Skin Proper

The skin overlying the calvaria is continuous with the skin of the face and neck. It is generally thinnest anteriorly at the forehead, where it measures approximately 3 mm, and it is thickest posteriorly at the occiput, where it measures up to 8 mm. The skin is usually slightly thicker in women than men. It gradually increases in thickness from childhood until age 35 years (women) or 55 years (men) and then begins to show thinning and atrophy.²

The epidermis is the outer squamous epithelium of the skin. It is composed predominantly of keratinocytes, with smaller populations of melanocytes, Langerhans cells, and Merkel cells. The superficial extensions of the dermal appendages pass through the epidermis to reach the skin surface.

The dermis is the underlying support layer composed predominantly of fibroblasts and ground substance. It houses the specialized dermal appendages (dermal adnexae), including the eccrine sweat glands, hair follicles, sebaceous glands, and apocrine units. These glands differ in their origin, distribution, and secretions. The apocrine glands, sebaceous glands, and hair follicles arise together from a common population of stem cells distinct from those giving rise to the eccrine glands. The eccrine glands produce mostly sweat, whereas the apocrine glands produce mostly scent. The apocrine and sebaceous glands feed their secretions toward the hair shaft with which they arise. The eccrine glands feed their secretions directly to the skin surface.

The follicles and sebaceous glands are influenced by the levels of androgens. With age, and under the influence of androgens, the diameter of the hair follicles shrinks in size, leading to loss of terminal hairs, a process called androgenetic alopecia (common pattern baldness). Conversely, increased levels of androgens cause enlargement of the sebaceous glands.

Superficial Fascia

The superficial fascia of the scalp contains dense adipose tissue that is continuous with the subcutaneous tissue of the face and neck. Anteriorly and laterally, it is continuous with the subcutaneous tissue over the frontalis and orbicularis oculi muscles. Anteriorly in the midline it stops at the bridge of the nose. Posteriorly, the superficial fascia is continuous with the subcutaneous tissues of the neck. Laterally it extends downward, superficial to the temporalis muscles, to attach to the external ears.

The superficial fascial layer varies in thickness. It measures 4 to 6 mm in people of normal weight but 20 mm or more in obese individuals. Within this layer, multiple, vertically oriented reticular fibers divide the layer into small compartments (Figs. 6-4 and 6-5). These septa form strong connections between the superficial dermis and the subjacent galea aponeurotica. As a consequence, the subcutaneous layer is relatively inelastic and the dermis, subcutaneous tissue, and galeal aponeurosis move together as one unit.

FIGURE 6-4 Sagittal reformatted noncontrast CT of the head. The skin (arrowheads) is thinner anteriorly and thicker posteriorly. A small cutaneous calcification (arrow) is noted.

FIGURE 6-5 Coronal reformatted noncontrast CT of the head depicts the isodense skin (left arrowhead) superficially, the thick lucent adipose tissue of the superficial fascial layer (right arrowhead) in the middle, and the thick isodense deeper tissue (arrow) representing the combined galea aponeurotica, subgaleal loose areolar tissue, and pericranium. Individual septations cross and partially subdivide the adipose tissue of the superficial fascial layer.

The subcutaneous layer has a rich network of arteries, veins, and lymphatics. The arteries are tethered to the fibrous septa, so they are relatively immobile and unable to constrict quickly after a laceration.³

Galea Aponeurotica

The galea aponeurotica is a continuous fibrous sheet intimately related to the muscles of the scalp. For that reason, it is described with the scalp muscles later in this chapter.

Subgaleal Connective Tissue

The subgaleal connective tissue is a largely avascular, filmy layer of loose fibroareolar tissue interposed between the galea aponeurotica superficially and the pericranium deeply. Anteriorly, the subgaleal layer extends deep to the frontalis and orbicularis oculi muscles. Posteriorly, it extends deep to the occipitalis muscle, as low as the superior nuchal line. Laterally, the subgaleal connective tissue is limited by the attachment of the galeal aponeurosis to the superficial temporalis fascia.

