364 EXCRETION BY THE SKIN AND KIDNEYS. 



ular cells. Near the bases of the pyramids the wide portion sometimes 

 forms the loop, but near the apices the loop is always narrow. The differ- 

 ence in the size of the epithelium is such, that while the diameter of the 

 tube is variable, its caliber remains nearly uniform. The membrane of these 

 tubes is quite thick, thicker, even, than the membrane of the tubes of Bel- 

 lini. 



Intermediate Tubes. After the narrow tubes of Henle have returned to 

 the cortical substance, they communicate with a system of flattened, ribbon- 

 shaped, anastomosing canals, y^- to -^^ of an inch (21 to 25 /x) in diam- 

 eter, with very thin walls, lined by rodded epithelium. These tubes take an 

 irregular and somewhat angular course between the true convoluted tubes 

 and finally empty into the branches of the straight tubes of Bellini, thus es- 

 tablishing a communication between the tubes coming from the Malpighian 

 bodies and the tubes of the pyramidal substance. They are called the inter- 

 mediate tubes, or the canals of communication. 



The tubes into which the intermediate canals open join with others gen- 

 erally two by two, and then pass in a nearly straight direction into the pyra- 

 mids, where they continue to unite. with each other in their course, becoming, 

 consequently, reduced in number until they open at the apices of the pyra- 

 mids, into the infundibula and the pelvis of the kidney. 



Distribution of Blood-vessels in the Kidney. The renal artery, which is 

 quite voluminous in proportion to the size of the kidney, enters at the hilum 

 and divides into four branches. A number of smaller branches penetrate 

 between the pyramids and ramify in the columns of cortical substance which 

 occupy the spaces between the pyramids (columns of Bertin). The main 

 vessels, which are generally two in number, occupy the centre of the columns 

 of Bertin, sending off in their course, at short intervals, regular branches on 

 either side, toward the pyramids. When these branches reach the boundary 

 of the cortical substance, they turn upward and follow the periphery of the 

 pyramid to its base. Here the vessels form an arched, anastomosing plexus, 

 the arterial arcade, situated between the rounded base of the pyramid and 

 the cortical substance. This plexus presents a convexity looking toward the 

 cortical substance, and a concavity, toward the pyramid. It is so arranged 

 that the interstices are just large enough to admit the collections of tubes 

 that form the so-called pyramids of Ferrein. 



From the arterial arcade, branches are given off in two opposite direc- 

 tions. From its concavity, small branches, measuring at first T ^ o- to -?%-$ of 

 an inch (21 to 34 ft) in diameter, pass downward toward the papillae, giving 

 off small ramifications at very acute angles, and becoming reduced in size to 

 about ^g-g- of an inch (10 /u,). These vessels, called sometimes the arteri- 

 olge rectse, surround the straight tubes, and pass into capillaries in the sub- 

 stance of the pyramids and at their apices. 



From the convex surface of the arterial arcade, branches are given off at 

 nearly right angles. These pass into the cortical substance, breaking up into 

 a large number of little arterial twigs, T fj nf to -^ of an inch (17 to 40 /A) in 

 diameter, each one of which penetrates a Malpighian body at a point oppo- 



