Fig 7. Haematoma on the forehead of a child.

2. Subaponeurotic abscesses come from infected wounds, erysipelas, or caries of the bones. It is not desirable to close deep wounds of the scalp too tightly. Some suppuration is liable to occur which, not finding an easy escape externally, may spread under the aponeurosis if the wound has been deep enough to divide it. Infection of wounds is the most frequent source of these abscesses, hence the desirability of providing for drainage for at least a short period. In erysipelas, serous effusion, which may become purulent, occurs in the subaponeurotic tissue, as well as in the layers above. It may sink downward and point in the temporal, occipital, or frontal region. In the temporal region the descent of the pus may be limited by the attachment of the lateral expansion of the aponeurosis to the zygoma. The attachment of the occipitalis muscle posteriorly to the superior curved line of the occiput prevents the effusion from coming to the surface at that point. The liquid accumulates low down on the forehead over the orbits, being prevented from entering by the attachment of the orbitotarsal ligament, and tends to point close to the median line. The frontal muscles of the two sides are apt to be slightly separated, leaving a weak spot just above the root of the nose, and this is where fluctuation can most easily be felt. These accumulations in the frontal, temporal, and occipital regions may require incisions for their evacuation and drainage. Suppuration arising from carious bone readily perforates the pericranium and then infiltrates the loose subaponeurotic tissue. The bones of the vault of the skull are not infrequently affected by syphilitic disease, producing caries and suppuration, which invade the subaponeurotic space.

3. Subpericranial abscesses are comparatively rare. They usually start from diseased bone and spread laterally beneath the pericranial tissue. The pus may be limited to a single bone on account of the firmer attachment of the pericranium at the site of the sutures. To avoid breaking into the subaponeurotic space, a free opening should be made into the abscess so as to allow the pus to drain externally.