266 ON THE EARLY STAGES OF INFLAMMATION 
are in all cases also stimulants, seems very doubtful. As regards the nerves, 
such does appear to be the case; for while many, and probably all influences 
which induce inflammatory congestion cause temporary paralysis of sensation 
in parts on which they act severely enough, they all stimulate the afferent 
nerves in the first instance, as is shown by the reflex changes in the calibre of 
the arteries which occur round about any irritated spot. The nervous centres, 
too, present an illustration of the same principle, not only in the effects pro- 
duced upon them by the nerves, as lately alluded to, but also in the excitement 
well known to be occasioned by small doses of many sedative narcotics, such 
as alcohol, opium, and chloroform, which may be regarded as special irritants 
of the nervous centres. In the case of the cilia, I have not observed primary 
increase of movement to be induced by any agent besides heat ; but I am not 
prepared to say that it might not by careful management be made to occur 
with some other irritants. 
The pigment-cells in the common frog give very little indication of the 
stimulating properties of irritants, as is evident from several of the experiments 
which have been recorded in this section. In the tree-frog, however, as we are 
informed by the German authorities, a part of the integument subjected to such 
influences rapidly assumes a pale tint, and that even in a portion of skin removed 
from the body. I have also several times noticed, after pinching the web of 
a common frog, that, although in the spot actually squeezed, the pigment-cells 
were deprived of their power of changing, a pale ring about one-sixteenth of an 
inch in breadth has gradually formed in its immediate vicinity in the course 
of the next hour; whence it seems probable that direct irritation tends to 
excite concentration in the English species as well as in the continental, but 
that in the former the effect is developed much more slowly, so that it is apt 
to pass unnoticed. I further, on one occasion, saw post mortem concentration 
greatly accelerated by heat.1 It is doubtful, however, whether these results 
are due to direct action upon the pigment-cells ; for in the tree-frog, as well 
‘ This observation was made as follows :—On the 2nd of December, 1857, having amputated the legs of 
a dark frog, I put them both into water of 100° Fahr., but removed the left limb within six minutes 
of its immersion and placed it in cold water, leaving the right for a quarter of an hour longer, at the 
end of which time it was considerably paler than the left, and the microscope showed that its pigment 
was more concentrated. But while the warm water had accelerated the process of concentration up 
to a certain point, it had ultimately paralysed the pigment-cells and rendered them incapable of further 
change; so that the right limb remained permanently of the same colour as when removed from it, 
whereas in the left, which had been subjected for a much shorter time to the noxious agency, concentra- 
tion continued to advance, so that in twenty-five minutes that leg was as pale as the other, and after 
an hour more it was a good deal the lighter of the two. I may further mention that rigor mortis was 
already carried to the extreme degree in the muscles of the right limb when taken out of the water, 
but in the left this change did not commence till about twelve hours later. At this period the pigment 
in the left limb still showed signs of retaining its functions, while that in the right had a dirty, indistinct 
appearance, indicating that it had lost its vitality. 
