THE SECRETION OF URINE 



1065 



slightly adherent to its substance and is continued onward as the 

 external coat of the upper and dilated segment of the ureter. In 

 transverse section each kidney presents two rather sharply differen- 

 tiated portions, namely, an outer or cortical and an inner or medullary. 

 This difference in the appearance of its cut surface is due to the peculiar 

 distribution of the urinary tubules, of which practically its entire 

 substance is composed. Consequently, it may be said that the 

 kidney is a compound tubular gland, the individual secretory units 

 of which are directed radially outward from a common central reser- 

 voir, known as the pelvis. For this reason, the beginning portion, or 

 glomerulus, of each urinary tubule must come to lie much closer to 



FIG. 522. FIG. 523. 



FIG. 522. DIAGRAMMATIC VIEW OF THE KIDNEY IN LONGITUDINAL SECTION, SHOWING 

 THE ARRANGEMENT OF THE URINIFEROUS TUBULES. 



G, Glomerulus; P, pelvis; V, ureter; C, cortical substance; M, medullary substance. 



FIG. 523. GLOMERULUS WITH THE BEGINNING SEGMENT OF THE URINIFEROUS TUBULE. 



G, Glomerulus; A and E, afferent and efferent blood-vessels; C, capsule of Bowman; 

 N, neck of uriniferous tubule; CT, distal convoluted tubule. 



the surface of the organ than its collecting segment. The renal cortex, 

 therefore, is made up principally of the glomeruli and distalmost por- 

 tions -of the uriniferous tubules, while the medulla contains chiefly 

 the smaller and larger collecting channels as they strive to attain the 

 cavity of the pelvis. 



Each tubule begins as a dilatation in which is suspended a coil of capillaries. 

 The former constitutes the capsule of Bowman and the latter the corpuscle of Mal- 

 pighi. At its point of exit from this enlargement the tubule is highly constricted, 

 forming here the so-called neck of the tubule. It then pursues a serpentine course, 

 this entire segment of it being known as the first or distal convoluted tubule. Then 

 follows a narrow, straight portion which actually enters the medulla but soon recurs 

 as a straight segment parallel to the former. These constitute the descending and 

 ascending limbs of the U-shaped loop of Henle. Having reentered the cortex, the 

 tubule again pursues a wavy course and forms the second or proximal convoluted 

 tubule. It now unites with others of the same kind into smaller collecting channels 



