To this class belong several anomalies to which we have already referred in the preceding pages; and of which we now proceed to consider the most important.

1. Contraction and final obliteration of the vein in consequence of persistent compression. - Such a form of compression is exerted by all tumors generally, but more especially by aneurisms; and we have frequently observed both a threatened and a complete destruction of the calibre of the vena cava superior from aneurisms of the ascending aorta. The vessel is first flattened at the spot exposed to the pressure, and when the latter is increased, the walls of the vein are at length brought into permanent contact with each other, and obliteration is thus established, consisting in a fusion or coalescence of the lining membrane of the vessel. The occlusion of the vessel above and below the coalescence by means of a coagulum (thrombus) is merely a secondary and unimportant occurrence, which, moreover, follows the known laws of a thrombus-formation.

2. Occlusion of the vein from coagulation of the blood. - To this class belong, independently of the coagula occasioned in varicose veins by the retarded flow or stagnation of the blood, the obstructing coagulum in phlebitis, and the coagulation arising at any part of the venous system from a diseased condition of the blood. Both (see pp. 257 and 260), may induce more or less complete obliteration of the vein. This is the most frequent form of occlusion and obliteration of the veins.

3. Occlusion arising from phlebolites seldom induces entire impermeability.

4. Occlusion of the veins from cancerous secondary formations is not of very rare occurrence, even in the venous trunks. (See p. 267).

The ordinary results of these anomalies are oedema and dropsies; the latter are, however, retarded in their development and healed by the establishment of a collateral circulation (even where the main trunks are closed) through the anastomoses of numerous veins. The obstruction which the valves must here present (Stannius) is undoubtedly overcome in many cases, or so far neutralized, that a collateral circulation is established, as in the arteries, by the anastomoses of very minute vessels having no valves. The closure of the portal vein can scarcely be compensated for, notwithstanding the numerous anastomoses of its roots with the systemic veins, and hence the dropsy which it induces does not admit of cure. Closure of the vena cava inferior is compensated* for by dilatation of the azygos and hemiazygos, and anastomoses of the epigastric vein with the internal mammary, and of the subcutaneous abdominal veins with the axillary: it is on the other hand less easy to compensate for the closure of the vena cava descendens, especially when the mouth of the vena azygos has at the same time become impervious; the collateral circulation is carried on by means of the venous plexus of the spinal canal and its anastomoses with the subclavian and hypogastric veins, by means of the anastomoses of the phrenic veins with the vena cava ascendens (and even with the great coronary vein of the heart, as occurred in a case of Raynaud's), and lastly, by means of anastomoses of the axillary and internal mammary, with the epigastric and circumflex iliac veins.

In rare cases we have also seen the circulation re-established in closed veins by means of canals, which have become developed near the centre of the occluding plug. This phenomenon either depends on the channelling of the thrombus - a process which we have already described in p. 248 - or it is due to the disintegration of the central portion of the coagulum into a molecular mass, which gradually becomes taken up into the blood, while the outer layers assume a fibroid character and coalesce with the venous walls. We have never observed such cases, but they have been described by Carswell and Hasse.

5. Separation Of Continuity

Under this head we include the various wounds and spontaneous lacerations of veins. The former, as is well known, heal by adhesion of the edges in cases of small incised or penetrating wounds; in cases, on the other hand, where the wound is larger, or where the vein is completely cut through, the cure is effected by obliteration of the vessel, consequent on a process which is generally much the same as that which occurs after an artery has been cut through.

Spontaneous lacerations of veins, if we except the bursting of true varices, are rare, in comparison with the lacerations of arteries. We have, however, observed them in the trunks of both venae cavae, in the azygos, the pulmonary, and other large veins.