152 GENERAL ZOOLOGY 



the capsules of uriniferous tubules, before breaking up into capil- 

 laries about the loops of the tubules. Thus the blood supply to 

 the tubule is entirely arterial. The tubule is somewhat more 

 complicated in form and structure than in the case of the frog and 

 consists of the following well-defined regions: (1) a capsule 

 enclosing a glomerulus; (2) a proximal convoluted portion; (3) a 

 U-shaped part composed of straight descending and ascending 

 limbs of Henle's loop; and (4) the distal convoluted portion (Fig. 

 98). The distal convoluted portion is joined by an arched 

 collecting tubule to a straight collecting tube, which leads to a 

 renal sinus or renal pelvis, a chamber in the concave region of 

 the kidney, drained by the ureter. The form of the tubule is 

 such that the distal and proximal convoluted portions are very 

 close to the capsule. The efferent arteries, after leaving the 

 glomerulus, are distributed among the tubules, forming a capil- 

 lary network leading to venous channels which eventually unite 

 to form the renal vein. 



Judging from experimental observations in rabbits, the 

 mammalian uriniferous tubule functions in much the same way 

 as the tubule of the frog's kidney. In the rabbit, excretion of 

 solids in solution occurs in the proximal convoluted segment and 

 in the descending limb of Henle's loop, and resorption in the 

 ascending limb and in the distal convoluted segment. From the 

 glomerulus water principally is excreted. It might also be added 

 that in certain fishes, Sygnathus and Hippocampus, the urinif- 

 erous tubules lack glomeruli, yet urine is excreted containing 

 dissolved substances in different concentrations than in the 

 blood. All of this strengthens the idea that certain portions of 

 the uriniferous tubule, exclusive of the capsule, excrete solids 

 in solution and that other portions of the tubule modify the 

 concentration of the solution by resorbing water. 



It should be clear from what has been said that the organ of 

 excretion, whether it be gill, lung, skin, or kidney, is merely a 

 means of ridding the body of waste matter. The circulatory 

 fluid transports excretory substances, formed by the body cells 

 and to a certain extent in the circulatory fluid itself, conveying 

 them to the excretory organ. The excretory organ is the elimi- 

 nator. In the absence of a circulatory system, the excretory 

 organ absorbs the metabolic products in a more direct manner 

 from the cells. 



