522 THE HUMAN BODY 



shiny than the medullary, and forms everywhere the outer layer 

 of the organ next its capsule, besides dipping in between the 

 pyramids in the way described. 



The renal artery divides in the hilus into branches (5) which 

 run into the kidney between the pyramids, giving off a few twigs 

 to the latter and ending finally in a much richer vascular network 

 in the cortex. The branches of the renal vein have a similar 

 course. 



The Minute Structure of the Kidney. The kidneys are com- 

 pound tubular glands, composed essentially of branched micro- 

 scopic uriniferous tubules, lined by epithelium. Each tubule 

 commences at a small opening on a papilla and from thence has 

 a very complex course to its other extremity : usually about twenty 

 open, side by side, on one papilla, where they have a diameter 

 of about 0.125 mm. (200 inch). Running from this place into 

 the pyramid each tubule divides repeatedly; the ultimate branches, 

 which are the secreting tubules, pursue a tortuous course (Fig. 140) 

 to terminations in the cortex of the kidney in peculiar spherical 

 dilatations, the Malpighian capsules, each containing a tuft of 

 capillaries, the glomerulus (Fig. 141). Throughout its course the 

 tubule is lined by a single layer of epithelium cells differing in 

 character in its different sections: they are flat and clear in the 

 capsules, and very granular in the convoluted parts, where their 

 appearance suggests that they are not mere lining cells but cells 

 with active work to do; in the collecting and discharging tubules 

 they are somewhat cuboidal in form and have no active secretory 

 function. All the tubes are bound together by a sparse amount 

 of connective tissue and by blood-vessels to form the gland. The 

 lymph-spaces are large and numerous, especially about the con- 

 voluted portions of the tubules. 



The Blood-Flow Through the Kidney. The amount of blood 

 brought to the kidney is large relatively to the size of the organ 

 and enters under a very high pressure almost direct from the aorta, 

 and leaves under a very low, into the inferior cava (Fig. 138). 

 The final twigs of the renal artery in the cortex, giving off a few 

 branches which end in a capillary network around the convoluted 

 tubules and in the pyramids, are continued as the afferent ves- 

 sels of Malpighian capsules, the walls of which are doubled in be- 

 fore them (Fig. 141); there each breaks up into a little knot of 



