46 
the marrow to hold good only in the case of rabbits, and has 
not succeeded in detecting them in the human body. This 
difference he regards as possibly explicable by the fact that the 
human structures examined were those of persons enfeebled 
by disease, and also that they were not examined till some 
hours after death. Besides that, the human marrow, even 
that of children, is very different in appearance, even to the 
naked eye, from that of young rabbits, which may often be 
separated completely from the surrounding bone in the form 
ofa red cylinder. ‘The & priori probability that similar rela- 
tions exist in the human marrow will, very possibly, be 
confirmed by researches conducted in some different method. 
Eales also insists upon the fact admitted by Neumann, 
that the yellow fatty marrow which fills all the long bones in 
adults plays, at best, a very subordinate part in blood for- 
mation; and that the observations must be understood to 
apply only to the red vascular marrow which is found in all 
bones in young animals, but in adults, in the spongy bones 
only. 
This structure is minutely described by Neumann, whose 
account is confirmed in the main by Eales. It consists of a 
remarkably developed capillary network, and of a special 
tissue, called by Neumann the medullary tissue, contained in 
the meshes of this network. The capillaries have this pecu- 
liarity, that their calibre is much greater than, on the aver- 
age, four times as great as that of the small arterial branches 
immediately supplying them, and this sudden enlargement of 
the blood channel must cause a considerable diminution in 
the velocity of the blood. The capillary network is also very 
close, its meshes being only about half-again as large, or less 
than twice as large as the diameter of the capillaries. Within 
these vessels, especially in the wider portions, is seen a 
great accumulation of white cells, as well as “ transitional 
forms”? in variable proportion, and it is these which espe- 
cially mark out the locality as the seat of blood metamorphosis. 
The tissue contained in the meshes of the vascular network 
consists of a stroma of delicate stellate cells, anastomosing by 
means of fine prolongations, and thus forming a reticulation 
within which are contained, in the red marrow, a large 
number of lymphoid cells; but these do not occur in the 
yellow marrow. To this tissue Neumann gives the name of 
medullary tissue; and to the red form, lymphoid medullary 
tissue ; it bears most resemblance to the cytogenous connec- 
tive tissue of Kolliker, or adenoid tissue of His. The yellow 
marrow being without lymphoid cells, and having its anasto- 
mosing cells filled with fat, resembles ordinary adipose tissue. 
