Microscopic studies show the presence of a network of fibrin within the tissues and numerous thrombosed lymphatics at the site of inflammation. Precipitated iron compounds, possibly coagulated horse serum, or particulate matter caught in this fibrinous reticulum will disseminate less readily than trypan blue from the site of inflammation.
Trypan blue injected at the periphery of an inflamed area fails to enter the site of inflammation. This failure of penetration is caused by the occlusion of lymphatic vessels and by the presence of a fine network of fibrin in the tissue spaces of the inflamed area.
Fixation of foreign substances by the inflammatory reaction is therefore primarily due to mechanical obstruction caused by a network of fibrin and by thrombosed lymphatics at the site of inflammation.
There is another phase of the problem which still requires more accurate information. This concerns the relation between exudation from blood vessels and changes in flow of lymph from the inflamed area. Further experiments are being conducted to investigate this question.
The reaction of fixation which occurs extremely early in the inflammatory process circumscribes the irritating substance and allows a definite period of time for the leucocytes to assemble for the purpose of phagocytosis.
It is through a delicate regulating mechanism of this kind that, to use the expression of Opie (7), "the vital organs are protected at the expense of local injury."