114 DR W. B. DRUMMOND. 
due to artificial or accidental extrusion, which, they admit, 
may readily be produced. 
The Arrangement of the Cells of the Marrow.—The arrange- 
ment of the different cells described in the marrow is not. 
by any means easy to make out, especially in mammals. In 
birds the marrow cells form a tolerably compact parenchyma, 
through which pass venous capillaries containing a moving 
stream of blood. ‘These capillaries are lined by layers of 
rounded cells which undergo subdivision, and of which the 
more superficial are gradually set free in the passing blood 
stream as nucleated red corpuscles. The marrow cells and 
the cells which give rise to the red corpuscles are thus 
separate from one another. 
In mammals the arrangement of the cells appears to 
be similar, but it is by no means easy to make this out 
distinctly. The different parts of the marrow are not. 
distinctly marked off from one another. The venous 
capillaries are represented by comparatively wide vascular 
channels, which in some parts appear to be bounded by 
nucleated red cells; in others, by the marrow cells 
proper ; in others, by a giant cell or a fat cell; while in 
some places they may have a proper endothelial lining. 
There is no absolute separation between the parenchyma 
of marrow cells and the groups of nucleated red cells. 
The latter may be found among the marrow cells, and do 
not anywhere form a complete boundary to the venous 
capillaries as they do in the marrow of birds. 
Seeing that the nucleated red corpuscles thus lie exposed 
in many places to the stream of circulating blood, one might 
expect to meet with them frequently in the general circula- 
tion. But this is not so, and their absence may be explained 
as follows. In the first place, the cells have probably a 
certain amount of cohesiveness, so that they are not readily 
loosened from their places until they are ripe—that is to 
say, until they have lost, or are just about to lose, their 
nuclei. In the second place, the stream of blood in the 
vascular channels must be exceedingly slow, and the current 
very feeble, for the total area of these channels is enormously 
greater than that of the small arterial twigs which supply 
the marrow with blood. 
