36 
HEATOLYMPH NODES OF THE SHEEP 
at the inner margin of the peripheral blood space, there were fewer poly- 
morphonuclear leucocytes and eosinophiles, but more lymphocytes. How- 
ever, in some of the largest blood islands, and in nodes in which blood 
had so extensively displaced the lymphatic tissue, extremely few lymph- 
ocytes were found among the solidly packed erythrocytes. 
Eosinophiles were present in the hemal areas in small numbers only, 
being confined almost entirely to the surrounding lymphatic tissue. In 
the case of very large blood islands the intermingling of erythrocytes and 
lymphatic tissue was occasionally so extensive, however, that there were 
approximately as many cells of the one kind as of the other. However, 
in some cases most of the blood was found distributed rather evenly 
among the lymphoid tissue. It seems probable that this condition results 
from a secondary invasion of the blood spaces by the surrounding lymph- 
ocytes, rather than by an invasion of the surrounding parenchyma by 
erythrocytes, although, to be sure, primarily the lymphatic tissue was 
displaced by blood cells. No matter what the character of a node, the 
boundaries between the blood islands and lymphoid tissues were ill- 
defined, as a rule (see fig. 20). Occasionally, however, a well-defined 
wall, of rather coarse reticulum, was present between the two (fig. 21). 
This bounding reticulum between blood islands and lymphatic tissue 
was seldom very apparent, except in a few nodes which contained so little 
blood and lymphatic tissue that they were practically empty. In these 
nodes the lymphatic tissue had been reduced to such an extent that one 
might rightly speak of islands of lymphatic tissue in a hemal area. As 
might be expected, however, there seemed to be very little intermingling 
of the blood and lymphocytes in by far the greater portion of some of 
these nodes. Moreover, since these walls of reticulum are often similar 
or identical to the walls of the apparently condensed reticulum seen at 
the periphery of a follicle, it is possible that the accumulation of blood 
in the empty reticulum may be an important factor in producing them. 
This supposition seems to be supported by the fact that they were seldom 
or only partially present, around very small blood islands. Moreover, 
whenever the total amount of blood was larger the contiguous surfaces 
of blood and lymphatic tissue were often indicated by a gently curving 
line, instead of by a very irregular and ill-defined one. However, here and 
there a portion of lymphatic tissue of irregular form might nevertheless 
extend boldly out into the surrounding blood. It is only occasionally 
that blood islands are found within the follicles ; and when so found they 
are generally small, and never seem to be continuous with those outside 
of them. Nevertheless their absence rather than their presence here 
would seem to need an explanation. 
