66 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
The pigmentary system also promises to render good service in toxological 
inquiry. Hitherto, in experiments performed upon animals with that object, 
attention has been directed chiefly, 1f not exclusively, to the effects produced 
upon the actions of the nervous centres, the nerves and the muscles. In the 
pigment-cells we have a form of tissue with entirely new functions, which, though 
apparently allied to the most recondite processes of the animal economy, yet 
produce very obvious effects, and thus afford great facilities for ascertaining 
whether or not they have been destroyed by any poison that may have been 
administered. 
An experiment of this kind which I once performed, though with a different 
object, may be mentioned by way of example. Being desirous of confirming 
the conclusion to which I had been led by experiments above related, viz. 
that diffusion always tends to take place when the influence of the nerves is 
withdrawn from the pigment-cells, it occurred to me that the urari poison 
might be brought into requisition for that purpose: for it has been shown by 
Professor Kolliker of Wirzburg, that this substance paralyses in the first 
instance the extremities of the motor nerves without affecting the contractility 
of the muscular tissue; and supposing the nerves concerned in regulating 
the pigmentary changes to be similarly deprived of their powers, while the 
pigment-cells themselves remained intact, diffusion should take place after 
exhibition of the drug, provided my view were correct. Accordingly, at 
2 tom p.m. on December 21, 1857, I introduced beneath the skin of the back 
of a pale frog a portion’‘of urari extract, for which I was indebted to the kindness 
of Dr. Christison. At 2! 25™ reflex action was entirely abolished, the creature 
being to all appearance dead, so far as could be judged by the naked eye, 
although the microscope showed that circulation continued in the webs. The 
pigment meanwhile had become stellate, but did not continue in that con- 
dition, being, half an hour later, found fully concentrated. Soon after this, 
however, diffusion again commenced, and continued to advance steadily till 
circulation ceased early the following morning, at which time the integument 
was almost black. In the course of a few hours, however, it was brought again 
back to the palest possible tint by post mortem concentration. 
The diffusion which ultimately took place in this case was no doubt due 
to loss of function on the part of the central ganglia or the nerves connecting 
them with the pigment-cells. But from the occurrence of concentration half 
an hour after the faculty for reflex action had ceased, we learn that these nerves, 
like the intrinsic motor nerves of the heart and intestines, remain unaffected 
by the urari poison for a considerably longer time than those which excite the 
contractions of the voluntary muscles. We further learn from the fact that 
