ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS 39 
are very few polymorphonuclear leucocytes (fig. 6) with ingested 
carbon. At this time only an occasional carbon-containing cell 
has migrated into the extravascular tissue, but the accumulation of 
these cells in the sinusoids explains their disappearance from the 
peripheral blood. After several days, as in dog 151, numerous 
carbon-containing cells are present in the periportal tissue and 
tubercle-like foci made up of carbon-containing mononuclear 
cells about which there is a small amount of fibrin (fig. 7) have 
formed in the sinusoids. In the liver of dog 209 and of other 
one-day animals an occasional small focus of carbon-containing 
mononuclear cells resembling the large foci of animal 151 may 
be seen in the sinusoids. Wide areas of surrounding liver 
parenchyma reveal no extravascular leucocytes, and the only 
possible source of these cells is the blood. In the capillaries of 
the spleen, bone-marrow, and lymph nodes and to a less extent 
in the capillaries of other organs there is a similar accumulation 
of leucocytes. In the one-day animals the reaction to the 
carbon is entirely intravascular and is limited to free and attached 
cells similar morphologically. 
That most normal tissue contains practically no extravascular 
phagocytic cells may readily be proved by injecting lampblack 
into organs and fixing the tissue after the cells of the tissue have 
had ample opportunity to ingest carbon, but before leucocytes 
have had time to pass from the blood stream into the tissue 
spaces. <A tissue or organ, such as the bone-marrow or spleen, 
is not suitable for such an experiment, for it is difficult’ to de- 
termine what cells are intravascular and what ones extravas- 
cular. In the intestinal wall, the subcutaneous tissue beneath 
the nipples and the kidney of animals 211 and 212 practically no 
extravascular carbon-containing cells are present thirty minutes 
and two hours after injection. There is a tendency to recognize 
a close relationship or identity of cells to which such terms as 
adventitial cells, gland connective tissue cells, areolar connective 
tissue cells, reticulum cells, ete., and the fibroblast. Certainly 
proof that. is positive and easily obtained shows that such 
extravascular cells do not ingest by phagocytosis and do not 
give rise to phagocytes. | 
