THE URINARY ORGANS. 



197 



parent, thin (0*0004 — 00008'"), but proportionally strong and 

 elastic coat, which, particularly in the straight tubules, may be 

 very easily isolated for a considerable extent, 

 when it is very prone to fall into folds, which 

 often present the striated aspect of connective 

 tissue. On the inner surface of this coat, 

 which in its chemical characters is very closely 

 allied to the sar oolemma (vid. § 58), lies a 

 single layer of polygonal, moderately thick cells, 

 surrounding the cavity of the tubulus, which, 

 from the readiness with which they alter, have 

 given rise to many erroneous representations 

 respecting the structure of the urinary ducts and 

 their contents. For instance, in the usual mode 

 of examination in water, they expand, owing to 

 its absorption and become vesicularly dis- 

 tended, so that their polygonal form and regular 

 arrangement are lost; the renal ducts, within 

 the structureless membrane, appearing to be 

 entirely filled with rounded larger cells and 

 no longer to possess any cavity. The cells, also, frequently 

 burst, in which case the tubuli contain nothing but a 

 fine granular substance, with nuclei and clear albuminous 

 drops escaped from the cells. In kidneys not quite fresh, 

 these changes proceed spontaneously; and, therefore, it is, 

 above all things, necessary to examine the organ as soon 

 as possible after death and to avoid all additions capable 

 of producing change. The contents of the epithelial cells 

 are, besides round nuclei of the usual kind, a most usually, 

 very finely- granular substance, which, on the addition of water, 

 affords clear, light-yellowish drops, probably of albumen, but 

 is not otherwise changed ; under acetic acid the contents, toge- 

 ther with the cell- membrane, are rendered pale, and soon dis- 

 solve ; whilst the nuclei at the same time soon become pale, 

 and finally, on the application of caustic alkalies, disappear in 

 the same manner as the membrane. Besides these granules, 

 which I do not hesitate to declare are a protein substance, and 

 the albumen in solution in the contents, the cells very corn- 

 Fig. 246. Two straight tubuli uriniferi of Man, one with perfect epithelium, the 

 other half empty : a, membrana propria ; b, epithelium. 



