXCRETION 149 



the tubule, lying in the dorsal region of the kidney is generally 

 referred to as the 'proximal convoluted tubule and the second loop, 

 the distal convoluted tubule. Bidder's canal is a longitudinal duct 

 lying within the inner margin of the kidney roughly parallel 

 to the ureter on the outer margin. The collecting ducts extend 

 from Bidder's canal to the ureter. There are many collecting 

 ducts and each duct has attached to it a number of uriniferous 

 tubules. The capsular ends of the tubules are located in the 

 ventral region of the kidney (Fig. 96). 



The walls of the capsule are composed of thin flat cells which 

 gradually pass over into a short columnar 

 form in the tube proper. The first part of 

 the tubule, next to the capsule, is the neck, 

 so-called because its diameter is smaller 

 than the rest of the tubule. The inner faces 

 of the cells of the neck are provided with 

 long cilia. Shorter cilia are also found in 

 the cells of the tubule a short distance 

 beyond the neck. 



The arterial blood supply to each kidney Fig. 96.— Uriniferous 



„ „ ... r^, tubule and collecting tu- 



comes from four to six renal arteries. 1 nese bule of frog - s kidney, c, 

 enter the ventral surface of the kidney and collecting tubule; d, dis- 



. i ,i ii i i ii_ tal convoluted tubule; G, 



send branches to both the capsular and the g i ome ruius; p, proximal 

 tubular portions of the renal tubules. Each convoluted tubule. 



. . ~ , , rr , (After Nussbaum.) 



glomerulus has an afferent and efferent 



artery. The kidneys also receive blood from the renal portal 

 veins, which enter the dorsal surface of the kidney and 

 form capillary networks about the tubules. The efferent 

 arteries from the glomeruli join the capillaries from the renal 

 tubules, from which the venous blood is drained by branches of 

 the renal veins. The glomeruli therefore receive blood only 

 from the arteries, but the tubular parts of the renal tubules 

 receive blood both from the renal arteries and from the renal 

 portal veins; mainly from the latter. 



Excretory material is removed (1) by the capsule from the 

 blood as it circulates through the glomerulus, and (2) by the 

 remainder of the tubule as the blood flows through the capillary 

 network about it. Opinion differs as to the manner in which the 

 tubule functions, but subsequent observations tend to confirm the 

 general conclusions reached by Nussbaum as the result of experi- 



