The soft tissue envelope of the cranial vault is called the scalp. The scalp extends from the external occipital protuberance and superior nuchal lines to the supraorbital margins. The scalp consists of 5 layers (seen in the image below): the skin, connective tissue, epicranial aponeurosis, loose areolar tissue, and pericranium. The first 3 layers are bound together as a single unit. This single unit can move along the loose areolar tissue over the pericranium, which is adherent to the calvaria.
Coronal section of scalp that shows layers of the scalp (From Snell RS, Clinical Anatomy for Medical Students, 5th ed)
The skin of the scalp is thick and hair bearing and contains numerous sebaceous glands. As a result, the scalp is a common site for sebaceous cysts.
Connective tissue (superficial fascia)
The superficial fascia is a fibrofatty layer that connects skin to the underlying aponeurosis of the occipitofrontalis muscle and provides a passageway for nerves and blood vessels. Blood vessels are attached to this fibrous connective tissue. If the vessels are cut, this attachment prevents vasospasm, which could lead to profuse bleeding after injury.
Epicranial aponeurosis (galea aponeurotica)
The epicranial aponeurosis is a thin, tendinous structure that provides an insertion site for the occipitofrontalis muscle. Posterolaterally, the epicranial aponeurosis attachment extends from the superior nuchal line to the superior temporal line. Laterally, the epicranial aponeurosis continues as the temporal fascia. Anteriorly, the subaponeurotic space extends to the upper eyelids due to the lack of a bony insertion. This loose areolar tissue provides a potential subaponeurotic space that allows fluids and blood to pass from the scalp to the upper eyelids.
Loose areolar tissue
Areolar tissue loosely connects the epicranial aponeurosis to the pericranium and allows the superficial 3 layers of the scalp to move over the pericranium. Scalp flaps are elevated along a relatively avascular plane in craniofacial and neurosurgical procedures. However, certain emissary veins traverse this layer, which connects the scalp veins to the diploic veins and intracranial venous sinuses.
The pericranium is the periosteum of the skull bones. Along the suture lines, the pericranium becomes continuous with the endosteum. A subperiosteal hematoma, therefore, forms in the shape of the skull bones.
The occipitofrontalis muscle consists of 2 occipital bellies and 2 frontal bellies. The occipital bellies arise from the superior nuchal lines on the occipital bone. The frontal bellies originate from the skin and superficial fascia of the upper eyelids. The occipital and frontal bellies insert into the epicranial aponeurosis.
Each occipital belly is innervated by the posterior auricular branch of the facial nerve, and each frontal belly is innervated by the frontal branch of the facial nerve. The frontal bellies can raise the eyebrows.