◊Outer 3 layers are often torn off as a unit in accidents; wounds do not gape if epicranius (occipitofrontal) m. not involved

Area: from supraorbital ridge to superior nuchal line, laterally to zygomatic arch and external auditory meatus 3 primary muscle groups: frontalis, occipitalis, temporalis

1. SKIN
   • linear hyperattenuating structure
2. SUBCUTIS
   = fibroadipose tissue closely adherent to skin and underlying epicranius
   • thick layer of fat attenuation
3. GALEA APONEUROTICA
   = layer of thick fibrous tissue continuous with epicranius (occipitofrontal) muscle consisting of
   1. frontal muscle belly + auricularis m. anteriorly
   2. occipital muscle belly posteriorly
   
   forming centrally the large epicranial aponeurosis
   • thin line of increased attenuation ← temporalis fascia
4. SUBGALEAL SPACE
   = subaponeurotic areolar tissue between periosteum of outer table and galea aponeurotica
   Histo: analogous to loose areolar tissue containing fat
   • allows the 3 superficial layers of scalp to move as a unit on the cranium → enabling the scalp to wrinkle
   • allows fluid to accumulate and spread across the cranium analogous to intracranial subdural fluid collections
5. PERICRANIUM = periosteum of outer table
   • firmly adheres to margins of underlying skull bones at suture lines
   • usually not visible by CT without subperiosteal hematoma
6. SUBPERIOSTEAL SPACE
   = potential space created when periosteum of outer table becomes detached from calvaria (= cephalohematoma); analogous to intracranial extradural hematoma