182 STUDENTS HISTOLOGY 



Henle; and, widening (25 AI), returns upon its course as the 

 ascending limb of Henle' s loop. It again enters the cortex, 

 keeping in a pyramid of Ferrein, and passes outward until it 

 approaches the outer limit of the cortex, near the capsule of the 

 kidney. Here the ascending limb widens (50 /A), forming the 

 distal convoluted tubule, which pursues a tortuous course in the 

 outer cortex. Many histologists also recognize an irregular tu- 

 bule, which pursues a zigzag course for a short distance between 

 the ascending limb of Henle 's loop and the distal convoluted tu- 

 bule. "The distal convoluted tubule then reenters a pyramid of 

 Ferrein, narrows (40 /*), and passes a second time into a Mal- 

 pighian pyramid, under the title of straight or collecting tube, or 

 tube of Bellini. The last, after reaching very nearly to the apex 

 of the pyramid, unites with others of a like character to form a 

 principal tube (85/*). Several principal tubes unite to form a 

 papillary duct (250/*). From one hundred to two hundred of 

 the last open upon the surface of the apical portion of a Mal- 

 pighian pyramid. 



It must be borne in mind that, in describing the tubular sys- 

 tem, although such terms as "convoluted tube," "looped tube," 

 etc., are employed, these are not separate tubes, but only names 

 applied to different portions of one long tube. A single tubule, 

 as we have seen, commences at Bowman's capsule, becomes 

 narrowed like the neck of a flask ; courses as the proximal 

 convoluted and spiral ; descends into, turns, and emerges from 

 a Malpighian pyramid, as Henle' s looped portion ; reaches the 

 extreme cortex, and swells as the distal convoluted ; and here 

 ends as a single or isolated tubule and enters a straight tube. 

 The straight tubes receive several distal convoluted termini, at 

 the cortical periphery, and pass in small bundles (forming the 

 pyramids of Ferrein) directly onward toward the apex of a 

 Malpighian pyramid, uniting with one another at very acute 

 angles, and the trunks formed by this union uniting until the 

 tube terminates as a papillary duct. 



The tubes are lined with epithelial cells, and these cell -elements 

 constitute the parenchyma of the kidney. The lining cells are, as 

 a rule, columnar or cuboidal. Two exceptions are presented, one 

 of which appears in the flattened cells lining Bowman's capsule, 

 and the other in the flattened cells of the descending limb of 

 Henle' s loop. The parenchyma will receive attention in our 

 practical work. 



