Congestion, strictly speaking, an accumulation of any liquid in an organ or tissue, but generally limited in medical works to an abnormal amount of blood in the vessels of a part otherwise healthy, and in most cases from an enlargement of the minute arteries and capillary blood vessels. Congestion may also be passive from obstruction to the circulation from external or constitutional causes. Congestion, called hyperemia by Andral, may be entirely independent, as to its cause, of the organ or tissue in which it is seated. The highly vascular organs, and those which receive the blood most directly from the heart, as the brain, the lungs, the liver, and the spleen, are the most frequent seats of congestion, together with their capillaries. Congestion differs from inflammation, to which, however it may lead, in its anatomical characters: in the former the organization and vital characters are unaltered, and the post-mortem appearance of sanguineous accumulation may be removed by the action of water; but in the latter the redness is permanent, the consistence is changed, and various morbid products are effused; though congestion precedes inflammation, it does not necessarily proceed to it.

There are certain conditions of the circulation in which congestion may be said to be normal and physiological; as, for example, in the erectile tissue of the nipple and other organs, and in the superficial coloration of the blushing cheek. Andral makes three degrees of congestion: 1, in which an increased amount of blood is sent to a part; 2, in which, in addition, the capillaries are dilated, with retardation of the circulation, a tendency to coagulation of the blood, and darker color of the tissues; this is the true type of congestion; 3, in which there is complete stagnation of the blood, with a darker coloration. As the first of these degrees is less than congestion, as ordinarily understood, so the last is more than congestion, involving a new condition of the affected parts. As congestion is a commencement of disease in many organs, the functional disturbances arising from it are various. It is not always easy to ascertain the predisposing and exciting causes of congestion,' though it may be stated as a general rule, that repeated stimulation of an organ or tissue predisposes it to congestion; inflammation in a neighboring part may induce congestion, as for instance in the brain during inflammation of the air passages; an unequal distribution of blood from cold or other causes may cause pulmonary or other visceral congestions.

The redness and swelling are in proportion to the accumulation of blood; the heat and pain are trifling, unless the congestion be extreme; the distention of the vessels may end in their rupture, and in circumscribed or diffused haemorrhage, though effusion of blood may also occur from a diseased state of the fluid, as in typhus, scurvy, and the oedematous congestions of chlorosis, without rupture of the vessels. It appears from the experiments of Magendie that a diminution in the proportion of fibrine in the blood, from any cause, predisposes to congestion; Andral noticed also a diminution of this element in many cases of cerebral congestion, beginning with headache, dizziness, and bleeding at the nose, and ending often in coma and apoplexy. Congestions of the brain and spinal cord, if of long duration, or ending in haemorrhage, are highly dangerous and frequently fatal; the spleen is sometimes congested to the point of rupture, causing death, without any premonitory symptoms. Frequent congestions of an organ bring on hypertrophy, thickening, and a disposition to inflammation; they are generally of short duration, and vary in severity from the apoplectic congestion of the brain to the simple swelling of a hemorrhoidal tumor; their seat is very frequently changed in many hysterical females.

The liver is almost always more or less congested at the moment of death; this condition may exist in its whole substance, the lobules presenting a nearly uniform dark color throughout; or the centres alone may be thus colored, the circumference being lighter; in this, the first stage of hepatic venous congestion, in which the hepatic veins are full and the portal plexus empty, the appearance is due to the continuance of capillary action after the general circulation has ceased; in the second stage, the portal as well as the hepatic venous system is congested, and the obstructing cause may be either in the liver, in the heart, or in the general venous system; occasionally the portal system is congested, the marginal portions of the lobules being darkest colored. A common form of venous congestion depends on deficiency of tone in the veins, which prevents the normal ascent of the blood from the lower parts of the body, thereby distending the vessels and causing an accumulation of blood; in this condition the serous parts of this fluid are prone to escape, forming dropsical effusions and anasarca.

This inability of the blood to ascend against gravity is found in a great variety of chronic diseases, the consequences of improper food, unhealthy habitations, or even the natural results of old age; the want of tone in the system is aggravated by ulcers, gangrene, and effusions of blood and other fluids into the cavities and tissues.