THE URINARY TRACT. 



smaller than that of the afferent vessel. Soon after having left 

 the tuft, it divides freely into capillaries for the supply of the 

 cortical as well as the pyramidal substance. So extensive is this 

 division that the kidney-tissue proper has no other capillaries 

 than those derived from the efferent vessels, the pyramidal sub- 

 stance being, besides, supplied with capillaries from a few inde- 

 pendent arterioles. The cap- 

 illary meshes are circular in 

 the region of the convoluted 

 tubules, and elongated squares 

 in that of the straight tubules. 

 (See Fig. 329.) 



The arterial arches send a 

 few small arterial branches 

 downward into the pyramidal 

 substance. The main supply 

 of this portion, however, is 

 capillary, and of two kinds: 

 First, the few elongated, 

 square meshes around the 

 straight collecting tubules ; 

 and second, the bundles of 

 very wide capillaries, accom- 

 panying the narrow tubules. 

 The latter are the so-called 

 vasa recta of the pyramidal 

 substance. Some authors 

 claim that ihey are arteries 

 produced directly from the 

 arterial arches, which, how- 

 ever, is not correct. Others 

 maintain that the vasa recta 

 arise immediately from the 

 efferent vessels of the tufts nearest the pyramidal substance 

 (C. Ludwig); again, others consider the vasa recta as veins 

 which originate from the capillaries of the cortical substance, 

 in a manner similar to the portal system of the liver (Huschke, 

 Hyrtl, Henle). We overcome all difficulties concerning the origin 

 of the vasa recta by considering them as considerably widened 

 capillary blood-vessels, which are prolongations of the narrow 

 capillaries of the cortical substance a fact which is easily dem- 

 onstrated in the injected kidney of the dog. (See Fig 332 ) 

 47 



CT 



FIG. 331. TUFT FROM THE KIDNEY OF 

 A DOG. INJECTED. 



T, capillary loops of tlie tuft, in connection 

 with the afferent artery, covered by E, flat epi- 

 thelia ; Ca, capsule, covered with flat epithelia, 

 in communication witli Co, the convoluted tu- 

 bule ; CL, convoluted tubule in longitudinal sec- 

 tion ; CT, convoluted tubule in transverse sec- 

 tion. Magnified 350 diameters. 



