198 THE BODY AT WORK 



is merely a thin basement membrane covered by epithelial 

 scales is involuted by the tuft of bloodvessels. The vessels 

 do not penetrate its capsule. Between the tessellated epi- 

 thelium which covers the tuft and the similar epithelium which 

 lines the capsule there is a space communicating by a narrow 

 aperture with the next portion of the tubule termed its 

 " contorted " part, because it is twisted about like a tangled 

 thread in the cortex of the kidney. The contorted tubule is 

 of relatively large calibre. The cells which line it are irregular 

 in form and indistinct in outline. The basal half of each 

 cell, between its nucleus and the basement membrane, is 

 vertically striated, or " rodded," as it is usually termed. Such 

 an arrangement of the protoplasm of a cell is commonly 

 associated with a habit of absorbing fluid. It would seem to 

 indicate in this case that the cells take water and various sub- 

 stances dissolved in water from the direction of the basement 

 membrane. After a time the contorted portion of the tubule, 

 although still sinuous, becomes more nearly straight the 

 " spiral portion " and assumes a radial direction. In the 

 zone between the cortex and the medulla, the spiral portion 

 tapers into an exceedingly slender tubule which, after running 

 some distance in the direction of the hilus, turns back again 

 towards the cortex, making a loop, known as the " loop of 

 Henle." The ascending limb of this loop is of larger calibre 

 than the descending limb. The descending limb is lined by 

 flattened epithelium, each cell so thin that (in microscopic 

 sections as ordinarily prepared) its nucleus bulges into the 

 lumen of the tube. The cells of the ascending limb are more 

 nearly cubical in form. On reaching the cortex, the tubule 

 again becomes contorted. The second contorted portion 

 narrows into a " collecting portion," which joins a ductule. 

 The ductules unite together, until at last a single duct is formed 

 which opens at the apex of a pyramid. The cells of the duc- 

 tules are cubical or columnar. Their cell-substance is clear, 

 whereas that of the cells lining other parts of the tubule is 

 cloudy in appearance. 



Such a tubule, viewed as a hydrostatic mechanism, presents 

 three portions, evidently fitted for different functions : (1) The 

 glomerulus is an apparatus which allows of the rapid exuda- 

 tion of water from blood. (2) The contorted portions of the 



