536 



THE UEINARY APPARATUS 



[CH. XXXVI. 



portions called respectively cortical and medullary; the latter is 

 composed of about a dozen conical bundles of urinary tubules, each 

 bundle forming what is called a pyramid. The upper part of the 

 ureter or duct of the organ is dilated into the pelvis ; and this, again, 

 after separating into two or three principal divisions, is finally sub- 

 divided into still smaller portions, varying in number from about 8 

 to 12, called calyces. Each of these little calyces or cups receives the 

 pointed extremity or papilla of a pyramid. The number of pyramids 

 varies in different animals ; in some there is only one. 



The kidney is a compound tubular gland, and both its cortical 

 and medullary portions are composed of tubes, the tubuli uriniferi, 

 which, by one extremity, in the cortical portion, commence around 



tufts of capillary blood-vessels, called Mal- 

 pighian bodies, and, by the other, open 

 through the papillae into the pelvis of the 

 kidney, and thus discharge the urine which 

 flows through them. They are bound 

 together by connective tissue. 



In the pyramids the tubes are straight 

 uniting to form larger tubes as they de- 

 scend through these from the cortical por- 

 tion ; while in the latter region they spread 

 out more irregularly, and become much con- 

 voluted. But in the boundary zone between 

 cortex and medulla, small collections of 

 straight tubes called medullary rays project 

 into the cortical region. 



Tubuli Uriniferi. The tubuli uriniferi 

 (fig. 417) are composed of a basement mem- 

 brane, lined internally by epithelium. They 

 vary considerably in size in different parts 

 of their course, but are, on an average, about 

 of an inch (^ mm.) in diameter, and are found to be made up 

 of several distinct portions which differ from one another very 

 markedly, both in situation and structure. 



Each begins in the cortex as a dilatation called the Capsule of 

 Bowman ; this encloses a tuft or glomerulus of capillaries called a 

 Malpigliian corpuscle. The tubule leaves the capsule by a neck, and 

 then becomes convoluted (first convoluted tubule), but soon after 

 becomes nearly straight or slightly spiral (spiral tubule) ; then rapidly 

 narrowing it passes down into the medulla as the descending tubule of 

 Henle ; this turns round, forming a loop (loop of Henle), and passes 

 up to the cortex again as the ascending tubule of Henle. It then 

 becomes larger and irregularly zigzag (zigzag tubule) and again con- 

 voluted (second convoluted tubule). Eventually it narrows into a 



stance of the kidney. B. The 

 epithelial or gland-cells, x 700 



