In almost all forms of anaemia the bone-marrow shows alterations which vary in different cases, and are most extreme in advanced cases of pernicious anaemia. The normal bone-marrow is of two sorts: the red, which occupies all the bones in the foetus and in young persons; and the yellow, which replaces the red in the shafts of the long bones in adult life, leaving the red in the cancellous tissue of the short bones and partly in that of the long bones. In anaemias, and more particularly in pernicious anaemia, there is a recurrence to the condition of red marrow in the shafts of the long bones, and there is frequently an atrophy of the bony lamellae, so as to accommodate more of the altered marrow. We have thus, instead of the yellow adipose marrow,, a red semi-fluid substance in which there may be no adipose cells, or only a few. As the normal red marrow is evidently an active tissue as compared with adipose tissue, and as the abnormal red marrow of anaemias is virtually of the same structure as the normal, it has been inferred that we have here a hypertrophy of the marrow. As the marrow is to be regarded as the principal source of the red corpuscles, the increase of its active tissue may be taken to imply an increased formation of red corpuscles. (See further under Bones and Joints).

In the altered bone-marrow there is an increased number of nucleated red corpuscles (Fig. 20 a, c), which, again, vary greatly in size, some of them reaching large dimensions. There are also large granular cells in large numbers, and many of these are eosinophil cells which are normal constituents of the marrow.