THE THIRD HUXLEY LECTURE 525 



cessive stages of concentration of the pigment admitted of only one just 

 interpretation.^ 



These changes in the disposition of the pigment accommodate the tint of the 

 animal to that of surrounding objects. A dark frog placed in a white earthen 

 basin in sunlight soon assumes a dull yellow colour, and a pale one is not long 

 in becoming black in a covered earthen jar. 



It was very interesting to find that light produces these effects, not by 

 direct action upon the skin, but indirectly through the retina and optic nerve. 

 A hood of black cloth, carefully arranged so as to exclude light from the eyes 

 without obstructing respiration, entirely prevented a dark frog from becoming 

 pale in bright sunlight. I was naturally desirous of ascertaining through what 

 efferent channels the nervous impulse that caused concentration of the pigment 

 on exposure to light was conveyed from the brain to the foot. Division of 

 the sciatic nerve had no effect whatever upon the colour of the limb. I then 

 tried cutting through all the structures in the thigh except the bone, the femoral 

 artery and vein, and the sciatic nerve. This also had no influence. But when 

 I added to the latter procedure the section of the sciatic, the animal being then 

 pale, it gradually grew dark below the seat of operation, till in no long 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 



' Max Schulze had not yet described the movements of animal protoplasm ; and if he had done 

 so, this could have gone but a little way in explaining the phenomena described in the text. The 

 gushing out of homogeneous pseudopodia from the granular body of an amoeba may, however, be of 

 an allied nature. I made attempts to sec the movement of the pigment-granules in cells in which 

 concentration was going on; but their extreme minuteness, together with the excessive rapidity of their 

 apparent motion under the high magnifying power requisite, made them generally elude observation. 

 1 fancied I saw an indefinite rush of something through the clear space around the already accumulated 

 mass, but I could not be sure. On one occasion, however, I saw some individual granules leave the 

 mass and make excursions into the colourless liquid, as T could not doubt it to bo. 



* Vide Phil. Trans., ibid, (see vol. i, p. 45). 



