166 



HUMAN PHYSIOLOGY. 



stitute the urine. If the apex of each pyramid be examined with 

 a lens, it will present a number of small orifices, which are the 

 beginnings of the uriniferous tubules. From this point the tubules 

 pass outward in a straight but somewhat divergent manner toward 

 the cortex, giving off at acute angles a number of branches (Fig. 20). 

 From the apex to the base of the pyra- 

 mids they are known as the tubules of Bel- 

 ^ n *' ^ n ^ e cort i ca l portion of the kidney 

 each tubule becomes enlarged and twisted, 

 and after pursuing an extremely convo- 

 luted course, turns backward into the med- 

 ullary portion for some distance, forming 

 the descending limb of Henle's loop : it then 

 turns upon itself, forming the ascending 

 limb of the loop, reenters the cortex, 

 ' again expands, and finally terminates in a 

 spheric enlargement known as Midler s or 

 Bowman's capsule. Within this capsule 

 is contained a small tuft of blood-vessels, 

 constituting the glomerulus, or Malpighian 

 corpuscle. 



Structure of the Tubules. Each tubule 

 consists of a basement membrane lined by 

 epithelial cells throughout its entire extent. 

 The tubule and its contained epithelium 



FIG. 20. DIAGRAMMATIC vary in shape and size in different parts 

 EXPOSITION OF THE . . 



METHOD IN WHICH of its course. Ihe termination of the 



convoluted tube con sists of a little sac 



THE URINIFEROUS 



1 USES UNITE T O 



FORM PRIMITIVE O r capsule, which is ovoid in shape and 

 CONES. (Tyson, 

 after Ludwig.) 



. 



measures about ^ of an mch - Thls ca P~ 

 sule is lined by a layer of flat- 

 tened epithelial cells, which is also reflected over the surface of the 

 glomerulus. During the periods of secretory activity the blood- 

 vessels of the glomerulus become filled with blood, so that the cavity 

 of the sac is almost obliterated ; after secretory activity the blood- 

 vessels contract and the sac-cavity becomes enlarged. In that 

 portion of the tubule lying between the capsule and Henle's loop the 

 epithelial cells are cuboid in shape ; in Henle's loop they are flat- 

 tened, while in the remainder of the tubule they are cuboid and 

 columnar. 



