This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Wounds of the scalp are common. Incised wounds bleed more freely and the hemorrhage is more difficult to control than in wounds elsewhere on the surface. This is due to the exceedingly free blood supply and to the peculiar arrangement of the blood-vessels in the tissues.
Small wounds of the scalp do not gape, particularly if they are longitudinal in direction and not very deep. The skin is so intimately bound to the aponeurosis beneath that displacement is impossible. If the cut is deep enough to divide the aponeurosis extensively, especially if the wound is transverse, gaping is marked. This is produced by contraction of the two bellies of the occipitofrontalis muscle, which pulls the edges apart.
Bleeding is apt to be persistent and hard to control because the arteries running in the deep layers of the skin and fibrous trabeculae are firmly attached and, therefore, when cut, their lumen cannot contract nor their ends retract. When large flaps are torn in the scalp, they rarely die because of their free blood supply, and sloughing is limited to the parts which are actually contused. As the subaponeurotic space is often opened, if the wound is sewed too tightly shut, subsequent bleeding instead of escaping externally may extend widely under the aponeurosis. Inasmuch as hair and dirt are often crushed into these wounds, great care should be taken to disinfect them. A cut will open the hair bulbs and sebaceous glands, and, as the hairs project into the subcutaneous tissue, they may serve as a starting point for infection.
Contraction of the occipitofrontalis muscle may prevent healing in extensive wounds. To avoid this the scalp is covered by a recurrent bandage or otherwise fixed.
Lacerated wounds do not bleed so freely as do incised wounds, but they are accompanied by a more extensive loosening of the scalp. Large flaps of tissue are frequently raised and turned to one side. The most severe of these injuries have been produced by the hair being caught by a revolving shaft, tearing nearly the whole scalp off. Its loose attachment to the pericranium and bone beneath by the loose subaponeurotic tissue, readily explains the reason of these extensive detachments.
Contusions cause only a moderate amount of swelling, which is usually circumscribed. While the skin is not broken, the blood-vessels and other tissues beneath are often ruptured, and, therefore, extravasation of blood occurs. When this is confined to the superficial fascia, it is small in amount and limited in area. It does not tend to work its way for any great distance beneath the skin. If the extravasation extends below the aponeurosis, it may cover a considerable area of the skull. When it occurs beneath the pericranium it is called cephalhoematoma, or in the new-born caput succedaneum. Caput succedaneum is found almost always on the right side, involving the parietal eminence. It is limited by the attachment of the pericranium at the sutures.
Hoematomas of the scalp possess the peculiarity of being soft in the centre and surrounded by a hard oedematous ring of tissue. In cephalhaematoma of long standing this ring may ossify, and the new bone may even extend and form a more or less perfect bony cyst. This, however, is very rare.
Haematomas produced by blows on the head are often mistaken for fractures. The raised edge is so hard as sometimes to be thought to be the edge of broken bone. The tissues beneath the skin at the site of impact seem to be pulpified and remain perfectly soft to the touch; the smooth unbroken skull can usually be felt over an area equal to the site of impact. Surrounding this soft area is the hardened ring, composed of tissues between the skin and the bone, into which serum and blood have been effused.
Inflammation and abscess are caused by infected wounds, furuncles, erysipelas, caries of the skull and suppurating sebaceous cysts.
The scalp is a favorite location for erysipelas; if not started primarily by an infected wound, the scalp may be involved secondarily by extension from the face.
Caries of the skull is often of syphilitic origin.
Abscesses may occur in three places:
1. Subcutaneous abscesses are usually small and do not tend to spread but rather to discharge through the skin. This is because the firm fibrous trabeculae prevent lateral extension. Furuncles are quite common in childhood; they are, of course, superficial to the aponeurosis. Sebaceous cysts are especially common in the scalp and they sometimes suppurate. The orifice of the obstructed duct is not usually visible. Sometimes in a small cyst a black spot on its surface indicates the opening of the duct. By means of a needle or pin this opening can be dilated and some of the contents expressed. Of course, if nothing further is done it will reaccumulate. When these cysts become inflamed they become united to the skin above so that it has to be dissected off. If pus forms, it either remains localized to the cyst or bursts through the skin and discharges externally. It does not tend to burrow under the skin laterally on account of the fibrous trabeculae uniting the skin and aponeurosis. The aponeurosis beneath is intact, therefore the pus does not get below it. The cyst, with the lining membrane entire, should be removed, otherwise it will recur.