34 FRANK ADAM McJUNKIN 
Further, it was found that large quantities of lampblack must 
be injected else most of it is removed from the plasma by 
endothelial cells rather than by free cells in the blood. Again 
as many as 90 per cent of the neutrophiles of the peripheral 
blood contain lampblack after its injection, and in rabbits 
especially the number of mononuclear carbon-containing cells 
is very small after the intravenous injection of non-citrated' 
carbon suspensions. By injecting sodium citrate at the same 
time that the lampblack is injected the percentage of neutro- 
philes with carbon is reduced to below one per cent and as many 
as 3.03 per cent of phagocytic mononuclear cells have been 
counted. It appears, therefore, to be necessary for the satis- 
factory intra vitam demonstration of so-called macrophages in 
blood not only to inject large amounts of microscopic particles, 
but also to inhibit the active phagocytosis of the neutrophiles by 
the use of sodium citrate. 
c. Conditions under which phagocytosis of carbon takes place 
in vivo 
In a previous report (McJunkin, 718) it is shown that the 
ingest on of carbon by certain cells in vitro may be controlled 
by the concentration of sodium citrate, and since both polymor- 
phonuclear leucocytes and the mononuclear ones ingest the 
carbon it became a matter of importance to employ certain con- 
centrations of sodium citrate in order to inhibit the activity of 
the neutrophiles and to obtain a clear picture. It was found 
that in vitro 15 mg. of this salt (Merck highest purity) per 
cubic centimeter of blood almost completely inhibits the ingestion 
by all cells under the conditions prescribed, while with 5 mg. 
per cubic centimeter of blood not only the phagocytic mono- 
nuclear cells, but more than half of the neutrophiles ingest the 
carbon. By using 7.7 mg. sodium citrate per cubic centimeter 
of blood the best differentiation of cells is obtained, since a small 
percentage of neutrophiles ingested the carbon and practically 
all of the phagocytic mononuclear leucocytes ingest it. 
