ORGANS OF THE BODY. 525 



in the dog and pig, for instance. In the latter animal the breaking up of 

 the collecting tubes into arching ramifications (b) is easily recognisable. 



It appears, moreover, that loops of communication never occur between 

 the ramifications of one collecting tube and those of another, although we 

 might sometimes be easily led to believe otherwise in thick sections of 

 injected kidneys. 



It was such deceptive appearances which tempted Henle, after he had 

 been successful in filling the renal tubuli so far, through the ureters, to 

 the conclusion that the terminations of the strait canals which open 

 at the apices of the papillae lay before him ; and farther, that a system of 

 tubes, distinct from these open passages, and in no way communicating 

 with them, is formed by the convoluted uriniferous canals, capsule of 

 glomerulus, and looped tubuli of the medulla. 



Both modes of procedure mentioned above, namely, that of macera- 

 tion in acid, and that of complete artificial injection, show that series 

 of passages, of various forms, spring from the arches just mentioned, and 

 also earlier still from the collecting tube itself. These it is (fig. 518, c) 

 which, arriving in the medulla, somewhat decreased in size (fig. 517, /?, g), 

 form there the descending arms of the looped tubes of Henle (fig. 517, 

 519, *,/). 



Here, then, we have the origin of the descending portion of the loops. 



If we now follow it still farther to repeat a former description we 

 find it (fig. 519, e) advancing into the medullary substance for a greater or 

 less distance, and then curving round on itself (f), pursuing the same 

 course back again to the medullary process (g, li). At the same time its 

 diameter increases, as already stated, and its lining of cells changes in 

 character. Arrived here it turns off sideways, sooner or later, to become 

 a convoluted tube of the renal cortex (/), terminating eventually as such in 

 one of Bowman's capsules. 



We now have the whole intricate course of the uriniferous tubes 

 before us. 



In some few instances we may be fortunate enough to succeed in driv- 

 ing the injection fluid as far as the capsules. 



It seems almost superfluous to add another diagram (fig. 520) for the 

 purpose of once more tracing the course which the secretion must take 

 from the glomerulus outwards, 



From Bowman's capsule (a) the fluid escapes into the convoluted tube 

 (&), which, after numerous twists and curls in the cortex, arrives in the 

 medullary substance, where it pur- 

 sues a straight course (c). Lined by 

 its own peculiar epithelium, it tra- 

 verses the medullary pyramid in a 

 direction more or less directly down- 

 wards, then forms a loop (c), and re- 

 turns to the cortex (d). The recur- 

 rent arm so formed alters sooner or 

 later in character : it becomes wider 

 and more tortuous (e), and, together 

 with other similarly constituted tubes, 

 empties itself into the collecting f 

 canal (/), which uniting with adja- 

 cent passages of the same order at acute angles (g, h~), pours out the 

 urine finally at the apex of the papilla (i). Many efforts have been made 



