526 THE THIRD HUXLEY LECTURE 



ramifications. Yet a sudden gush of tears or outburst of perspiration, although 

 famihar, 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 agenc}'^ that caused the grouping of the molecules ceased to 

 operate. The transference of the granules from the bod}" of each cell to its 

 remotest ramifications, and their close packing there, were an act such as a living 

 organism alone could have effected. Pigmentary concentration and diffusion 

 were vital functions of a profound character concerned with the relative dis- 

 tribution of different constituents of the cells. Yet from the very happy 

 circumstance of the conspicuousness of the pigment, the results of their activity 

 could be observed with the utmost facility, and their behaviour in relation to 

 inflammatory congestion easily studied even under a low magnifying power. 



The pigment-cells pervade the skin and subcutaneous tissue of which the 

 frog's web consists, and are especially numerous about the blood-vessels, round 

 which their branches twine abundantlv. Thev must, therefore, be acted on 

 along with the vascular parietes b}" anything applied to the surface of the mem- 

 brane. And to state shortly the result of many experiments, I found that 

 any agency, physical or chemical, which caused the blood-corpuscles to lag 

 behind the liquor sanguinis in the part on which it operated rendered the pigment 

 cells in that particular area incapable of discharging their functions. Whatever 

 might be their state at the time of the experiment, whether in full diffusion, 

 complete concentration, or any intermediate condition, so they remained in 

 the irritated spot, while in surrounding parts of the web, as in the body generally, 

 they changed as usual in obedience to differences of illumination or other circum- 

 stances. At the same time they were not killed : for if the irritation had not 

 been too severe, they recovered their full activity when resolution occurred.^ 



^ I have in rare instances seen an irritant cause diffusion from a state of concentration as a pre- 

 liminary effect. This was unmistakably the case on one occasion when mustard was employed. The 

 pigment was in an intermediate (stellate) state when the appUcation was made. In a narrow ring 

 round the mustard, where the volatile oil could only act extremely mildly on the web, the stellate con- 

 dition gave place to complete diifusion ; whereas under the mass, where the irritant had acted at once 

 with full energy, the stellate appearance remained unchanged. Inflammatory congestion, however, 

 had been produced in the ring of full diffusion as well as in the more strongly irritated area. It happened 

 that complete concentration afterwards took place in the rest of the web, while the irritated areas 

 retained the appearances above described. It seems probable that the diffusion under the slighter 

 irritation may have been the result of the nerves in the irritated part being paralysed before the 

 pigment-cells. 



As is commonly the case with more specialized structures, the pigment-cells are a delicate form of 

 tissue, and are more readily killed than other constituents of the web. Hence, if care is not taken to 

 avoid pushing the action of the irritant too far, it will be found, after resolution has taken place, that 



