Scalp Anatomy: Boundaries and Layers

Scalp Boundaries

• The scalp's soft tissue covers the cranial vault.
• Anterior boundary: supraorbital margin (eyebrows)
• Posterior boundary: level of external occipital protuberance and superior nuchal line
• Lateral boundary: level of superior temporal line, continuous with hairy temple area up to zygomatic arch

Scalp Layers

• The layers of the scalp, from superficial to deep, S.C.A.L.P.:
  • Skin
  • Connective tissue
  • Aponeurosis (Epicranial)
  • Loose Areolar tissue
  • Pericranium
• Scalp is a common site for sebaceous cysts.

Skin (1st Layer)

• The skin layer of the scalp is thick, hairy, and adherent to the epicranial aponeurosis (3rd layer).
• Rich in sebaceous glands, making it a common site for sebaceous cysts.

Subcutaneous Connective Tissue (2nd Layer)

• Fibrous, dense, connecting skin to the epicranial aponeurosis.
• Rich in blood vessels and nerves.
• Scalp wounds bleed profusely due to the inability of torn blood vessels to contract/retract because of closely attached thick fibrous septa.

Epicranial Aponeurosis and Occipitofrontalis Muscle (3rd Layer)

• A flat tendon on the cranial vault
• It is adherent to the first and second layers
• These layers form a unit that moves on the pericranium.

Epicranial Aponeurosis Attachments

• Anteriorly: Receives insertion of the frontal bellies of the occipitofrontalis muscle.
• Posteriorly: Receives insertion of the occipital bellies of the occipitofrontalis and is attached to the external occipital protuberance and the highest nuchal lines.
• Laterally: Attached to the superior temporal line, sending a thin extension down to the zygomatic arch.

Occipitofrontalis Muscle

• Has frontal and occipital bellies connected by the epicranial aponeurosis
• Frontal Belly: No bony attachment, attached to subcutaneous tissue in the region of eyebrows, supplied by the temporal branch of the facial nerve
• Occipital Belly: Arises from the lateral 2/3 of the highest nuchal line, supplied by the posterior auricular branch of the facial nerve
  • Action: Produces wrinkles in the forehead and raises the eyebrows

Loose Areolar Tissue (4th Layer)

• Continuous with loose tissue of the upper eyelid.
• Plane of movement for the first three layers of the scalp.
• Considered the "dangerous layer" of the scalp.
  • Emissary veins can spread infections from the scalp to:
    • Diploe of the skull (osteomyelitis)
    • Intracranial venous sinuses (thrombosis)
• Hemorrhage in this layer gravitates to the upper eyelid, causing a black eye.

Emissary Veins

• Valvless veins connecting veins outside the skull with venous sinuses inside the skull.
• Equalizes venous pressure outside and inside the skull.

Pericranium (5th Layer)

• Loosely attached to the surface of the skull bones.
• Easily separated from the skull bones.

Clinical Anatomy

• Bleeding from scalp wounds is difficult to stop because arterial walls are attached to fibrous septa in the subcutaneous tissue
• This attachment prevents contraction/retraction, inhibiting blood clotting
• Loose areolar tissue facilitates the spread of bleeding and infection.

Nerve Supply of the Scalp

• Anterior Half of Scalp:
  • Sensory: Branches of the trigeminal nerve
    • Supratrochlear N: Skin of the forehead.
    • Supraorbital N: Skin of the forehead up to the vertex of the scalp.
    • Zygomaticotemporal: Hairless area of the temple.
    • Auriculotemporal: Hairy area of the temple.
  • Motor: Temporal branch of facial N. (motor) to frontal belly of occipitofrontalis muscle.
• Posterior Half of Scalp:
  • Sensory: Branches of the cervical plexus
    • Great auricular N. (C2,3): Skin over mastoid process.
    • Lesser occipital N. (C2): Scalp behind the auricle.
    • Greater occipital N. (C2 dorsal ramus): Occipital scalp up to the vertex.
    • Third occipital N. (C3 dorsal ramus): Skin of the lower occipital region.
  • Motor: Posterior auricular branch of facial N.(motor) to occipital belly of occipitofrontalis.

Blood Supply of the Scalp

• Anterior Half:
  • Supratrochlear branch of ophthalmic artery
  • Supraorbital branch of ophthalmic artery
  • Superficial temporal branch of external carotid artery (ECA)
• Posterior Half:
  • Posterior auricular branch of external carotid artery (ECA)
  • Occipital branch of external carotid artery (ECA)
• In an emergency, remember superficial arteries ascend from the face and neck
• In an emergency, encircle the head just above the ears and eyebrows with a tie to apply pressure to the arteries.

Overview of the scalp's anatomical boundaries and layers. Includes the anterior, posterior, and lateral limits of the scalp. Describes the five layers: skin, connective tissue, aponeurosis, loose areolar tissue, and pericranium.