CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 643 



descending limb, and hence in properly stained specimens can be 

 easily distinguished. 



The discharging tubule, in which the collecting tubule passing 

 straight down into the pyramid ends after several junctions with 

 its fellows, has much the same characters as the collecting tubule, 

 save that it becomes increasingly larger, and the cells lining it 

 are taller and more columnar. 



At the mouth of the ultimate discharging tubules as they 

 open on the papilla of a pyramid, the single layer of columnar or 

 cubical cells lining the tubule becomes continuous with the 

 epithelium coating the papilla; and this, like the epithelium 

 lining the calyces, pelvis and ureter, consists of two or three 

 layers of cells of whose characters we shall speak later on. 



396. Bearing in mind what we have previously learnt 

 concerning secreting epithelium in other glands, it is obvious that 

 the cells of the convoluted and irregular tubules are cells which 

 exhibit to an eminent degree the characters of active secreting 

 cells. The same may be said, though less emphatically, of the cells 

 of the spiral tubule, and of the ascending limb of the loop of Henle. 

 The cells of the collecting and discharging tubules on the other 

 hand possess those characters which we associate with cells lining 

 the conducting portions of a gland ; but in saying this we may 

 repeat the caution 240, that we must not assume that the cells 

 in such a situation do nothing else than afford a smooth lining for 

 the passage of material secreted elsewhere. 



The cells of the descending limb of the loop of Henle are 

 peculiar ; they are certainly conducting rather than secreting 

 cells; but the meaning of this remarkable loop of Henle is at 

 present obscure. Its presence in the mammalian kidney is in 

 part but only in part explained by the characters of the urinary 

 tubule in the lower animals. In the frog, newt and other amphibia 

 the tubule begins as in the mammal, in a Malpighian capsule, 

 and the first part of the tubule succeeding the Malpighian 

 capsule is lined by clear cells leaving a narrow lumen into which 

 project from the cells remarkably long cilia directed downwards 

 and moving, in the living kidney, with an undulatory movement. 

 This first part is obviously a conducting part, and is represented 

 in the mammalian kidney by nothing more than the constricted 

 neck which joins the capsule to the convoluted tubule ; we may 

 speak of it as the first conducting portion. The succeeding part 

 is a wider tube lined by cells which bear no cilia, but whose 

 free border is beset by short, rigid narrow processes, like short 

 bristles. This is obviously a secreting portion, and we may speak 

 of it as the first secreting portion; it corresponds to the first 

 convoluted and the spiral tubule of the mammalian kidney, 

 though the cell-substance is not striated as in the mammal. 

 There next follows a section in which the tubule is of much 

 narrower diameter and the cells, formed of clear cell-substance, 



412 



