It was found that the tissues were attacked by phagocytic cells that became enlarged and carried away fragments of the tissue; the cells were subsequently identified as leucocytes or blood-cells.
In the " seventies " of the 19th century feeling ran somewhat high over the rival doctrines concerning the origin of puscorpuscles, Cohnheim and his school maintaining that they were derived exclusively from the blood, that they were leucocytes which had emigrated through the walls of the vessels and escaped into the surrounding tissue-spaces, while Stricker and his followers, although not denying their origin in part from the blood, traced them, in considerable proportion, to the fixed elements, such as fibrous tissues and endothelia.
It is largely to researches on the bone marrow that we owe our present knowledge of the origin and the classification of the different cellular elements of the blood, both erythrocytes or red corpuscles, and the series of granular leucocytes or white corpuscles.
If the white cells be required, as in local suppurating abscess, general septicaemia, acute pneumonia, &c., there is an active proliferation of the myelocytes to form the polymorpho-nuclear leucocytes, so that we have in this condition a leucoblastic transformation of the fatty marrow.
Absorption of this infarcted zone is carried on by means of leucocytes and other phagocytic cells, and by new blood-vessels.