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The seat of greatest congestion will always be found in the neighborhood of the toe, because of the increased vascularity of that part, and, although at times it is limited to the podophyllous tissue alone, any or all parts of the keratogenous membrane may be affected by the congestion and followed finally by inflammation.

When, from direct communication with the blood-stream, due to extensive hæmorrhage, bacteria from the outside gain entrance, this simple inflammation is further complicated by the formation of pus, or a limited gangrene of the keratogenous membrane.

In cases of great severity the gangrene of the keratogenous membrane spreads until the deeper structures are involved.

Posteriorly the cavity is bounded by the front of the os pedis, and is lined by a thin growth of horn secreted by the keratogenous membrane covering the bone.

It will here be seen that the injury to the keratogenous membrane has led to great interference with the secretion of horn from the sensitive laminæ.