REGULATING THE CONTRACTIONS OF THE ARTERIES 45 
only temporary dilatation of the arteries, provided that a part remains which 
furnishes roots of nerves for the posterior extremities, is in harmony with the 
transient effects which were seen to be produced upon the vessels by partial 
division of the roots of the nerves within the spinal canal in the experiments 
of April 8 and 11, 1857." In both these cases the arteries of the webs appeared 
to recover their contractile power completely, although the leg remained nearly, 
if not entirely, paralysed ; which seems to indicate that a few fibres of the 
nerves for the blood-vessels of a part can supply the place of the rest more 
perfectly than is the case with the ordinary nerves of sensation and motion. 
This peculiarity of the ‘ vaso-motor’ nerves is more strikingly illustrated by 
the first experiment mentioned in this paper”, in which 1t may be remembered 
that the arteries of the webs completely recovered their usual powers of varying 
their calibre within half an hour after division of the sciatic, although this is 
an operation which abolishes for days at least all sensation and voluntary 
motion in the leg. I have since seen yet more remarkable instances of the same 
thing. On October 10, 1857, with the view of investigating the nature of the 
control exercised by the nervous system over the actions of the pigment-cells,* 
I divided all the soft parts in the middle of the thigh of a frog, except the main 
artery and vein. The first effect upon the arteries was full dilatation; but 
about twenty-four hours later they were again of moderate size, while the 
circulation was still active. After the death of the animal, I examined with 
the microscope the coats of the artery and vein, and also the periosteum, 
together with a very slight amount of muscular tissue adhering to it, but could 
detect no nerves in any of them, although from the method of examination 
I could hardly have missed branches containing more than very few nerve- 
tubes. Comparing the result in this case with the permanent dilatation which 
always occurred after removal of the spinal cord, so long as the circulation 
continued active, it was evident that the slender filaments contained in the 
coats of the vessels, or possibly in the bone, had served as an efficient means 
of communication between the cerebro-spinal axis and the arteries of the foot. 
On the 13th of the same month I repeated the experiment upon another 
frog, operating in this case upon both thighs. In the first place, I divided 
thoroughly all the soft parts except the artery, vein and nerve, the circulation 
remaining unaffected. The nerves were then successively cut, full dilatation 
of the arteries and rapid flow through the capillaries being the immediate result. 
An hour and a half later, however, the flow was observed to be less rapid, no 
* Vide pp. 30, 31. *WNide p.520; 
* Further information regarding this experiment, as respects the pigmentary system, will be found 
in the next paper (p. 48 of this volume). 
