REGULATING THE,;CONTRKACTIONS OF THE ARTERIES oT 
dilatation of the arteries of the right foot took place, one which had a few 
minutes previously varied from I to 2 degrees of the eyepiece micrometer being 
now 33° in diameter, and remaining so for ten minutes together. Half an hour 
later, however, I was astonished to find the artery again contracted to 2°, 
and not quite constant in calibre. But next day, on dissecting the animal, 
I found that some branches of considerable size between the cord and the sciatic 
plexus remained entire. 
This experiment, while confirming the proof of the influence exerted by the 
cord over the arteries of the feet, convinced me how difficult it is to make sure 
of dividing all the roots of the nerves for the hind legs within the spinal canal ; 
the operation being a very delicate one, while the parts are obscured by the 
bleeding which occurs in the living animal. At the same time the speedy 
recovery of function after partial division of the roots, pointed out a ready 
source of fallacy in such experiments. Had I deferred the examination of the 
web for half an hour in this case, there would have been no evidence of any 
effect produced on the vessels by the operation, and yet, had it not been for 
dissecting the frog, I should not have doubted that all the roots had been severed. 
Dilatation of the vessels of the webs having been found to follow division 
of the roots of the spinal nerves, it appeared important, in order to complete 
the evidence on the point at issue, to observe the occurrence of contraction 
in the arteries on irritation of the cerebro-spinal centre. For this purpose, 
on the 14th I laid open the cranium of a frog under chloroform and thrust 
a very fine needle into the cerebral hemispheres, while one of the feet was 
stretched under the microscope: no effect was, however, produced upon the 
arteries ; one selected for micrometrical observation, the largest of the web, 
measuring, as it had done before, nearly 4°, which was a state of full dilatation. 
I then treated in a similar manner the posterior dark-coloured portion of the 
brain, including the optic lobes, cerebellum, and medulla oblongata, which 
were not distinguished from one another in the experiment. As I continued 
this treatment for a few seconds, keeping my eye over the microscope, the artery 
became contracted to 1°, which was the length of a red corpuscle. The leg 
then became spasmodically extended, and the artery was carried out of the field ; 
but when I next looked at the web after removal of the needle, the vessels had 
dilated again to pretty full size. Having selected a main artery of another web 
more conveniently placed, I repeated the experiment of thrusting the needle 
into the posterior portion of the brain. This vessel, which just before, though 
by no means at its largest size, measured 23°, became contracted to almost 
absolute closure, and remained so till the needle was removed, after which it 
gradually dilated, and in three minutes measured 2°; forty seconds later 23° ; 
