SCALP
The scalp extends from the superciliary arches anteriorly to the external occipital protuberance and superior nuchal lines posteriorly and to the temporal lines laterally. It consists of five layers (see Fig. 12.2). Conveniently, the first letters of each layer together spell SCALP, making recall easier. The first three layers are intimately bound together and move as a unit.
Skin. This is thick and hair bearing and contains numerous sebaceous glands. Connective tissue beneath the skin. This is a dense fibrofatty layer containing fibrous septa that unite the skin to the underlying epicranialaponeurosis. This layer contains numerous blood vessels. The arteries are derived from both the external and internal carotid arteries, and free anastomoses occur between them. Aponeurosis (epicranial). This is a thin, tendinous sheet that unites the occipital and frontal bellies of the occipitofrontalis muscle (see below and Fig. 12.14). The lateral margins of the aponeurosis are attached to the temporal fascia. The subaponeurotic space is the potential space deep to the epicranial aponeurosis. It is limited in front and behind by the origins of the occipitofrontalis muscle, and it extends laterally as far as the attachment of the aponeurosis to the temporal fascia. Loose areolar tissue. This occupies the subaponeurotic space (see Fig. 12.2) and loosely connects the epicranial aponeurosis to the periosteum of the skull (the pericranium). This is the plane of movement of the scalp, that is, when the scalp moves, the first three layers (SCA) slide along this layer relative to the underlying periosteum. The areolar tissue contains a few small arteries, but it also contains some important emissary veins. The emissary veins are valveless and connect the superficial veins of the scalp with the diploic veins of the skull bones and with the intracranial venous sinuses. Pericranium. The pericranium is the periosteum covering the outer surface of the skull bones. The pericranium is continuous with the periosteum on the inner surface of the skull bones (endosteum) at the sutures between the individual skull bones.
Clinical Significance of the Scalp Structure It is important to realize that the skin, the subcutaneous tissue, and the epicranial aponeurosis (scalp layers SCA) are closely united to one another and are separated from the periosteum by loose areolar tissue. The skin of the scalp possesses numerous sebaceous glands, the ducts of which are prone to infection and damage by repeated hair treatment (e.g., combing and brushing). For this reason, sebaceous cysts of the scalp are common.
Scalp Lacerations
The scalp has a profuse blood supply to nourish the hair follicles. Even a small laceration of the scalp can cause severe blood loss. It is often difficult to stop the bleeding of a scalp wound because the arterial walls are attached to fibrous septa in the subcutaneous tissue (C layer) and are unable to contract or retract to allow blood clotting to take place. Local pressure applied to the scalp is the only satisfactory method of stopping the bleeding (see below). It is common for large areas of the scalp to be cut off the head as a
person is projected forward through the windshield in automobile accidents. Because of the profuse blood supply, it is often possible to replace large areas of scalp that are only hanging to the skull by a narrow pedicle. Appropriate vascular suturing reduces the chance of necrosis. The tension of the epicranial aponeurosis, produced by the tone of the
occipitofrontalis muscles, is important in all deep wounds of the scalp. If the aponeurosis has been divided, the wound will gape open. For satisfactory healing to take place, the opening in the aponeurosis must be closed with sutures.
Often, a wound caused by a blunt object such as a baseball bat closely
resembles an incised wound. This is because the scalp is split against the unyielding skull and the pull of the occipitofrontalis muscles causes a gaping wound. This anatomic fact may be of considerable forensic importance.
Life-Threatening Scalp Hemorrhage Note that all the superficial arteries supplying the scalp ascend from the faceand the neck. Thus, in an emergency situation, encircle the head just above the ears and eyebrows with a tie, shoelaces, or even a piece of string and tie it tight. Then, insert a pen, pencil, or stick into the loop and rotate it so that the tourniquet exerts pressure on the arteries.
Scalp Infections
Infections of the scalp tend to remain localized and are usually painful because of the abundant fibrous tissue in the subcutaneous layer. Occasionally, an infection of the scalp spreads by the emissary veins,
which are valveless, to the skull bones, causing osteomyelitis. Infected blood in the diploic veins may travel by the emissary veins farther into the venous sinuses and produce venous sinus thrombosis. Blood or pus may collect in the potential space deep to the epicranial
aponeurosis (the L layer of the scalp). It tends to spread over the skull, being limited in front by the orbital margin, behind by the nuchal lines, and laterally by the temporal lines. On the other hand, subperiosteal blood or pus is limited to one bone because of the attachment of the periosteum to the sutural ligaments.