PIGMENT CELLS. 89 



removal, are determined by the altered states of 

 capillary circulation, which are themselves dependent 

 upon the degree of contraction of the small arteries 

 which carry the blood to them ; and this is affected 

 by the changes in nerve centres from which the 

 arterial nerves are derived. In this way, I think, 

 may be explained the concentration and diffusion of the 

 pigment in these remarkable cell spaces and channels, 

 which formed the subject of a highly interesting me- 

 moir by Professor Lister, who, however, inferred that 

 the nerves exerted some direct action upon the diffusion 

 of pigment. In order to make this view appear plausible 

 he was, however, obliged to assume the presence of an 

 " apparatus, probably ganglionic in structure, co- 

 ordinating the action of the pigment cells,"* which 

 neither he nor any one else has been able to discover, 

 and the existence of which is for many reasons most 

 doubtful. The fact is easily explained by the altera- 

 tion in the amount of fluid traversing the tissues at 

 different times. When the circulation is diminished 

 or arrested, the fluid in the cells passes into the 

 tissues, the tubes become nearly emptied, and the 

 walls in apposition. The solid particles become con- 

 centrated in the large central cavity of the cell, 

 which is the last part to lose its fluid ; but when the 

 circulation is free, and the tissues a're abundantly 

 irrigated, the particles spread into the radiating tubes 

 now filled with fluid. Professor Lister himself 

 remarks that post-mortem secondary diffusion occurs 

 in a piece of web " cut out and placed in a drop of 

 water on a plate of glass." Instead of accounting 

 for this fact by the changes resulting from the 

 imbibition of water, he invents an explanation which 

 I must venture to say does not appear to be justified 

 by observation or experiment, attributing it to the 

 action of hypothetical nerve cells "disseminated among 



* "On the Cutaneous Pigmentary System of the Frog." 

 Phil. Trans., 1857. 



