Such a continued pressure may be exerted by different tumors, as goitres, encysted tumors, cancerous products, and aneurisms of neighboring arteries. Complete obliteration is very rarely induced by these causes; at any rate, in the larger arteries. The vessel becomes obliterated at the spot exposed to pressure in consequence of the coalescence of the lining membrane of the vessel; above this point the occlusion is affected by means of a plug reaching to the nearest branch, and beyond this the vessel is finally obliterated in the same manner as after tying the artery, as we shall have occasion to revert to in the sequel.

Besides these different modes of contraction, occlusion, and obliteration, we further noticed the following conditions when treating of aneurisms. (See p. 216).

a. An impermeability of the mouths of the branches passing from an aneurismal vessel in consequence of their contraction into fissure-like openings.

b. An impermeability of the mouths of these vessels, induced under certain conditions already indicated at the above page, by the fibrinous layers filling the aneurismal sac, and the shrinking and obliteration of the vessel consequent upon it.

We shall consider the establishment of the circulation consequent on the obliteration of an artery when we treat of the healing of cut arteries, and the process of obliteration that follows the tying of an artery.