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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 smaller vessels that absence of adhesiveness 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 capillaries, 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 chlorophyl 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
