9 6 
Scientific Proceedings (43). 
A, mm. 
B, mm. 
C, mm. 
Proximal convoluted portion with medullary seg- 
11 .3 
9.4 
10.2 
(Specific renal epithelium of cortex.) 
1.4 
6.7 
15.0 
(Pavement epithelium.) 
Thick segment of medullary loop and distal con- 
7.8 
6.9 
3.6 
( Cubic or short columnar epithelium.) 
Total length of tubules, exclusive of collecting duct. 
20.5 
23-0 
28.8 
The tubules selected represent type tubules and were selected 
with a view of drawing attention to the fact that the character 
of the tubule, and perhaps also its functions, changes with the 
position of the renal corpuscle in the cortex. Tubules with renal 
corpuscles situated near the periphery of the cortex possess short 
medullary loops with short segments lined by pavement epithe- 
lium. The deeper in the cortex the renal corpuscle is situated the 
longer becomes the medullary loop and the longer the segment of 
the loop lined by pavement epithelium. These statements pertain 
not only to the renal tubules of rabbits but also to those of other 
mammals including man. It may be observed from a study of the 
table that that portion of the renal tubule lined by the specific 
renal epithelium of the cortex, namely the proximal convoluted 
portion and its medullary segment, does not vary materially in 
length in tubules of different types. If one may assume a specific 
excretion for the special renal epithelium of the proximal convoluted 
portions and their medullary segments, it is evident that this 
specific excretion would be essentially of the same extent for all 
the renal tubules, irrespective of type. If, on the other hand, one 
may assume an absorption of water and perhaps certain salts 
in the thin segments lined by the clear, pavement epithelium the 
extent of this absorption must differ widely in tubular segments 
having this structure, and that, therefore, urine of different de- 
grees of concentration must as a consequence enter the collecting 
ducts. 
