THE KIDNEY. 205 



It is round or elliptical in shape, and has a diameter of about 

 0.2 mm. Where the capsule empties its contents into the 

 tubule, there is a slight constriction known as the neck ; it is 

 very distinct in some of the lower animals. The canal then en- 

 larges and begins to pursue a tortuous course in the cortical 

 substance ; it is now called a convoluted tube 1 (Fig. 91, F). It 

 next undergoes sudden diminution in size and passes straight 

 through the medulla until, at a variable point, it bends upon 

 itself, forming a loop ; then, ascending, it increases in calibre, and 

 in the cortical substance becomes convoluted for the second time. 

 Those canals that are nearest the glomeruli are called convo- 

 luted tubules of the first order, the others convoluted tubules 

 of the second order. Between these two are the looped tubules 

 of ffenle, just described, each being divided into a descending 

 and ascending branch (Fig. 91, G and H). The convoluted 

 tubules of the second order terminate by emptying into tubules 

 of greater diameter, called collecting tubules,* which descend 

 through the cortical and medullary substances, and, receiving 

 other collecting tubules on the way, finally empty into the 

 pelvis of the kidney (Fig. 91, J). * 



At the base of each pyramid there are a vast number of col- 

 lecting tubules, but as they successively empty into larger 

 collecting tubes, the area they occupy is thereby diminished ; 

 at the apex of the papillae, where they ultimately discharge 

 the urine into the pelvis of the kidneys, there are only about 

 twenty in number. This gradual coalescence of the tubes gives 

 to the pyramids a conical shape, but the breadth of the base is 

 also partly due to the presence of the looped tubules which 

 pass down into the pyramids for a varying depth. 



The larger collecting tubules may be readily injected with 

 Beale's blue fluid 3 or carmine-gelatine, either directly or from 

 the ureters ; it will be found, however, that the injection will 

 seldom extend beyond the looped tubules, owing to the small 

 diameter of the descending branches. 



1 Tubulus contortus. 2 Straight tubules of Bellini. 



3 Glycerine, pure, 2 oz. ; tr. perchloride iron, 10 drops ; ferrocyan. potassium, 3 

 grains ; strong hydrochl. acid, 3 drops ; water, 1 oz. Mix the tincture of iron with 

 one ounce of the glycerine ; and the f errocyanide of potassium, first dissolved in a little 

 water, with the other ounce ; mix gradually, and shake during admixture ; add the 

 iron to the ferrocyanide ; lastly, add the water and hydrochloric acid. Beale : 

 Microscope, p. 87. 



