Page:The third Huxley lecture.pdf/29

25 time it presented from the toes to the wound as great a contrast with the rest of the body as if that part had been covered with a miniature black stocking. Thus the regulation of this function, which is probably closely allied to the action of the cells in nutrition, was not carried on exclusively through special nervous channels, as is the case with the contractions of the voluntary muscles, but one nerve could take the place of others in the duty.

Light was not the only agency that induced pigmentary concentration. It might take place rapidly during struggling of the animal, and I once saw a frog grow pale in its efforts to avoid capture. Here mental emotion perhaps came into play, if we may use such an expression regarding the frog.

It seems quite astonishing that nervous action should make the pigment molecules rush thus rapidly to the centre of the cell from its remotest and finest ramifications. Yet a sudden gush of tears or outburst of perspiration, although familiar, is perhaps not less wonderful.

Concentration of the pigment took place, as we have seen, under nervous influence, and diffusion on its withdrawal. But diffusion was no mere passive phenomenon, such as might follow according to any ordinary arrangement of matter, when the agency that caused the grouping of the molecules ceased to operate. The transference of the granules from the