External hernias involve protrusion of peritoneum ± omentum and bowel into the layers of the abdominal wall (particularly at the site of a previous surgical incision), inguinal or femoral canals. The hernial sac is usually thin walled comprising fibrous connective tissue lined by peritoneum. It may become irreducible and undergo secondary ischaemia of the contents and with ulceration and infection of the overlying skin. Internal hernias into anatomical spaces (e.g., the lesser omental sac) or across fibrous bands (congenital or acquired) may also obstruct and become ischaemic but are rare.

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