426 ANIMAL PHYSIOLOGY. 
THE SECRETION OF URINE. 
Among experimental facts from which important conclu- 
sions have been drawn are the following (when blood-pressure 
within the kidney is referred to, it will be understood that the 
glomeruli are meant): 1. Section of the spinal cord, which 
greatly lowers the general blood-pressure, is followed by dimi- 
nution or total arrest of the secretion of urine. 2. Section of 
the renal nerves, and to a less extent of the splanchnics de- 
creases the flow of urine. 3. Stimulation of the spinal cord 
after section of the above nerves (which raises the blood-press- 
ure in the kidney by elevating the general blood-pressure) in- 
creases the flow of urine. 4. Certain diuretics increase the 
blood-pressure, either generally or in the kidney, while others 
act on the renal epithelium, apparently independently of blood- 
pressure. 
By means of apparatus adapted to register the changes of 
volume the kidney undergoes, it is found that the kidney not 
only responds to every general change in blood-pressure, but 
Bee. ARIE SPO URED ICUIRV ES 
pth ng 
KIDNEY CURVE 
APU % Wie, m, ) ha it 
US " : ( / 
) / im 
U4, MIN 
Fic. 328.—BP, Ls arta pea curve ; K, curve of the volume of the kidney; T, time-curve, 
intervals indicate a quarter of a minute; A, abscissa (Stirling, after Roy). 
to each heart-beat—that is, its volume varies momentarily. 
This shows how sensitive it is to variations in blood-pressure. 
From the above and other experiments it has been concluded 
that the secretion of urine is largely dependent on blood-press- 
ure. Until very recently certain experiments (of Nussbaum) 
were considered as favoring the view that the activity of the 
glomeruli was of a wholly or greatly different character from 
that of the tubules. In the amphibia (frog, newt, etc.) there is 
a double renal blood-supply. The glomeruli derive their blood 
from the renal artery, and the tubules from the renal-portal 
system. The vein returning the blood from the lower extrem- 
ity divides (Fig. 231) at the upper part of the thigh into two 
branches, one of which, entering the kidney, breaks up into 
