Layers of the Scalp - Mnemonic: SCALP

1. S - Skin

• The outermost layer
• Similar to skin elsewhere but bears hair follicles over most of its surface
• Contains sebaceous glands and sweat glands

2. C - Connective Tissue (Dense, subcutaneous)

• Lies deep to the skin; anchors skin firmly to the galea below
• Contains all the arteries, veins, and nerves supplying the scalp
• The fibrous septa surrounding vessels hold cut vessels open, preventing normal retraction - this is why scalp wounds bleed profusely
• Clinically: even a small laceration in this layer can cause significant hemorrhage

3. A - Aponeurosis (Galea Aponeurotica / Epicranial Aponeurosis)

• The tendinous sheet connecting the frontal belly (anteriorly) and occipital belly (posteriorly) of the occipitofrontalis muscle
• Together, layers 1-3 are firmly adherent and move as a single unit - called the "scalp proper"
• This is the tissue avulsed in "scalping" injuries
• Clinical key: a wound that gapes open = the galea has been cut. This requires separate suturing of the galea to achieve hemostasis and prevent infection spreading intracranially

4. L - Loose Areolar (Connective) Tissue

• A potential space between the galea and pericranium
• Allows the scalp proper to move freely over the skull
• Contains emissary veins - valveless veins draining through diploic channels into the intracranial venous sinuses
• Called the "danger zone" of the scalp:
  • Infections here can spread rapidly across the entire scalp
  • Bacteria can be carried via emissary veins to cause meningitis, osteomyelitis, or brain abscess
  • Blood from trauma collects here as a subgaleal hematoma

5. P - Pericranium (Periosteum of skull)

• The deepest layer; periosteum covering the outer surface of the calvaria
• Firmly adherent at suture lines of the skull (hence subperiosteal collections are limited by suture lines - as in cephalhematoma)
• Thin and flimsy - cannot hold sutures; often mistaken for the galea clinically
• Can be elevated off the bone except at suture lines

Key Clinical Correlations

Blood Supply

• Supratrochlear and supraorbital (from ophthalmic branch of internal carotid)
• Superficial temporal, posterior auricular, and occipital arteries (from external carotid)

Innervation

• Anterior to vertex: branches of CN V (trigeminal) - supratrochlear, supraorbital, zygomaticotemporal, auriculotemporal nerves
• Posterior to vertex: cervical nerves - lesser occipital (C2), greater occipital (C2/C3), third occipital (C3)
• Motor (occipitofrontalis): facial nerve CN VII

Sources: Gray's Anatomy for Students, p. 1062-1063 | Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 814