36 ON THE PARTS OF THE NERVOUS SYSTEM 
the posterior segment of the cord, and introduced the point of a needle a short 
distance into its anterior extremity and withdrew it immediately, keeping 
my eye over the microscope. The effect upon the artery was immediate con- 
striction, causing a retrograde stream of the blood in it for about a second, 
and then absolute obliteration of calibre. At o 49™ the artery allowed single 
corpuscles to pass through it with considerable difficulty. At 1 o’clock the 
arteries of the web were still small, but I noticed that they were undergoing 
very remarkable oscillations in calibre, just as occurred on one occasion in 
the frog last operated on, but in the present case they were more striking. I 
noted the variations for some time, and give in the following table a specimen 
of those which occurred during one minute : 
nM. Ss 
At I 2 57 the diameter of the artery was 1}°. 
At 1 3. 9g the diameter of the artery was 1°. 
At I 3 20 the diameter of the artery was 3°. 
At I 3 25 the diameter of the artery was o'. 
At r 3 38 the diameter of the artery was 3°. 
At I 3 45 the diameter of the artery was 0°. 
At I 3 50 the diameter of the artery was 4°. 
These oscillations continued for upwards of half an hour, but during the latter 
part of that time the extreme degrees of constriction were not observed. 
At 15 43™ p.m. I raised the vertebral arches from the end of the spinal canal, 
and removed the posterior half of the cord together with the cauda equina ; 
immediately after which, the artery, which for the last hour had not exceeded 
Iz, became expanded to 24°, a dimension which it had never before been 
observed to attain, except during the secondary dilatation that ensued after 
the first division of the cord when the heart was in powerful action. All the 
other arteries of the web became dilated at the same time, and remained of 
perfectly constant diameter during the hour that I continued to observe them. 
Finally, at 2h 4o I introduced a needle into the anterior part of the spinal 
canal which had hitherto been undisturbed, and irritated both the anterior 
portion of the cord and the brain, but no effect whatever was produced upon 
the vessels. 
The constriction of the arteries, which resulted in this case from irritation 
of the posterior half of the cord isolated from the rest, and the permanent 
dilatation which ensued on removal of the same part, prove that this portion 
of the cerebro-spinal axis certainly contains a nervous centre for regulating 
the contractions of the arteries of the feet. The frequently alternating contrac- 
