(A) Active Hyperemia

Active Hyperemia is produced when from any cause a general relaxation of the arteries in the sac occurs. Leaving inflammation out of account, this will hardly occur except as a result of sudden removal of extra pressure from these vessels. If a large ovarian tumour be removed from the abdomen, or ascitic fluid drawn off, the arteries and capillaries which have previously accommodated themselves to the undue pressure on their walls, relax, and hyperemia occurs. The hyperemia may result in the occurrence of a peculiar form of Chronic haemorrhagic peritonitis, comparable with haemorrhagic pachymeningitis (see p. 698), the effused blood being here also sometimes a prominent feature (Hematoma of the peritoneum). As in the other case it may be a question whether there is an inflammation preliminary to the haemorrhage, or whether the hyperemia induces bleeding, the succeeding organization of the clot leading to the formation of a membranous layer on its surface. Where ascitic fluid has been drawn off many times, there may be several layers of soft membrane on the surface of the peritoneum, the innermost being the most delicate and the newest.

(B) Passive Hyperemia

Passive Hyperemia occurs as a result of obstruction to the portal circulation, either alone or as part of a general venous hyperemia. It is chiefly important in relation to ascites, of which it is the most frequent cause.

(C) HaeMorrhage

There may be unimportant haemorrhages in the substance of the peritoneum in scurvy, hemophilia, etc. More considerable haemorrhages occur in consequence of rupture of considerable vessels. Aneurysms of the abdominal aorta not infrequently rupture. The blood accumulates at first behind the peritoneum, but it may also pass into the .cavity. Traumatic rupture of liver, spleen, or kidneys may also cause haemorrhage although these lesions, especially in the case of the kidney, are liable to produce subperitoneal haemorrhage. Then there are frequent haemorrhages from the female generative organs, as from the rupture of the cyst in extra-uterine pregnancy, but also without apparent cause at menstrual periods. (See Periuterine Hematocele.) There are also haemorrhages in tuberculosis and cancer of the peritoneum.

The effused blood is in many cases readily absorbed, but when the collection of blood is local, as in the pelvis, or the peritoneum is altered by inflammation, it may remain. If there is a considerable mass the name Haematoma is applied. The blood becomes decolorized and surrounded or encapsuled by new-formed connective tissue in the ordinary way. The blood thus sets up a chronic inflammation which, in the case of the pelvic haematoceles, may have serious consequences by interfering with the uterus and ovaries.

(D) Ascites

This name is given to dropsy of the peritoneal cavity. It is sometimes, although rarely, a part of a general oedema, occurring in disease of the heart or lungs, in Bright's disease, and in anaemic states. It is peculiarly prone to occur when the portal circulation is specially obstructed. This may happen by the portal vein being obstructed by thrombosis, or pressed on from without, but more frequently it is by some lesion in the liver itself, such as cirrhosis or cancer, which obstructs the portal vessels, as it were, in detail.

In these cases the ascites is from increased transudation, and it is much less common to have it occurring from interference with absorption. This may have something to do with it, however, in cases of cancer of the peritoneum, where the numerous cancerous tumours originate from material carried into the lymphatic channels and growing there. These tumours will necessarily obstruct the lymphatics to a large extent in detail, whilst they may also produce by their irritation a hyperaemia of the peritoneal vessels and so lead to increased transudation.

The fluid in ascites is contained in the general sac of the peritoneum, but where adhesions have previously existed it may be confined to particular parts, and sacculated. Sometimes also, in children, there is a dropsy in the sac of the omentum (Hydrops omenti).

The character of the fluid in ascites is that of ordinary transudations, a clear, slightly yellow, limpid fluid of low specific gravity. After it has stood for a time it often deposits a very gelatinous coagulum of fibrine.

In prolonged ascites the peritoneum is apt to get somewhat thickened, especially when puncture has been frequently performed. The great omentum is not infrequently gathered up so as to be thicker and shorter than usual, and in that case it will act less efficiently as a drain.

Chylous ascites has already been referred to as occurring in connection with obstruction of the thoracic duct (see under (Edema). The fluid in the abdomen is milky, containing finely dissolved fat. Of a different nature is Ascites adiposus, which may be confused with clrylous ascites, as in it also the fluid is milky. In this case the milki-ness is from fatty degeneration of cells, either the endothelium in ordinary ascites or the cells in cancer. There is always the distinction between the two conditions that in chylous ascites the fat is free, while in ascites adiposus it is in cells.