404 The Arteriole Recte of the Mammalian Kidney 
capillaries about uriniferous tubules without being directly associated 
with glomeruli. 
There are, as is well Known, two leading and opposing theories on the 
nature of urinary secretion tersely stated by Hans Meyer as follows in a 
recent summary of observations on renal function: “According to one of 
these theories, which was developed most fully by Heidenhain, we have to 
deal with a true secretory process by which water and perhaps the salts pass 
through the glomerulus, whereas the specific constituents of the urine are 
liberated from the tubules, so that the sum of both secretions is represented 
by the outflowing urine. According to the other hypothesis, which was first 
proposed by Ludwig and subsequently modified by his successors (in a bio- 
logical sense), there goes on in the kidney, side by side with the glomerular 
activity, dependent essentially on the mechanical conditions of the circulation, 
and independently also on the secretion of certain urinary constituents, a 
process of resorption in the urinary tubules. Through this resorption the 
slightly concentrated secretion of the glomerulus, corresponding to the water 
of the blood, undergoes concentration to a point characteristic of the urine.’ 
It is not my purpose here to discuss either of these theories. It may, how- 
ever, be permitted to call brief attention to certain points in the structure of 
the uriniferous tubules in connection with an account of the relations of ter- 
minal branches of the renal arteries, points which, it seems to me, shoul: 
be considered by the followers of either of the leading theories on the nature 
of urinary secretion. 
In each uriniferous tubule, including the glomerular capsule, there may 
be recognized four types of epithelium with distinct regional distribution. 
(1) The flattened epithelial cells lining the glomerular capsule continuous 
with the flattened epithelium, probably syncitial in character covering the 
glomerulus; (2) the epithelium of the proximal convoluted portion, colum- 
nar in shape with striated protoplasm and striated inner border; (3) the 
peculiar flattened epithelium of the descending limb of Henle’s loop; (4) the 
short columnar epithelium of the ascending limb, the distal convoluted por- 
tion and a portion of the junctional tubule, an epithelium with indistinct 
cell outline, with basal striation, but differing in structural detail and in re- 
action to stains from the epithelium of the proximal convoluted portion. 
These four types of epithelium are found, not only in the uriniferous tubules 
ot the mammalian kidney, as determined by reconstruction in this laboratory, 
but also in the tubules of the simpler reptilian kidney (recently reconstructed 
in this laboratory and to be described in another conimunication) as also in 
the tubules of the amphibian kidney (mesonephros). The epithelium of the 
neck of the uriniferous tubules is here not especially considered; it differs 
from the other epithelia described, though it probably has little functional 
significance. If it is true, as stated by Starling in introducing the section on 
“The Mechanism of the Secretion of Urine” in Schaefer’s Text-book of 
Physiology, tuat “a difference of function is invariably associated with a 
difference of structure, so that the interdependence of function and structure 
has become an axiom,’ we should be justified in postulating a difference of 
function to the different parts of the uriniferous tubules lined by the differ- 
ent types of epithelium, and the extent to which this may be done is, as it 
