The CSF returns to the venous system through arachnoid villi. These project as clumps (arachnoid granulations) into the superior sagittal sinus, which is a dural venous sinus, and its lateral extensions, the lateral lacunae (Fig. 8.35).
The brain is a component of the central nervous system.
During development the brain can be divided into five continuous parts (Figs. 8.36 and 8.37). From rostral (or cranial) to caudal they are:
The telencephalon (cerebrum) becomes the large cerebral hemispheres. The surface of these hemispheres consists of elevations (gyri) and depressions (sulci), and the hemispheres are partially separated by a deep longitudinal fissure. The cerebrum fills the area of the cranial cavity above the tentorium cerebelli and is subdivided into lobes based on position.
The diencephalon, which is hidden from view in the adult brain by the cerebral hemispheres, consists of the thalamus, hypothalamus, and other related structures, and classically is considered to be the most rostral part of the brainstem. (However, in common usage today, the term brainstem usually refers to the midbrain, pons, and medulla.)
The mesencephalon (midbrain), which is the first part of the brainstem seen when an intact adult brain is examined, spans the junction between the middle and posterior cranial fossae.
The metencephalon, which gives rise to the cerebellum (consisting of two lateral hemispheres and a midline part in the posterior cranial fossa below the tentorium cerebelli) and the pons (anterior to the cerebellum, and is a bulging part of the brainstem in the most anterior part of the posterior cranial fossa against the clivus and dorsum sellae).
The myelencephalon (medulla oblongata), the caudalmost part of the brainstem, ends at the foramen magnum or the uppermost rootlets of the first cervical nerve and to which cranial nerves VI to XII are attached.
The brain receives its arterial supply from two pairs of vessels, the vertebral and internal carotid arteries (Fig. 8.38), which are interconnected in the cranial cavity to produce a cerebral arterial circle (of Willis).
The two vertebral arteries enter the cranial cavity through the foramen magnum and just inferior to the pons fuse to form the basilar artery.
The two internal carotid arteries enter the cranial cavity through the carotid canals on either side.
Each vertebral artery arises from the first part of each subclavian artery (Fig. 8.38) in the lower part of the neck, and passes superiorly through the foramen transversarium of the upper six cervical vertebrae. On entering the cranial cavity through the foramen magnum each vertebral artery gives off a small meningeal branch.
Continuing forward, the vertebral artery gives rise to three additional branches before joining with its companion vessel to form the basilar artery (Figs. 8.38 and 8.39):
The first is a posterior inferior cerebellar artery.
A second branch is the posterior spinal artery, which passes posteriorly around the medulla and then descends on the posterior surface of the spinal cord in the area of the attachment of the posterior roots—there are two posterior spinal arteries, one on each side (although the posterior spinal arteries can originate directly from the vertebral arteries, they more commonly branch from the posterior inferior cerebellar arteries).
A third branch joins with its companion from the other side to form the single anterior spinal artery, which then descends in the anterior median fissure of the spinal cord.