The injection into the peritoneal cavity of a urea solution (30 or 50 per cent) together with or after an inflammatory irritant (aleuronat) prevents wholly or in part the local fixation of graphite particles or ferric chloride introduced subsequently. The histologic picture in the retrosternal lymphatics explains how this comes about. When free dissemination of graphite to the retrosternal nodes occurs, the lumen of the lymphatic vessel is unobstructed, whereas partial dissemination is accompanied by small fibrinous thrombi occluding the lumen in part only.

Trypan blue injected at the periphery of an inflamed skin area treated with a concentrated urea solution and bacteria (Staph. aureus) penetrates readily into the area, whereas it fails to do so when introduced around an inflamed area consequent on the injection of distilled water and bacteria (Staph. aureus).

Concentrated urea per se is an inflammatory irritant. Graphite particles injected into a peritoneal cavity previously treated with concentrated urea penetrate freely to the retrostemal lymphatic nodes; the lymphatic vessel is relatively unobstructed. Trypan blue injected into the circulating blood accumulates rapidly in cutaneous areas almost immediately after the latter have been treated with concentrations of urea ranging from 50 per cent down to 20 per cent.

The results of this study furnish evidence, in addition to that already provided, that fixation of foreign substances is primarily due to mechanical obstruction caused by a fibrin network and by thrombosed lymphatics at the site of inflammation. The significance of fixation in relation to immunity and its bearing upon some of the other processes involved in the inflammatory reaction have been stressed.