Retroperitoneal Hernia includes cases in which the intestine passes into a pre-existing pouch in the peritoneum, greatly enlarging and filling it. The hernial sac hence lies behind the peritoneum of which it is an offset. There are three principal pouches in the peritoneum which are capable of giving rise to such hernias.

The fossa jejwno-duodenalis is the most important. It exists just where the jejunum arises from the duodenum, and lies between the last part of the duodenum, which bounds it on the right, and the aorta, which bounds it on the left. The pouch was present, according to Waldeyer, in about 70 per cent, of the bodies which he has examined, and is generally large enough to admit the terminal phalanx of the thumb. It is best seen when the jejunum and small intestine generally are raised and carried to the right, so that the origin of the mesentery may be exposed. The little pouch, if present, is then seen lying in the posterior wall of the abdomen with a sharp sickle-like margin. Sometimes a fold of the jejunum passes into this-pouch, constituting a hernia. The pouch may be greatly enlarged by the protrusion of further portions of the intestine into it, and cases have been recorded in which the entire intestine has passed into the greatly distended sac.

The fossa sicbccecalis has its seat between the folds of the meso-colon ascendens. Into this pouch the intestine is very rarely protruded, and the pouch itself only occurred in about 30 per cent, of the bodies examined by Waldeyer.

The fossa intersigmoidea is a pouch in the meso-colon of the sigmoid flexure lying between its two folds. The aperture is in the under layer. This is the commonest of these pouches, occurring in about 80 per cent, of the bodies, but from the position of the aperture it does not appear ever to become the seat of hernia.

Contents Of Hernias

The parts protruded are usually the intestine, and, for the most part, the more movable small intestine. Sometimes also the great omentum is carried into the sac. The urinary bladder, large intestine, or any part of the contents of the abdomen, may, under exceptional circumstances, pass into the sac.

As the contents of the sac are unduly exposed to pressure, stretching, and friction, there is apt to be a Chronic peritonitis set up in the wall, especially in old cases. This may unite the loops of intestine together. In the case of large hernias with wide necks there may thus be produced complete matting of the intestine. It even happens that if the hernia be such as to allow successively of the descent of any part of the small intestine, the whole of the small intestine may be mutually adherent.

If the intestine be long retained the chronic inflammation may induce adhesion of the intestine to the internal wall of the sac, and the hernia becomes Irreducible. It may be irreducible from other causes, such as narrowness of the neck, protrusion of an excessive bulk of viscera, etc.

The mode of descent of the Large intestine merits special notice. The sigmoid flexure, being freely movable, may be protruded just like the small intestine, and the Caecum or transverse colon may also sometimes pass into a sac in a similar fashion, but otherwise the large intestine having no mesentery and being only partially covered with peritoneum does not usually descend. When it does its descent is in some respects comparable with that of the testis in the foetus. Before its descent the testis lies behind the peritoneum and is only partially covered by it. As it descends it remains with only a partial peritoneal covering, and even in the tunica vaginalis, after the sac has separated from the general peritoneum, the testis lies behind with its posterior aspect free of peritoneum. And so in a hernial sac, the caecum may be carried down, but in its new position it remains only partially covered with peritoneum, and really forms, as it were, a part of the wall of the sac. This will only occur in very large hernias as a rule, but when it does occur the piece of intestine will be irreducible.

A still more peculiar condition sometimes occurs. The intestine may be protruded mainly at the part where it is uncovered by peritoneum, and instead of pushing a peritoneal sac before it, it may, as it were, drag one after it. As the gut is protruded it may even get more and more, stripped of peritoneum, so that the hernia may be much more extensive than the sac. This, however, is a very exceptional occurrence, and it is more common to find that as an ordinary hernia advances it drags the colon into it, so that besides free loops of small intestine there may be, fixed in the wall and only partly covered by peritoneum, a piece of the caecum, or the sigmoid flexure, or even the fundus of the bladder.

In a similar fashion to that just described, the ovary may be protruded. A large majority of cases of Ovarian hernia are congenital, and they appear to arise by a fault of development by which the ovary descends as the testis does normally. The ovary passes through the inguinal ring and takes a sac with it, but just like the testis it is itself attached to the wall. The sac remains open like the tunica vaginalis in a congenital inguinal hernia. The ovary in that case will be irreducible, unless, as sometimes happens, the broad ligament is so long as to allow the ovary to pass back through the neck. In this case, however, the ovary will still have its fixed attachment in the sac. Apart from this congenital inguinal form, ovarian hernias may be acquired, and these may be either inguinal or femoral.

Strangulation And Incarceration

These terms express a condition in which the contents of the sac are caught tightly at the neck, so that there is not only a hindrance to their return but an excessive pressure interfering with the circulation.

This mostly occurs when, on account of some peculiarity in the situation of the intestine as it issues from the sac, there is, to begin with, a partial obstruction. If the intestine at its entrance into the sac be free, while at its exit it makes a sudden bend so as to cause a partial obstruction, then the faeces will pass readily in, but will accumulate inside as they do not find free exit. The mere loading with faeces may cause irreducibility, and if the faeces decompose the development of gas may still further increase the bulk of the contents. In this way the sac will become too full, and as the neck is narrow there will be special constriction here. Again, the intestine already in the sac, by its peristaltic movement, may drag more and more of the gut after it, till the intestine may become impacted at the neck.

In any case the neck of the sac constricts the portion of intestine concerned, and the most direct effect is Obstruction of its veins. This itself, by producing hypersemia, and, it may be, oedema of the mucous membrane, leads to swelling and further constriction. The whole protruded piece becomes of a dark colour from venous engorgement and haemorrhage. Finally the pressure may be enough to close even the arteries.

The venous obstruction alone seems sufficient, if complete, to cause necrosis of the intestine, and so Gangrene is an occasional result. If the arteries are also obstructed there is still greater probability of the occurrence of gangrene.

Sometimes the obstruction is relieved before gangrene has occurred, and yet in some of these cases a severe inflammation results after the intestine has been returned to the abdomen, leading on, it may be, ultimately to gangrene of the affected piece of gut. The probable explanation of this is that, during the incarceration, the blood-vessels have been so damaged that, on the restoration of the circulation, they are no longer able to recover. It has been shown by experiment (in the ear of the rabbit) that if, by ligaturing the main arteries, the vessels of the part are deprived of blood for a time, and then the circulation restored by loosing the ligature, the result is active hypersemia, acute inflammation, or the hremorrhage infarction, according to the time during which the ligature has acted. So in the case before us, the release of the constriction, by allowing of the re-establishment of the circulation in vessels seriously compromised, may lead to inflammation or gangrene.

Literature

For external hernias see surgical and anatomical works. Wood, in Ashurst's Encycl. of Surg., 1885, v.; Bowditch, Diaphragmatic hernia, 1853; Garlick, Path, trans., 1878, xxix.; Baker, (Pericardial diaphragmatic) ibid., 1877, xxviii.; Adams, Glasg. Med. Jour., 1880, xiv., p. 353; Waldeyer, Virch. Arch., 1874, lx., p. 65; Balfour, Edin. Med. Jour., 1869; Eve, Lancet, 1885; Enolisch, (Ovarian hernia) Strieker's Med. Jahrbucher, 1871, p. 335; Albert, (Hernia inflammata) ibid., 239.