64 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
had the pigment in the same condition throughout at one time implies that 
a large number of pigment-cells were acting in concert, and therefore probably 
under the control of the nervous system, although, as the leg had been amputated, 
they were of course freed from the influence of the central ganglia. Hence 
we are led to suspect the existence in the limb of an apparatus, probably gan- 
glionic in structure, co-ordinating the actions of the pigment-cells, just as we 
know that the muscular contractions in the mammalian intestine are harmonized 
by a local mechanism of that nature, while we have reason to think that the 
same thing holds regarding the arteries in the frog’s web.’ Such a view is in 
accordance with the results of recent anatomical discovery, which has revealed 
nerve-cells in many parts where their occurrence had not previously been con- 
jectured. But in the absence of more positive evidence, we must be careful not 
to trust too much to analogy on such a point; for it by no means necessarily 
follows, that, even if muscular fibre-cells are incapable of acting in mutual 
harmony without the aid of the nervous system, the same must be the case 
with pigment-cells, which, it is to be remarked, resemble ganglion corpuscles 
in being connected together by anatomosing offsets. The nerve-cells, if such 
be really the means by which the harmonious actions of the pigment-cells in 
an amputated limb are induced, must be disseminated among the tissues of the 
web itself ; for both post mortem concentration and secondary diffusion occur 
in a piece of web cut out and placed in a drop of water on a plate of glass. 
This was ascertained on September 4, 1858, in the case already alluded to as 
an instance of rapid occurrence of concentration. About half an hour after 
removal from the body, the pigment, previously reticular, was in the dotted 
state, and three hours later it was found to be again stellate. 
The case of the pigment-cells 1s analogous to that of the arteries in this 
respect, that, so long as circulation is going on, they are generally in entire 
subjection to the central ganglia, and act only when stimulated by their in- 
fluence. But as, in the arteries, it appears to be by the independent action 
of the local nerves that a contraction caused by direct irritation spreads to: 
a considerable distance from the part operated on, so it is probably by local 
means that the pallor induced by pinching the web affects a circle of surrounding 
tissue. If this be true, the case of direct irritation will be an exception to the 
general rule, that, while circulation continues healthy, concentration always 
implies the operation of the central organs of the nervous system. 
Comparing the changes in the pigment in an amputated limb with those 
which take place under similar circumstances in the arteries,” it appears that 
* See the preceding paper ‘On the Parts of the Nervous System which regulate the Contractions. 
pr therArtertes” sp: Ar: * See the preceding paper before referred to, p. 39. 
