252 ON THE EARLY STAGES OF INFLAMMATION 
to very different degrees in different instances under the action of this substance, 
but that in all cases, after reaching a certain point, it becomes incapable of ad- 
vancing further in the irritated part, however much it may increase in the body 
generally, in case of the animal changing to a darker colour. These differences 
depend partly upon the strength of the mustard, the diffusion being least when the 
irritant is most potent. Thus, on one occasion, when a solution of the volatile 
oil in spirit of wine was applied to a web in which the pigment was fully con- 
centrated, congestion was very rapidly developed, without any alteration in 
the appearance of the chromatophorous cells. That the diffusion is in inverse 
proportion to the energy with which the mustard acts, was well illustrated by 
the experiment which furnished the drawing given in Plate V, Fig. 1.'_ In that 
case, a frog having been prepared in the manner mentioned in the note to p. 32, 
a portion of very strong mustard was placed upon the middle of one of the webs, 
the pigment being in the stellate condition, such as is seen on the left-hand side 
of the sketch, which represents a part of the edge of the spot to which the irritant 
was applied, together with an adjoining portion of the web. Shortly after 
this had been done, I noticed that the pigment was in a state of full diffusion in 
a ring round about the opaque mass, producing the reticular appearance shown 
in the stripe down the middle of the sketch. I had in a previous case seen 
a similar ring become affected with congestion, when a portion of mustard 
had been applied for a long time, in consequence of the pungent vapour of the 
volatile oil playing upon the neighbouring parts of the web, and there could 
be no doubt that the effect on the pigment in the present instance was due to 
the same cause ; but in the latter no material change was as yet visible in the 
blood except close to the edge of the mustard, where the corpuscles were seen 
to be abnormally adhesive. After the lapse of about an hour, the area on which 
the irritant had lain being examined, was found to be the seat of the most intense 
inflammatory congestion, indicated in the drawing by the crimson colour of the 
vessels, but the pigment there had experienced only an exceedingly slight degree 
of diffusion, being, in fact, almost exactly in the same state as at the commence- 
ment of the experiment. Thus the vapour of the volatile oil, though operating 
too mildly to cause inflammatory congestion, nevertheless induced the highest 
possible degree of pigmentary diffusion ; but the mustard, where it lay actually 
in contact with the web, and acted energetically upon it, arrested that very 
process of diffusion to which its gentler operation gives rise. 
In the progress of the case it happened that the animal changed from the 
medium tint which it had at first to a very pale colour, the pigment, in the web 
generally, assuming the dotted condition depicted on the right-hand side of 
' This experiment was performed subsequently to the reading of the paper. 
