True hernia consists in a protrusion of the intestine, omentum, or other abdominal organ into a sac formed by a prolongation of the peritoneum. The sac may project externally, or it may bo contained within the abdomen, and so we may distinguish External and Internal hernias. The hernias, especially the external ones, are of so much importance in a surgical point of view that full descriptions are given in the surgical and anatomical text-books, and need not be repeated here, except in.outline.
For the most part the sac is an entirely abnormal projection of the peritoneum. An exception to this occurs in the case of congenital inguinal hernia, in which the sac is formed by the persistence of a foetal condition. There is a partial exception also in the case of most internal hernias, where the sac usually arises by the exaggeration of an existing normal pouch.
Hernias are usually ascribed to the abdominal contents being subjected to undue pressure. In severe muscular efforts, such as are involved in lifting heavy weights, the glottis is closed, and the muscles of expiration fix the chest and abdomen, the contents of the abdomen being subjected to severe pressure by the contraction of the muscles of the abdominal wall. If there is any part of the wall which is unduly weak a bulging outwards may occur here, and so give the starting point for the hernial protrusion.
In this connection the greater frequency of hernia on the right side may be noted. In violent exertions the right arm is usually more used than the left, and as the chest is bent over to the left side to counterbalance the strain on the right, the lower surface of the diaphragm faces more to the right and presses the viscera towards that side. It is clear that straining at stool or otherwise will also increase the pressure on the abominal contents, and any excess will predispose to hernia.
The protrusion takes place where there is any Weakness of the abdominal wall. The external hernias occur at specially unsupported parts of the wall, while the internal ones have usually a pouch ready made as a starting point. The abdominal wall from its anatomical conformation is weak at certain points in every person, but there may be congenitally a special weakness, which in some cases seems to be hereditary. On the other hand, when the abdominal contents are increased, as a result of tumours, fluid accumulation, or pregnancy, the stretched wall may be weakened. It may be so also from direct injury to the wall.
Another cause is sometimes assigned for the production of hernias, namely, an abnormal elongation of the mesentery. It is supposed that such a lesion will allow the intestine to impinge unduly against the abdominal wall, especially at its lower parts.
The viscera nearly always push the peritoneum before them, and the proper sac is formed by the peritoneum, which shows a remarkable power of stretching. But there are cases of protrusion in which the aperture has been produced by actual rupture of the peritoneum, and in these cases the hernia may be devoid of a proper sac. These cases, however, of what may be called False hernia, are exceedingly rare, as an injury, although tearing the muscular wall and other tissues, generally leaves the elastic peritoneum uninjured and capable of protrusion.
In Congenital hernias the sac is formed of peritoneum, but there has been no actual protrusion. In congenital inguinal hernia the sac is formed by the tunica vaginalis, whose connection with the peritoneum has remained patent. In congenital umbilical hernia the peritoneum is prolonged into the umbilical cord (see p. 55).
The hernial sac usually acquires adhesions to the structures among which it is protruded, and it does so by a chronic inflammation. It very often happens also that the contents of the sac become adherent to its internal surface by inflammation, and in that case the hernia is irreducible.
It is not necessary to enter fully into the individual forms of hernia, and of the external ones little more than an enumeration will suffice.
The external hernias are, (1) Inguinal hernia in the congenital and acquired forms, or, as otherwise divided, direct and oblique; (2) Femoral hernia. These two are by far the commonest forms. Of comparatively rare occurrence are, (3) Hernia of the sciatic notch: (4) Perineal hernia, protruded between the fibres of the levator ani; (5) Vaginal hernia; (6) Hernia of the foramen ovale; (7) Umbilical hernia, which is congenital or acquired, in the former case arising by protrusion into the dilated umbilical cord; (8) Abdominal hernia occurring in various parts of the abdominal wall, chiefly towards the edges of muscles, and arising by tearing of tendons or muscular fibres, hence frequently traumatic; its commonest situation is near the linea alba; (9) An interesting form of. hernia is that in which the testicle having descended imperfectly the protrusion takes place into the sac around the misplaced testis. This sac will occupy at first the position of the inguinal canal, but it is liable to enlarge so that there may be a considerable sac in the substance of the abdominal wall.
Internal hernia comes less frequently into sight, and the possibility of its existence is apt to be forgotten.