METABOLISM 



245 



about 0.8 gram per liter of dissolved ammonia. The taste is largely 

 that ofsodium chloride plus a bitterness whose source is complicated. 

 leTHEJEXCRETiON OF URINE from the renal epithelium to the distal 

 end j)f the urethra involves both the secretory function of protoplasm 

 and a complicated series of muscular and recoiling movements; the latter 

 part of the process, the expulsion of the urine from the bladder, is speci- 

 fied as micturition. 



FIG. 131 



Junctional 



convo- tubule, 

 luted \ 



tubule. 



Proximal \ 

 convoluted >** 

 tubule, j 



I Spiral part 

 -< of ascending 

 ( limb. 



Descending limb } 

 of Henle's loop, j 



Diagram of a uriniferous tubule, suggesting vaguely the varieties of epithelium composing 

 itt| A, flattened cells with oval nuclei; B, polyhedral, striated cells; C, polyhedral, striated cells, 

 but with their nuclei near the lumen of the tubule; D, polyhedral cells striated only in the outer 

 part and with flattened and angular nuclei; E, variable cells: polyhedral, columnar, angular 

 with short processes, and fusiform; F and G, columnar and variable cells; H, angular cells with 

 conspicuous rodded striations. (Gray.) 



The first matter to be examined into, then, is the manner in which 

 some of the numerous complex substances recently enumerated are taken 

 from the blood-stream and collected, dissolved in water, in the receiving 

 hilum of the kidney. Not until all the secrets of protoplasmic secretion 

 are unravelled will the details of these versatile chemical reactions be 

 known, and here as elsewhere in discussing secretion it is only the gross 



