254 ON THE EARLY STAGES OF INFLAMMATION 
shown elsewhere! that concentration is the invariable result of the action of 
the nerves upon the chromatophorous cells, and that diffusion takes place 
whenever they are liberated from nervous influence. Also in the tree-frog of the 
Continent, which is much more liable to changes in the colour of the integu- 
ment, in consequence of direct irritation, than our own species, the invariable 
experience of the German observers was, that concentration followed the applica- 
tion of a local stimulus, while secondary diffusion sometimes occurred in the 
irritated spot, depending apparently upon exhaustion.2. From these facts, 
diffusion ensuing on irritation cannot well be regarded as an increased action 
excited by the stimulus, but rather as an evidence of diminished vigour, With 
croton oil and cantharides, which have not an irritating vapour, the diffusion 
is exactly limited to the extent of the irritant, showing that it is due to a direct 
action on the tissues; and the most probable explanation of its occurrence 
appears to be that mustard, croton oil, and cantharides have the peculiarity 
among irritants of affecting the nerves of the pigment-cells in the part they act 
on, somewhat more rapidly than the cells themselves, and, paralysing the former 
while the latter still retain their powers more or less intact, permit diffusion 
to go on unrestrained by nervous influence, till the further operation of the 
irritant completely suspends the pigmentary functions. It may be objected 
to this view, that diffusion occurs on the application of these substances to an 
amputated limb, but, from evidence given elsewhere,’ it is probable that the 
pigment-cells possess a local nervous apparatus, on which the occurrence and 
maintenance of post mortem concentration depend, and the paralysis of which, 
while the pigment-cells retain their powers, would give rise to diffusion in an 
amputated limb. Be this as it may, the fact that the state of full diffusion 
continued in the ring around the congested area in the last mustard experiment 
for hours after the irritant had been removed, although, during that time, com- 
plete concentration occurred in the web generally, is pretty clear evidence that 
the pigment-cells in that part had not merely been stimulated to increased 
action (for in that case they would have returned to their former condition 
soon after the stimulus had ceased to operate), but had suffered a loss of the 
faculty of concentration. Whether the loss of power resided in the nerves 
of the pigment-cells, or in those cells themselves, is a matter of indifference as 
regards the objects of the present inquiry ; the important fact being that an 
action of the mustard so mild as to give rise to little or no derangement of the 
blood, nevertheless produced a certain degree of loss of power in the part on 
which it operated. There can be no doubt that the same principles apply to 
the cases of croton oil and cantharides ; and thus the diffusion caused by these 
+ See the paper ‘On the Pigmentary System ’, p. 64. *SSEP pis; * See p. 63. 
