B. NORMAL AND PATHOLOGICAL HISTOLOGY OF THE BLOOD.
In satisfactorily prepared dry specimens =the red blood corpuscles= keep their natural size and shape, and their biconcavity is plainly seen. They present a distinct round homogeneous form, of about 7.5 µ in diameter. They are most intensely coloured in a broad peripheral layer, and most faintly in the centre corresponding to their depression. With all stains mentioned above the stroma is quite uncoloured, and the hæmoglobin exclusively attracts the stain, so that for a practised observer the depth of stain gives a certain indication of the hæmoglobin equivalent of each cell, and a better one than the natural colour of the hæmoglobin in the fresh specimen. Corpuscles poor in hæmoglobin are easily recognised by their fainter staining, especially by the still greater brightness of the central zone. When somewhat more marked, they present appearances which from the isolated staining of the periphery Litten has happily named "pessary" forms. The faint staining of a red corpuscle cannot be explained, as E. Grawitz assumes, by a diminished affinity of the hæmoglobin for the dye. Qualitative changes of this kind of the hæmoglobin, expressing themselves in an altered relationship towards dyes, do not occur, even in anæmic blood. If in the latter, the blood discs stain less intensely, this is due exclusively to the smaller amount of hæmoglobin.
A diminution in the hæmoglobin content can in this way be shewn in all anæmic conditions, especially in posthæmorrhagic, secondary and chlorotic cases. On the contrary, as Laache first observed, in the pernicious anæmias, the hæmoglobin equivalent of the individual discs is raised.
To appreciate correctly pathological conditions, it must always be borne in mind, that in normal blood the individual red blood corpuscles are by no means of the same value. Step by step some of the cells are used up and replaced by new. Every drop of blood contains, side by side, the most various stages of life of fully formed erythrocytes. For this reason influences which affect the blood--provided their intensity does not exceed a certain degree--cannot equally influence all red corpuscles. The least resistant elements, that is, the oldest, will succumb to the effect of influences, to which other and more vigorous cells adapt themselves.