30 ON THE PARTS OF THE NERVOUS SYSTEM 
permanent dilatation whatever from the division of the sciatic nerve, a result 
quite at variance with the experience of previous observers. 
The explanation of this will shortly appear. On April 8, 1857, I laid 
open the spinal canal of a frog in its entire length, and divided, as I supposed, 
all the roots of the nerves coming off from the left side of the cord from 
the occiput to the sacrum, and immediately examined the webs of both feet, 
the frog being under the influence of chloroform. In the right limb the circula- 
tion was almost entirely arrested, while in the left it was going on freely. My 
attention was then diverted for half an hour, when the arteries of the right 
foot were found of medium size ; but in all the three webs of the left foot they 
were extremely dilated, appearing to have two or three times the diameter of 
those of the right limb.t This observation was of itself sufficient to prove 
that the spinal system, as distinguished from the sympathetic, does influence 
the contractions of the arteries of the frog’s foot. Here, however, as in the 
case of the divided sciatic nerve, the effects were not permanent. Six hours 
later the arteries on the left side appeared smaller than they had been, though 
still bearing marks of the operation by remaining constant in calibre, whereas 
those of the right foot exhibited very frequent variations, from pretty full 
dilatation to almost absolute closure. Next day the same state of things 
continued, the vessels of the left foot being constant in size for four minutes 
together, while in the right foot an artery exhibited about eight distinct varia- 
tions of calibre per minute as observed by micrometer; but after three days 
more they had become both small and variable in the left foot, and seemed 
to have quite recovered. On the application of galvanism to the cord, however, 
both legs were thrown into violent spasm, showing that communications still 
existed between the left limb and the nervous centre ; and it appeared probable 
that the branches which remained undivided had come after a while to supply 
more or less perfectly the place of those which had been cut. “A similar ex- 
planation seemed applicable to the speedy recovery of contractility in the vessels 
after cutting the sciatic, other nerves in the limb supplying the place of the 
divided trunk. 
In another experiment, performed on the 11th of April, the roots of the 
nerves on the right side were divided within the spinal canal, beginning at the 
head and proceeding gradually backwards. No enlargement of the vessels 
of the webs occurred until the roots of the sciatic plexus were cut, when full 
* In this and other cases of division of roots of the spinal nerves, I observed that the skin of the 
limbs supplied by the nerves cut became perfectly smooth, instead of being, as usual, rough with 
minute papillae. This appears to show that the unstriped muscular tissue of the skin is under control of 
the spinal system. 
