124 PRACTICAL MICROSCOPY. 



voluted then re-enters a pyramid of Ferrein, narrows (-g-J-g- "), and 

 passes a second time into a Malpighian pyramid, under the title of 

 straight or collecting tube, or tube of Bellini. The last, after reach- 

 ing very nearly to the apex of the pyramid, unites with others of a 

 like character, and forms principal tubes (TUT")- Several principal 

 tubes unite to form a papillary duct ( T -J- ") From 100 to 200 of the 

 last open upon the surface of the apical portion of a Malpighian 

 pyramid. 



It must be borne in mind that, in describing the tubular system, 

 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, then, com- 

 mences 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 por- 

 tion; 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; the resulting 

 trunks uniting until the tube terminates as a papillary duct. 



The tubes are lined with epithelia; and these cell elements con- 

 stitute the parenchyma of the kidney. The lining cells are, as a rule, 

 of the columnar variety. Two exceptions are presented, one of 

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

 other in a like form, in the descending limb of Henle's loop. The 

 parenchyma will receive attention in our practical work. 



BLOOD-VESSELS. 



The vascular arrangement is complex. The most prominent and 

 essential feature is afforded in the existence of two distinct capillary 

 plexuses. 



The renal artery, as already described, sends branches into the 

 substance of the kidney. These pass between the Malpighian 

 pyramids, and.m the cortical columns. These arterial trunks arch 

 over the bases of the pyramids of Malpighii, forming the arterial 

 arcade. From these arches small straight branches are sent outward 

 toward the capsule of the kidney, occupying a position midway be- 

 tween the pyramids of Ferrein, in the labyrinths. The last are the 

 interlobular arteries. During their course, they send off side 

 arterioles which penetrate the capsule of the Malpighian bodies. 



