400 EXCRETION. 



clear pavement-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 establishing 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. Some observers have described them as 

 forming an anastomosing plexus, but this disposition is not definitely established. 



The tubes into which the intermediate canals open join with others, generally two by 

 two, and then pass in a nearly straight direction into the pyramids, where they continue 

 to unite with each other in their course, becoming, consequently, less and less numerous, 

 until they open at the apices of the pyramids into the infundibula and the pelvis of the 

 kidney. 



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

 nous in proportion to the size of the kidney, enters at the hilum and divides into four 

 branches. By numerous smaller branches it then penetrates between the pyramids and 

 ramifies in the columns of cortical substance which occupy the spaces between the pyra- 

 mids (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, situated exactly at the 

 boundary which separates the rounded base of the pyramid from 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 directions. From 

 its concavity, numerous small branches, measuring at first from T ^Vo to yfo f an ^ nc ^ m 

 diameter, pass downward toward the papillaB, giving off small ramifications at very acute 

 angles and becoming reduced in size to about ^Vff of an inch. These vessels called 

 sometimes the arteriolas rectaB surround the straight tubes and pass into capillaries in 

 the substance of the pyramids and at their apices. 



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

 nearly right angles. These pass into the cortical substance, breaking up into a large num- 

 ber of little arterial twigs, from ^-^Vo to -g^-g- of an inch in diameter, each one of which 

 penetrates a Malpighian body at a point opposite to the origin of the convoluted tube. 

 Once within the capsule, the arteriole breaks up into from five to eight branches, which 

 then divide dichotomatously into vessels measuring from ^Vo to TTOTJ- f an i ncn i Q diame- 

 ter, arranged in the form of coils and loops, constituting a dense, rounded mass (the 

 Malpighian coil), filling the capsule. These vessels break up into capillaries without 

 anastomoses. Their coats are amorphous and are provided with numerous nuclei rather 

 shorter than those found in the general capillary system. 



The blood is collected from the vessels of the Malpighian bodies by veins, sometimes 

 one, and frequently three or four, which pass out of the capsule and form a second capil- 

 lary plexus surrounding the convoluted tubes. When there is but one vein, it generally 

 emerges from the capsule near the point of penetration of the arteriole. The walls of 

 the vein are much more fragile than those of the arteriole, and, consequently, in ordinary 

 microscopical preparations of the cortical substance, the arteriole is left attached, while 

 the veins are torn off. 



The efferent vessels, immediately after their emergence from the capsule, break up 

 into a very fine and delicate plexus of capillaries, closely surrounding the convoluted 

 tubes. These form a true plexus, the branches anastomosing freely in every direction ; 

 and the distribution of vessels in this part resembles essentially the vascular arrangement 

 in most of the glands. Bowman has called the branches which connect together the 



