156 EXCKETTON. 



lishing a communication between the tubes coming from 

 the Malpighian bodies and the tubes of the pyramidal sub- 

 stance. They are called the intermediate tubes, or the 

 canals of communication. Some observers have described 

 them as forming an anastomosing plexus, but this disposi- 

 tion is not definitely established. 



The tubes into which the intermediate canals open join 

 with others, generally two by two, and 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 blood- 

 vessels of the kidney present certain interesting peculiarities 

 in their distribution, which have been very successfully stud- 

 ied by Bowman, Isaacs, and many other anatomists, by means 

 of minute injections of the renal arteries and veins. With the 

 improved methods of injection now employed, their arrange- 

 ment can be readily followed. 



The renal artery, which is quite voluminous in propor- 

 tion to the size%f 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 col- 

 umns 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 col- 

 umns 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, which is situated exactly at the boundary which sep- 

 arates 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 pyra- 



