490 



KIDNEY. 



[CHAP, xxxiv. 



that the elements characteristic of the urinary secretion are selected 

 by the epithelium lining this part of the tube. The diameter of the 

 tube is less immediately after leaving the tuft, than in the rest 

 of its course further down (Fig. 234) . The epithelium in the con- 



.Ftflr.234. 



Entire uriniferous tube of the large black newt (Triton cristatns, female), a. Artery having upon its 

 walls numerous branched pigment cells. The commencement of the tube, and that part near the tuft, 

 are of less diameter than the central portion, which is the most active part of the secreting tube. 

 Towards its termination in the upper part of the oviduct, d, the tube becomes straight and much nar- 

 rowed, c. Magnified about 30 diameters, from a drawing of Dr. Beale's. 



voluted portion of the tube presents an excellent example of the 

 spheroidal, or glandular variety. It consists of polyhedral particles 

 rather less than 10 * 00 of an inch in diameter, with a distinct nucleus, 

 and containing numerous granules, and occupying as much as one- 

 third or more of the total diameter of the tube. 



The extreme diameter of the convoluted tube is about T -i-^ of an 

 inch, while the diameter of the central canal is not more than 

 from y^o-o to -gi^ of an inch. 



Straight Portion of the Tube. The straight portion of the tubes 

 of which the medullary cones or pyramids are composed form 

 anastomoses, or if traced from the papilla towards the base of the 



