The _urinary canals_ are divided into two kinds, namely, the outer and the inner tubular systems. The outer canals, called the _tubulæ uriniferi corticalis_, are very small in caliber and are located in the lobules. The renal artery (Fig. 53, No. _B_, 3) breaking up into arterioles in the kidney, and finally reaching the cortical portion of the lobules, form capillary plexuses in the shape of minute spheres, which are the glomerules (Fig. 53, No. _C_, 3 and _B_, 5). Around each glomerule there is formed a capsule called _Bowman’s capsule_, which is the beginning of the uriniferous tubule. This entire mass is called the _Malpighian body_, or _renal corpuscle_. This capsule then extends as the urinary, or secreting tubule, being at first constricted, then convoluted, and terminates into a second portion, the _descending limb of Henle_. This portion of the tubule becomes constricted. It then forms the loop of Henle and ascends as the second portion, or second limb of Henle, which is again of greater diameter. The top of the secreting tubule is slightly wavy and empties into the collecting tubule along with many others. These collecting tubules in turn merge into large tubules which finally empty into the ureter (Fig. 53, No. 14). These collective bundles correspond to the pyramids of the kidneys of mammals. [Illustration: FIG. 51.—The kidneys. 1_a_, The posterior. 1_b_, the middle and 1_c_, the anterior lobes of the kidney. 2, The posterior aorta. 3, The external iliac or crural artery. 4, The ischiadic artery. 5, The sacralis media artery. 6, The ureter which empties into the cloaca at 7. 8, The external iliac vein. 9, The internal iliac vein. 10, The iliacus communis. ] [Illustration: FIG. 52. _A._ Photomicrograph of a section of a kidney showing three lobules. 1, Cortical portion of lobule showing glomeruli. 2, The central or medullary portion of the lobule. 3, The outer surface of the kidney. _B._ Salts from the urine of a hen. 1, Uric acid crystals. 2. Sodium urate crystals. ] [Illustration: FIG. 53.—The renal structure. _A._ 1, Capillary blood-vessel. 2, Descending limb of Henle. 3, Collecting tubule. 4, Ascending limb of Henle. _B._ 1, Branch of renal artery. 2, The descending or medullary branch. 3, The ascending branch. 4, The arteriole taking part in the formation of the glomerule. 5, The glomerule. 6, Bowman’s capsule. 7, Blood-vessel extending from the glomerule. 8, The vein of the medullary part of the lobule. 9, The neck. 10, The convoluted tubule. 11, The descending limb of Henle. 12, Henle’s loop. 13, The ascending loop of Henle. 14, The collecting tubule. _C._ 1, Section through a convoluted tubule. 2, Bowman’s capsule. 3, The glomerule. ]