32 ON THE PARTS OF THE NERVOUS SYSTEM 
and about a minute afterwards 3°. The experiment was repeated several times 
with similar results, ‘the invariable rule’ (to quote from my notes) ‘ being 
contraction of the artery up to a certain point, and maintenance in the contracted 
state during the whole time, often several minutes, that the needle was stirred 
about in the brain; and then expansion, beginning almost immediately after 
withdrawal of the needle, and advancing to a certain point at which it remained 
till the needle was again introduced.’ As the brain became more and more 
broken up, the contractions grew less and less energetic, and the dilatations 
were increased, till the needle failed to produce greater contraction than from 
4° to 3°. I then thrust the needle into the spinal canal and withdrew it im- 
mediately. The hind legs started, and, after a few seconds, when I first caught 
sight of one of the webs, it was almost bloodless, and the arteries were invisible 
through extreme constriction. Four minutes later the artery before observed 
had begun to dilate and measured 1°, and after five minutes more it was 3°. 
A repetition of this experiment produced similar effects.’ 
Abundantly sufficient proof had now been obtained that the cerebro- 
spinal axis does contain a nervous centre for regulating the contractions of the 
arteries of the feet. But it was uncertain whether that centre were confined 
to any one part of the cord, or diffused extensively through it and the brain ; 
or even whether a similar office might not also be discharged by some of the 
sympathetic ganglia. With a view to determining these points, which are 
of great physiological interest, several experiments were performed, some of 
which it will be necessary to relate; but in order to make their description 
intelligible, it will be well to say a few words regarding the arrangement of 
the spinal cord in the frog. It does not occupy the entire length of the spinal 
canal, but extends backwards only seven-tenths of the distance from the occiput 
* The constriction of the arteries of the webs on irritation of the cord may be readily demonstrated 
in the following simple manner. The head of the frog being depressed so as to stretch the ligament 
between the occiput and first vertebra, a sharp knife is carried across the spinal canal immediately 
behind the head, so as to divide the cord from the brain. The toes may now be tied out and any 
observation made upon the web without the inconvenience generally produced by voluntary struggles 
on the part of the animal, while at the same time the use of chloroform is avoided; which is very 
desirable, on account of the irritating effect of its vapour on the web and the constant care required 
for its administration. If the webs be examined immediately after the operation, they will be found 
exsanguine from extreme constriction of the arteries ; but in a few minutes this state will give place 
to dilatation with free flow of blood. If now a fine needle, curved at the end, be introduced through 
the wound into the spinal canal, so that its point may penetrate a short distance into the cord, while 
the eye of the observer is kept over the microscope, the arteries will be seen to become constricted to 
absolute closure, and dilate again after withdrawal of the needle. The experiment may be repeated 
as often as may be desired till the cord becomes disorganized. 
I have lately found the above-mentioned mode of preparing the frog the best adapted also for 
experiments elucidating the nature of inflammation Little if any reflex action of the limb occurs when 
irritants are applied to the web; and if no great amount of blood have been lost in the operation, the 
creature will survive it a long while, e.g. eight days in one case. 
