328 EXCRETION 



some of the cells of the pancreas and of the salivary glands. The 

 nucleus usually is situated between the granular and the rodded zones. 



The pyramids of Ferrein extend into the cortical substance to form 

 what are known as the medullary rays. The spaces between the medul- 

 lary rays constitute the labyrinth. The division between the cortical 

 and the medullary substance is known as the boundary zone. 



The greatest part of the solid excrementitious constituents of the 

 urine, such as urea and the urates, is separated from the blood by the 

 cells of the convoluted tubes of the cortical substance and perhaps 

 by the dilated portions of the tubes of Henle, while the water and a 

 certain portion of the inorganic salts of the urine transude through the 

 bloodvessels in the Malpighian bodies. 



Narrow Tubes of Henle. The convoluted tubes above described, 

 after a tortuous course in the cortical substance, become continuous, 

 near the pyramids, with the tubes of much smaller diameter, which form 

 loops extending to a greater or less depth into the pyramids. The loops 

 formed by these canals (the narrow tubes of Henle) are nearly parallel 

 with the tubes of Bellini and are much greater in number near the bases 

 of the pyramids than toward the apices. The diameter of these tubes 

 is variable, and they present enlargements at irregular intervals in their 

 course. The narrow portions are about g oVo ^ an m ch (12 A 1 ) in diame- 

 ter, and the wide portions, about twice this size. The narrow portion is 

 lined with small clear cells with very prominent nuclei. The wider por- 

 tions are lined with larger, granular cells. Near the bases of the pyra- 

 mids the wide portion sometimes forms the loop, but near the apices the 

 loop is always narrow. The difference in the size of the epithelium is 

 such that while the diameter of the tube is variable its calibre remains 

 nearly uniform. The membrane of these tubes is quite thick, thicker, 

 even, than the membrane of the tubes of Bellini. 



After the narrow tubes of Henle have returned to the cortical sub- 

 stance, they communicate with a system of canals, -j^Vo to ToVo" ^ an 

 inch (21 to 25/it) in diameter, with very thin walls, lined by rodded 

 epithelium. These are known as the second convoluted tubes. They 

 take an irregular and somewhat angular course between the first con- 

 voluted 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 sometimes called the intermediate tubes, or the canals of com- 

 munication. 



The tubes into which the second convoluted tubes open join with 

 others, usually, two by two, and then pass in a nearly straight direc- 

 tion into the pyramids, where they continue to unite with each other in 