The areolar tissue in this layer is only loosely connected to the subjacent pericranium, so the subgaleal layer serves as a “gliding plane” for motion of the scalp over the calvaria. As a consequence, it is easy to establish a subgaleal plane for dissection at surgery and postmortem examination.⁴

Pericranium

The pericranium is the dense membranous outer periosteum of the calvaria. It covers the external surfaces of the frontal, parietal, and occipital bones deep to the subgaleal areolar tissue and extends as far laterally as the superior temporal line on each side. The pericranium varies in thickness from individual to individual and from region to region. Generally, however, the pericranium is thicker frontally than at the vertex. In anatomy texts, the pericranium is described as firmly adherent to the skull, especially at the suture lines. However, the surgical literature indicates that it can easily be dissected from the skull as an intact sheet.

Occipitofrontalis

The occipitofrontalis is the major muscle complex of the epicranium. It is composed of the paired frontalis muscles anteriorly and the paired occipitalis muscles posteriorly. These muscles are interconnected by the galea aponeurotica (Fig. 6-6).⁵ Each frontalis muscle is a thin quadrilateral sheet of muscle that is attached to the overlying superficial fascia externally (especially at the eyebrows) and that ascends into the galea aponeurotica anterior to the coronal suture. The frontalis muscle has no bony attachment.⁴ The medial margins of the two frontalis muscles blend together above the nasal root. The medial fibers of each frontalis muscle continue inferiorly to become continuous with the procerus (see later). The intermediate fibers of the frontalis blend with the orbicularis oculi and the corrugator supercilii (see later). The lateral fibers of the frontalis muscles blend with the lateral portions of the orbicularis oculi over the zygomatic processes of the frontal bones.⁵ Because the frontalis has no bony anchor, hematomas and other pathologic processes may extend anteriorly and inferiorly, deep to the aponeurotic sheath, to reach the eyelids. Posteriorly, the paired quadrilateral shaped occipitalis muscles arise by tendinous fibers from the lateral two thirds of the superior nuchal lines of the occipital bone and mastoid processes of the temporal bones. They insert into the galea aponeurotica at a level slightly above the superior aspect of the auricles.

FIGURE 6-6 Muscles of the scalp. The largest scalp muscle is the epicranius, composed of paired anterior muscle bellies, named the frontalis, and paired posterior muscle bellies, named the occipitalis. Between these muscles is an aponeurotic sheath, named the galea aponeurotica. The temporalis muscle lies deep to the temporal fascia. The auricular muscles arise from this sheath to insert on the external ear.

(Modified from Lewis WH. Henry Gray’s Anatomy of the Human Body, 20th ed. Philadelphia, Lea & Febiger, 1918, p 379.)

The paired frontalis muscles work in conjunction to elevate the eyebrows and forehead and to produce the characteristic transverse creases of the forehead. The occipitalis acts in conjunction to tighten the scalp. In many individuals they do not demonstrate any motion.⁶ Both the frontalis and the occipitalis are innervated by the facial nerve—the frontalis by temporal branches of the facial nerve and the occipitalis by occipital branches of the facial nerve.

Procerus

The procerus is a small pyramidal slip of muscle that extends downward from the skin of the forehead between the eyebrows and from the medial portions of the frontalis muscles to join a fascial aponeurosis that covers the lower nasal bones and the adjacent lateral nasal cartilages (see Fig. 6-6).⁵

Orbicularis Oculi

The orbicularis oculi is a broad flat elliptical muscle that extends into the eyelids, surrounds the orbital aperture, and spreads circumferentially into the anterior temporal region, the infraorbital cheek, and the superciliary region (see Fig. 6-6).⁵ It has three parts:

1. The thick orbital portion of the orbicularis oculi attaches medially to the nasal process of the frontal bone, the frontal process of the maxilla, and the intervening medial palpebral ligament. Its upper portion blends with the frontalis muscle and the corrugator supercilii on each side.

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