224 ON THE EARLY STAGES OF INFLAMMATION 
a given time through the capillaries, but neither full dilatation, extreme con- 
striction, nor any intermediate state of the former is capable fer se of producing 
accumulation of corpuscles in the latter. 
SECTION ITI 
On the Effects of Irritants upon the Circulation. 
It is well known that the application of an irritant substance to the web 
of the frog’s foot is followed by changes of calibre in the blood-vessels, and also 
by an abnormal accumulation within them of the corpuscular elements of the 
blood. The first experiments which I performed upon the frog were directed 
to the solution of the much-debated question, whether or no the latter were 
a mere consequence of the former ; and although it has, I think, been sufficiently 
shown at the conclusion of the last section that such cannot be the case, yet 
it will be well to allude shortly to these experiments on account of their further 
bearing upon the subject of this inquiry. 
It occurred to me that if, instead of the powerful irritants commonly used 
in these investigations, some exceedingly mild stimulant were employed, the 
changes in the calibre of the vessels might perhaps be produced without con- 
comitant alterations of the blood. The material which appeared most suitable 
for this purpose was warm water, which is known to cause, in the human subject, 
increased redness without inflammation of the part to which it is applied. 
Accordingly, in September 1855, I endeavoured to ascertain its effects 
upon the frog. In most of the experiments, the foot of the animal being stretched 
under the microscope upon a glass plate somewhat inclined, so that any fluid 
upon it might run off quickly, an assistant threw a stream of water of known 
temperature upon it by means of a syringe, the eye of the observer being kept 
over the microscope, which was provided with a micrometer in its eyepiece. 
In this way the effects produced by the water could be seen almost immediately 
after it had ceased to play upon the web, and the changes of calibre in any 
artery selected for observation were noted with precision. It was found that 
the result of the warm application was constriction of the arteries to absolute 
closure, generally lasting for several seconds,' and then giving place to dilatation 
beyond their original dimensions, to which they afterwards gradually returned. 
‘ The period of constriction varied much in different instances, and it sometimes passed off (if it 
occurred at all) before it could be observed. It was best marked in a case, in which, the animal being 
very quiet, chloroform was not employed. The anaesthetic appears to impair the functions of the 
spinal cord as a regulator of the calibre of the vessels ; and its administration is generally followed by 
their dilatation. 
