540 THE THIRD HUXLEY LECTURE 



vein was the result of isolation of the vessel from other structures. For 

 adhesiveness of corpuscles is not occasioned in the frog's web by amputation 

 of the Hmb ; nor is it produced in the human subject by complete detachment 

 of a portion of tissue ; as is clearly shown by the persistent healthiness of a piece 

 of skin entirely transplanted in skin-grafting. We are therefore forced to the 

 conclusion that the adhesiveness of the red discs in the horse's jugular, as con- 

 trasted with its complete absence in the vein of the bat's wing, was due to the 

 larger size of the vessel in the former case. And the only way in which it 

 seems possible to interpret this difference of behaviour of the corpuscles in the 

 two cases is to suppose that they possess an innate and normal viscosity which 

 is kept in abeyance by some action of the healthy tissues ; this action having 

 a limited range of operation, so that, while effective for vessels of small size, 

 it fails to influence the mass of blood in a large venous trunk. And I may 

 remark, in passing, that it is only in the smaher vessels that absence of adhesive- 

 ness of the corpuscles is essential for the free transmission of the blood. 



The mobility of the black pigment-granules of the frog has often struck 

 me as extremely remarkable. Perfect absence of any tendency to aggregate 

 on their part must be fully as essential to the freedom with which they move 

 through the exquisitely delicate ramifications of their containing cells as want 

 of adhesiveness of the blood-corpuscles is to their free transit through the capil- 

 laries, and I cannot but think that the two phenomena must be analogous. 

 It may be, for aught we know to the contrary, that the pigment-granules may 

 be themselves living entities. Their uniformity in size is in favour of such 

 an idea. Our fathers would have been greatly astonished to learn that the 

 chlorophyll grains of vegetables were, as has been shown in recent years, living 

 organisms, multiplying by division like the nuclei of their containing cells.; 

 and though the pigment-granules are much smaller, they must be greatly 

 surpassed in minuteness by many microbes which, though hitherto invisible 

 to us, we believe from analogy to be the causes of some infective diseases. But 

 however this may be, the perfect mobility of the pigment-granules seems to 

 me a special property which they possess as constituents of the healthy living 

 body ; in other words, to use once more the expression which in the present 

 state of our knowledge is indispensable, a vital property. 



If this be so, we understand what would otherwise be very unintelligible, 

 viz. that when the pigment-cells have their functions temporarily suspended 

 by a noxious agent, the granules do not become diffused as they do when simply 

 withdrawn from the influence of the nervous centres, but remain exactly as 

 they were before the irritant was applied, whether fully concentrated, com- 

 pletely diffused, or in any intermediate state. If we suppose that the pigment- 



