The scalp is composed of five layers; skin, subcutaneous tissue, galea, areolar tissue, and the pericranium. There is potential for severe blood loss due to the heavy blood supply in the subcutaneous tissue and the loose areolar connection to the pericranium.
The skull is a rigid container made up of eight major bones. The bones of the skull have two solid layers separated by cancellous bone, which adds further rigidity and strength. The inner surface of the skull is lined by the dura. While protecting the brain from external forces, the rigid skull does not allow for the expansion of the intracranial contents. Fractures of the skull, particularly of the temporal bone can disrupt the enclosed meningeal artery and lead to epidural hematoma.
The adult brain weighs between 1300 and 1500 g and occupies 80 percent of the total volume of the skull. The brain's three basic structures—the cerebral hemispheres, the cerebellum and the brainstem—are divided by two major fixed dura attachments. The falx cerebri vertically separates the two major halves of the cerebrum down to the level of the brainstem. The tentorium cerebelli separates the cerebellum and brainstem from the cerebrum at the base of the skull. The inner edge of the tentorium cerebelli is the site of the most common brain herniation syndrome, uncal herniation. The cerebrum is further anatomically divided into major lobes named after the bones overlying them: frontal, temporal, parietal, occipital.
The brain is covered with multiple anatomic layers and potential spaces. The outermost layer next to the skull is the dura. It is between the dura and the skull where bleeding from the meningeal arteries causes epidural hematomas. Underneath is a thinner fibrous material called the arachnoid. The pia is closely associated with the gray matter of the brain and is the inner most layer. Between the arachnoid and the pia is the subarachnoid space where cerebrospinal fluid (CSF) circulates. In the average adult, there is 150 mL of CSF surrounding the brain and spinal cord. Approximately 500 mL of CFS is produced in the choroid plexus of the lateral ventricles each day.
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