Marrow

Marrow is a substance of low specific gravity filling the cells and cavities of the bones of mammals. There are two varieties, which are known as red or watery marrow and yellow or oily marrow. In some of the short bones, as the bodies of the vertebrae and the sternum, the marrow has a reddish colour, due to the presence of cells which have this colour and are supposed to be transition forms between the proper marrow cells and red blood-corpuscles. On analysis it is found to contain 75 per cent. of water, the remainder consisting of albuminous and fibrinous matter, with salts and a trace of oil. In the long bones of a healthy adult mammal the marrow occurs as a yellow, oily fluid, contained in vesicles like those of common fat, which are imbedded in the interspaces of the medullary membrane—i.e. a highly vascular membrane lining the interior of the bones. This marrow consists of 96 per cent. of oil and 4 of water, connective tissue, and vessels.