526 
Miller and Carlton—The Cat's Kidney. 
the medulla and cortex could in this way be plainly seen. With 
a planimeter the area of the entire section and the entire area 
less cortex was found in square decimeters, the needle of the 
planimeter passing over the surface of the glass. Readings 
were taken on both sides of each section, although theoretically 
the areas of two sections on the same line ought to be the same. 
These double readings served as checks on any error. While 
theoretically the readings on each side of the section line should 
be the same, in practice it was found there was a slight vari¬ 
ation due to the fact that it is next to impossible to follow the 
lines exactly with the planimeter twice in succession. The sec¬ 
tions of the arteries and veins, as well as the difference in color, 
served as a guide for the dividing line between cortex and 
medulla; the former had been previously determined as an 
accurate guide by the careful study, under the microscope, of 
numerous stained sections. 
The entire area of one side was then multiplied by the thick¬ 
ness of the section. This ought to give the exact volume of 
the section if the area of both sides was the same; but a section 
was seldom found where both sides did not show considerable 
variation in area. The exact figure is not the mean of the two 
areas multiplied by the thickness for this leaves out all curve, 
which in some sections is great. The truth, as near as we can 
get at it, is the mean of one-half of the two areas multiplied by 
the thickness and the greater area multiplied by the thickness. 
This is shown by the following figure: 
multiplied by kl divided by 2 is represented by the lines ef 
