THE KIDNEY. 213 



7. The papillary ducts, the final segment of the renal tubule, number from ten to 

 eighteen for each papilla, at the apex of which they open into the calyx. Each is 

 formed by the junction of from ten to thirty of the larger collecting tubules and attains 

 a diameter from 200-300 /*. The lining epithelium is composed of conspicuous clear 

 columnar cells, about 20 fi in height and one-third as much in width, which rest upon 

 a basement membrane almost as far as the end of the canal. At this point the simple 

 columnar epithelium of the duct becomes continuous with the stratified squamous 

 epithelium that covers the free surface of the papilla and lines the calyx. 



The Supporting Tissue. The uriniferous tubules and the blood-vessels are held 

 in place by a delicate interstitial stroma of reticular connective tissue, elastic fibres 

 being relatively very few. At the surface of the kidney this tissue is condensed into 

 a compact fibrous stratum, the tunica albuginea, containing scattered bundles of 

 unstriped muscle and an increasing number of elastic fibres, not to be confounded 

 with the fibrous tunic that envelops the organ and may be stripped off without dis- 

 turbing the renal substance. Although forming a continuous framework throughout 

 the kidney, the interstitial stroma is not uniformly distributed, being most abundant 

 along the path of the interlobar and the larger blood-vessels, from whose adventitia 

 delicate trabeculae extend in all directions to form the meshes lodging the tubules, the 

 smaller vessels and the capillaries. Within the cortex the supporting tissue is meagre, 

 being best developed along the interlobular vessels and around the Malpighian 

 bodies. The interstitial tissue is much more plentiful within the medulla than the 

 cortex, its amount increasing towards the apex of the papilla, where considerable 

 tracts of stroma-tissue separate the papillary ducts. Not only the blood-vessels, but 

 also the nerve-trunks are provided with sheaths of renal stroma. 



The blood-vessels supplying the kidney, branches of the renal artery, 

 enter the renal substance through the vascular foramina surrounding the 

 papillae on the wall of the sinus. As they pass along the sides of the papillae, 

 their positions correspond to the primary interlobar tracts of connective 

 tissue of the foetal kidney. On reaching the level of the bases of the renal 

 pyramids, each interlobar artery breaks up into twigs, some of which pursue 

 an irregularly arched course across the bases of the pyramids, thereby pro- 

 ducing in places the impression of "arcades" at the junction of the medulla 

 and cortex. From these arcuate arteries or their divisions arise the terminal 

 branches which supply the cortex. The cortical twigs radiate generally per- 

 pendicular to the free surface, towards which, as the interlobular arterioles, 

 they pass, giving off the short lateral twigs that end in the vasa afferentia 

 of the glomeruli. These are arranged in columnar groups along the path of 

 the interlobular cortical arterioles. Some of the latter continue to the free 

 surface where, in conjunction with direct capsular branches from the renal 

 artery, they supply the capsule of the kidney. After traversing the capillary 

 complex of the glomerulus, the blood is carried off by the vas efferens, which 

 on its exit immediately resolves into the cortical capillaries, whose meshes 

 about the convoluted tubules are round and about the tubules of the medul- 

 lary rays are elongated. In addition to the usual path through the glomer- 

 ulus, the kidney substance is supplied also by terminal vessels that pass 

 directly into the intertubular network. The course of the medullary twigs 

 is influenced by the radial disposition of the tubules between which they run; 

 they are, therefore, relatively straight and, hence, known as arteriola redes. 

 They arise only exceptionally from the arcuate arteries, and chiefly from the 

 afferent branches of low lying, perhaps atrophic (Huber), Malpighian 

 bodies. The supply of the medulla is, therefore, less independent than 

 formerly believed. 



The veins of the kidney are also disposed as cortical and medullary 

 branches, which empty into larger stems, the so-called vena arciformes, that 



