THE URINARY ORGANS. 163 



The collecting tubes, which open into the calix at the apex of the 

 pyramid, are straight, and lie nearly parallel to each other and to 

 the axis of the pyramid, and, therefore, nearly perpendicular to the 

 base of the pyramid. As they are followed from the apex, in a 

 direction the reverse of that taken by the urine in flowing through 

 them, they branch dichotomously, and the branches become pro- 

 gressively smaller. At the base of the pyramid these straight 

 tubules are collected into bundles that radiate toward the convex 

 surface of the kidney, and are called the " medullary rays." In 

 these, and in the part of the pyramid that is near the boundary- 

 zone, the collecting tubes are associated with other straight portions 

 of the tubules, " Henle's tubes," which will be described pres- 

 ently. From the medullary rays the tubules pass into the region 

 between those rays in the cortical portion of the kidney. This 

 region of the cortex is known as the " labyrinth." Here the tub- 

 ules lose their straight character and become much contorted, form- 

 ing the "second convoluted tubules." They then re-enter the 

 medullary rays, which they descend for a variable distance into the 

 pyramid, constituting the "ascending branches of Henle's tubes," 

 which make a sharp turn, " Henle's loop," and then retrace their 

 course up the medullary rays into the cortical portion of the kidney, 

 "descending branches of Henle's tube." They then pass again 

 into the labyrinth and form the "first convoluted tubules," which 

 finally merge into the structure of the Malpighian bodies, also 

 situated in the labyrinth. In consequence of the passage of tubules 

 from them into the surrounding labyrinth the medullary rays become 

 smaller as they are followed from the base of the pyramid, and 

 eventually disappear before the capsule of the kidney is reached. 

 They are completely surrounded by the labyrinth. 



If we now follow the course of the urine in its way from the 

 Malpighian body to the outlet of the tubule, we shall find that it 

 passes through the following divisions of the tubule : 1, the "first 

 convoluted tubule;" 2, the "descending branch of Henle's tube;" 

 3, "Henle's loop;" 4, the "ascending branch of Henle's tube;" 

 5, the "second convoluted tubule;" 6, the "collecting tube." Of 

 these, the two convoluted tubules are situated in the labyrinth ; all 

 the rest in the medullary rays and pyramid. All of the portions, 

 with the exception of the convoluted tubules and the loop, are 

 straight and lie parallel to each other (Fig. 139). 



Before entering more particularly into the structure of the renal 



