Basilar sinuses connect the inferior petrosal sinuses to each other and to the vertebral plexus of veins. They are on the clivus, just posterior to the sella turcica of the sphenoid bone (Fig. 8.44).
The 12 pairs of cranial nerves are part of the peripheral nervous system (PNS) and pass through foramina or fissures in the cranial cavity. All nerves except one, the accessory nerve [XI], originate from the brain.
In addition to having somatic and visceral components similar to those of spinal nerves, some cranial nerves also contain special sensory and motor components (Tables 8.4 and 8.5).
The special sensory components are associated with hearing, seeing, smelling, balancing, and tasting.
Special motor components include those that innervate skeletal muscles derived embryologically from the pharyngeal arches and not from somites.
In human embryology, six pharyngeal arches are designated, but the fifth pharyngeal arch never develops. Each of the pharyngeal arches that does develop is associated with a developing cranial nerve or one of its branches. These cranial nerves carry efferent fibers that innervate the musculature derived from the pharyngeal arch.
Innervation of the musculature derived from the five pharyngeal arches that do develop is as follows: first arch—trigeminal nerve [V3], second arch—facial nerve [VII], third arch—glossopharyngeal nerve [IX], fourth arch—superior laryngeal branch of the vagus nerve [X], sixth arch—recurrent laryngeal branch of the vagus nerve [X], posterior arches—accessory nerve [XI].
The olfactory nerve [I] carries special afferent (SA) fibers for the sense of smell. Its sensory neurons have: peripheral processes that act as receptors in the nasal mucosa, and central processes that return information to the brain.
The receptors are in the roof and upper parts of the nasal cavity, and the central processes, after joining into small bundles, enter the cranial cavity by passing through the cribriform plate of the ethmoid bone (Fig. 8.53). They terminate by synapsing with secondary neurons in the olfactory bulbs (Fig. 8.54).
The optic nerve [II] carries SA fibers for vision. These fibers return information to the brain from photoreceptors in the retina. Neuronal processes leave the retinal receptors, join into small bundles, and are carried by the optic nerves to other components of the visual system in the brain. The optic nerves enter the cranial cavity through the optic canals (Fig. 8.53).
The oculomotor nerve [III] carries two types of fibers:
General somatic efferent (GSE) fibers innervate most of the extra-ocular muscles.
General visceral efferent (GVE) fibers are part of the parasympathetic part of the autonomic division of the PNS.
The oculomotor nerve [III] leaves the anterior surface of the brainstem between the midbrain and the pons (Fig. 8.54). It enters the anterior edge of the tentorium cerebelli, continues in an anterior direction in the lateral wall of the cavernous sinus (Figs. 8.53 and 8.54; see Fig. 8.45), and leaves the cranial cavity through the superior orbital fissure.
In the orbit, the GSE fibers in the oculomotor nerve innervate levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.
The GVE fibers are preganglionic parasympathetic fibers that synapse in the ciliary ganglion and ultimately innervate the sphincter pupillae muscle, responsible for pupillary constriction, and the ciliary muscles, responsible for accommodation of the lens for near vision.