ORGANS OF THE BODY. 



which leave the cortex of the kidney the term 

 curving back again that of recurrent tubes. 



In fig. 509 we have a representation of these 

 looped canals (d) lying between the widely sepa- 

 rated tubes (b, c). It shows likewise the in- 

 equality in the distances from the papillae, at 

 which the smaller tubes turn on themselves. 



It seems hardly necessary to remark that the 

 number of looped tubes increases the nearer 

 we approach the cortex of the organ. This is 

 shown by transverse sections of the medullary 

 pyramids taken at varying heights. Near the 

 apices of the papillae but few cross-sections of the 

 looped tubes of Henle are to be seen around the 

 wide openings of the straight canals. But nearer 

 the bases of the pyramids of Malpighi the 

 small lumina of the former become more and 

 more numerous. Again, while the open urini- 

 ferous tubes are at first arranged close to one 

 another, surrounded by circles of the orifices 

 of the looped canals, we meet them further out- 

 wards with larger intervals between them, which 

 are occupied by cross-sections of the tubes of 

 Henle in great numbers. But it is not only 

 a difference in diameter which distinguishes 

 these two systems of uriniferous tubes from one 

 another : the glandular epithelium in the open 

 canals is of a species entirely distinct from 

 that in the looped, and the so-called mem- 

 brana propria presents several points of dif- 

 ference, though of a less marked kind. 



The short trunks of the open canals have 

 at their commencement no membrana propria ; 

 they are simply bounded by the fibrous frame- 

 work of the apex of the papillae. Further on 

 a delicate, transparent, limiting membrane be- 

 gins to be apparent. This remains throughout 

 the ramifications thin and fine, presenting always 

 a single outline under the microscope. The case, 

 however, is quite different with the looped 

 canals (fig. 510, a, b, c). Here the membrana 

 propria is stronger and thicker, and exhibits 

 under high magnifying power a double contour. 



In the short primary trunks of the open canals 

 we find an epithelial lining continuous with 

 that covering the surfaces of the papillae. 

 But the cells here are clearer, and of the low 

 columnar type, with broad bases turned towards 

 the walls of the tube. A considerable lumen 

 is still left, however, for the height of the 

 cells is only 0'0300-0'0201 mm. They re- 

 main thus as far as the branches of the first 

 and second order (Henle). The last system of 



descending, afid to those 

 J 



Fip. 509. Vertical section 

 through a medullary pyra- 

 mid of the pie's kidney (half 

 diagrammatic), a, trunk of 

 uriniferous tube opening on 

 the tip of a pyramid ; b and 

 r, branches of the same; 

 rf, looped tubes, or Henle $ 

 canals ; e, vascular loops; and 

 j\ branches of the rasa recta. 



branches which run, as 



