44 ON THE PARTS OF THE NERVOUS SYSTEM 
stance found to be still 4° in diameter. I may mention that I measured the 
posterior segment of the cord immediately after its removal, and found its length 
to be one-sixth of that of the whole cord ; it was in fact little more than the 
tip of it; but allowing for a certain amount of contraction, it may be reckoned 
as one-fifth. 
This case shows that the extremity of the cord acts as a nervous centre 
for the arteries. But the experiment of the 2nd of June proved that after the 
removal of the greater part of the posterior half of the cord, the vessels still 
remained under the control of the nervous system.1 Hence it is clear that the 
nervous centre for the arteries is not confined to any limited region of the cord. 
This experiment also indicates, in a very striking manner, how small 
a piece of the cord will suffice to regulate the calibre of the arteries, and how 
little effect may be produced, even in the first instance, by the removal of 
a large portion which also possesses that function. For it was shown, by the 
absence of contraction in the vessels when the anterior segment was irritated, 
and still more conclusively by the absence of dilatation when the anterior 
segment and the brain were removed, that the posterior segment was the only 
part capable of acting on the arteries after the removal of the middle segment ; 
or, in other words, that this operation deprived the arteries of the influence 
of the whole cerebro-spinal axis, except the posterior fifth of the cord. Yet, 
although the heart was acting powerfully at the time, the dilatation produced 
by this procedure was only moderate in amount, and very transient. Hence it 
follows that the mere fact of the speedy return of the arteries to their former 
state of contraction, after removal of an anterior portion of the cerebro-spinal 
axis, aS seen in the experiment of April 16, 1857,? is no ground whatever for 
believing that such a portion does not act as a nervous centre for the arteries. 
This being clearly understood, the invariable occurrence of contraction, when 
the posterior part of the brain or the anterior half of the cord was irritated, 
in the experiments of April 14 and 16, 1857,> must be regarded as strong pre- 
sumptive evidence, if not absolute proof, that they as well as the posterior 
half of the cord preside over the arterial contractions in the feet, although, 
as shown at p. 37, they appear to exert their influence only through those 
roots of nerves which take origin from the posterior regions of the cord. On 
the other hand, the cerebral hemispheres seem to take no part in this function, 
so far at least as it is safe to draw any inference from the negative evidence 
derived from a single experiment performed upon them, viz. that mentioned 
ab p. 31. 
The fact that the removal of a large portion of the cord is followed by 
. Midéeup. 37. . Wide: Spirsise * Vide ppengity sae 
