=Passive Hyperemia.=--Passive hyperemia is most often a mechanical consequence of obstruction of the return of blood, which can be imitated at will, and which is not infrequently the result of carelessness, as when an injured limb is bandaged too tightly. Experiment shows that when such mechanical obstruction has taken place there is temporary increase of intravenous pressure, which soon returns to the normal standard, such readjustment meaning that blood has found its way back by collateral circulation. Only when such rearrangement is possible do we have anything like permanent passive hyperemia. In organs with a single vein, such as the kidneys, the question of obstruction may assume a very important aspect. Under these circumstances the appearance of the involved part, when visible, is spoken of as _cyanotic_, while its surface, instead of being abnormally warm, is the reverse, due to impeded access of warm blood and more rapid surface cooling. The blood under such conditions is often darker than natural, because, remaining longer in the part, it absorbs more carbonic dioxide, or at least gives up more of its oxygen. As long as actual gangrene is not threatened, the blood column has a communicated pulsation, at least in the large veins. Escape of corpuscular elements may occur after the phenomena above noted have been present for some time; but the corpuscles rarely, if ever, escape until there has been more or less copious transudation of the fluid portion of the blood--_i. e._, the serum. When anatomical changes can be grossly, yet carefully, observed, as in the fundus of the eye, it is seen that under these circumstances the arteries become smaller, although whether this is a primary or secondary change is not to be determined. Discoloration of the integument is the frequent result of leakage of blood corpuscles and their pigmentary substance into the tissues, and is consequently a frequent accompaniment of chronic passive edema. It is seen often in connection with varicose veins of the legs. Another form of passive congestion or hyperemia is that due to enfeeblement of the heart’s action by serious injury or wasting disease. When under these circumstances the lung has become more or less infiltrated with fluid, with hemorrhagic extravasation, the condition is known as _hypostatic pneumonia_--a misnomer, nevertheless indicating a condition which is only too frequent in the aged and feeble. RESULTS OF HYPEREMIA AND CONGESTION. These may be-- 1. Speedy subsidence of all hyperemic phenomena--resolution. 2. Acute swelling. 3. Chronic swelling. 4. Gangrene. 5. Nutritional changes--atrophy and hypertrophy.