EXCRETION AND THE EXCRETORY ORGANS 467 



of the pyramid, which projects, as the papilla, into a calyx of the 

 ureter. At its outer end each pyramid separates into smaller 

 portions, the pyramids of Ferrdn, 2", separated by thin layers of 

 cortex and gradually spreading everywhere into the latter. The 

 cortical substance is redder and more granular looking and less 

 shiny than the medullary, and forms everywhere the outer layer 

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

 pryamids 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 diam- 

 eter of about 0.125 mm. (^-Q inch). Running from this place into 

 the pyramid each tubule divides repeatedly ,"the ultimate branches, 

 which are the secreting tubules, pursue a tortuous course to ter- 

 minations in the cortex of the kidney in peculiar spherical dila- 

 tations, ihe^Malpighian capsules, each containing a tuft of capil- 

 laries, the glomerulus. 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 convoluted 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). 



