672 STRUCTURE OF TUBULI URINIFERI. [BOOK n. 



In the first convoluted tubule the basement membrane is 

 distinct and conspicuous ; indeed we may say at once that this 

 distinctness and sharpness of outline of the basement membrane 

 holds good for the whole length of the tubulus uriniferus until 

 we reach the discharging tubules in the medulla ; and here the 

 basement membrane is lost to view simply because it becomes 

 fused with the connective-tissue groundwork or stroma which is 

 especially well developed in the lower part of a pyramid. Else- 

 where the basement membrane may easily be recognized as an 

 independent membrane. 



The epithelium of the first convoluted tubule has the following 

 characters. The outlines of the cells are very indistinct, so that 

 not unfrequently the tubule seems to be lined by a layer of 

 cell-substance in which rounded nuclei are imbedded at intervals. 

 When the outlines are made out it is seen that each cell, which 

 has a rounded nucleus placed at about its middle, is more or less 

 cubical, sometimes of such a height as to leave a narrow, some- 

 times so low as to leave a fairly wide lumen. The outer portion 

 of the cell next to the basement membrane is in many specimens 

 striated radially ; the appearance suggests that the cell substance 

 is here composed of prisms or rods stretching radially from the 

 basement membrane to or beyond the region of the nucleus and 

 united together by some substance of a different nature ; but 

 in many good specimens such a striation may be absent or 

 indistinct. The inner portion of the cell, next to the lumen, is 

 of a more ordinary granular appearance, but the free border is 

 frequently jagged, bearing irregular processes projecting into the 

 lumen, and having somewhat the appearance of broken cilia, though 

 they are not of the nature of cilia. In the frog and some other 

 animals the first portion of the urinary tubule bears long, active 

 cilia ; but, as we shall see, this ciliated portion corresponds to the 

 short constricted neck of the tubule in the mammal, and is 

 succeeded by a non-ciliated portion which corresponds to the 

 portion which we are now describing. The whole cell stains readily 

 and deeply with the ordinary staining reagents. It may contain 

 fat globules arranged in rows, leaving spaces or vacuoles when 

 the fat is removed; sometimes these are very numerous. The 

 appearances in fact presented by the cells in this first part of 

 the tubule differ very much in different specimens ; but we have 

 at present no exact knowledge which will enable us to correlate any 

 of these differences with varying conditions of functional activity. 



The spiral tubule, which is as wide as or even wider than the 

 convoluted tubule, possesses a wide and regular lumen. The cells 

 which line the tubule have much the same character as in the 

 convoluted, but are lower and more regular in form ; hence the 

 wider and more regular lumen ; their striation also is less distinct 

 and may be absent. The rounded nuclei of the cells are very 

 conspicuous. 



