The abdominal cavity is a large enclosed space lined by a sheet of tissue. The muscles of the abdominal wall resist the varying changes of pressure within the cavity. Increased pressure may force the protrusion of a portion of the lining tissue through a weak spot in the muscles of the abdominal wall. This forms a pouch and usually, sooner or later, some part of the abdominal contents will be pushed into the pouch. It may appear at the navel or through an operation scar, but the commonest position is in the groin. The weakness may have been present from birth, but it may be brought on by a chronic cough or strain. At first, a rupture is noticed under the skin as a soft rounded swelling, which is often no larger than a walnut, but it may become very much bigger after some months. The swelling tends to disappear when the patient is lying down, but it reappears when he stands up or coughs. Normally there is no severe pain, but usually a sense of discomfort and dragging is present.
When a hernia is suspected, the patient must always be examined while standing. In the groin (inguinal hernia), the swelling of a rupture must not be confused with swollen lymph glands. Usually there are several of these swollen tender glands, and they do not disappear when the patient lies down.
It is sometimes possible to see and to feel an impulse transmitted to the hernia swelling, if the patient is asked to cough forcibly several times.