Skull: Midsagittal Section

The occipital bone bounds most of the posterior cranial fossa. It is pierced by the foramen magnum, through which the medulla oblongata and spinal cord, surrounded by their meninges, become continuous; it also transmits the vertebral arteries, a few small veins, the spinal roots of the accessory (XI) nerves, and the recurrent meningeal branches from the upper spinal nerves. The occipital condyle articulates with the homolateral superior atlantoarticular process. The hypoglossal (XII) nerve passes through the corresponding canal. The jugular foramen lodges the superior bulb of the internal jugular vein (in which the sigmoid and inferior petrosal sinuses end); the glossopharyngeal (IX), vagus (X), and accessory nerves pass through it anteromedial to the bulb, and it provides an entry for the recurrent meningeal branches of the vagus and small meningeal branches of the ascending pharyngeal and occipital arteries. The basilar part of the occipital bone unites with the body of the sphenoid to form a sloping platform anterior to the pons and medulla oblongata.

The squamous part of the temporal bone is grooved by the posterior branches of the middle meningeal vessels and the sulcus along the superior border of its petrous part is for the superior petrosal sinus. The inferior petrosal sinus lies in the sulcus between the petrous temporal and occipital bones. The internal acoustic meatus is a canal about 1 cm long, ending in a cribriform septum that separates it from the internal ear. It transmits the facial (VII) nerve and its nervus intermedius, the vestibulocochlear (VIII) nerve, and the internal auditory (labyrinthine) artery.

The sphenoid bone has a central body from which two greater and two lesser wings and two pterygoid processes arise. The body contains two air sinuses separated by a septum that is often incomplete. Its concave upper surface, the sella turcica, houses the pituitary gland. The optic canal transmits the optic (II) nerve and the ophthalmic artery.

The nasal cavity is roofed over mainly by the cribriform plate of the ethmoid bone, augmented anteriorly by small parts of the frontal and nasal bones, and posteriorly, by the anteroinferior surface of the sphenoidal body. Its floor is formed by the palatine processes of the maxillae and by the horizontal plates of the palatine bones. The incisive canal transmits the nasopalatine nerves and branches of the greater palatine arteries. Each lateral wall is formed above by the nasal surface of the ethmoid bone that covers the ethmoidal labyrinth and supports thin, shell-like projections, the superior and middle nasal conchae. These overhang the corresponding nasal meatuses. Below, each lateral wall is formed by the nasal surface of the maxilla, the perpendicular plate of the palatine bone and the medial pterygoid plate. The maxillary and palatine bones articulate with a separate bone, the inferior nasal concha, overhanging the inferior nasal meatus. The sphenoidal air sinuses open into the nose through the sphenoidal aperture in the sphenoethmoidal recess posterosuperior to the superior concha. The frontal and maxillary air sinuses open into the middle meatus through a semilunar hiatus, and the multiple air cells forming the ethmoidal labyrinth open into the superior and middle meatuses. The lower opening of the nasolacrimal duct is near the anterior end of the inferior meatus. The sphenopalatine foramen behind the middle concha transmits the nasopalatine nerve.

The nasal cavity is subdivided by a more-or-less vertical septum formed by the perpendicular ethmoidal plate and the vomer. The triangular gap between them anteriorly is filled in by the nasal septal cartilage (not shown in the illustration).

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