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## Internal View of Cranial Base When the top of the skull is cut and removed, the internal view of the cranial base (figure 8.8) is revealed. Here we see the frontal bone surrounding the delicate cribriform (krib'ri-förm; cribrum = sieve, forma = form) plate of the ethmoid bone. The plate has numer...

## Internal View of Cranial Base When the top of the skull is cut and removed, the internal view of the cranial base (figure 8.8) is revealed. Here we see the frontal bone surrounding the delicate cribriform (krib'ri-förm; cribrum = sieve, forma = form) plate of the ethmoid bone. The plate has numerous perforations called the cribriform foramina, which provide passageways for the olfactory nerves (CN I; see sections 13.9 and 16.3a) into the superior portion of the nasal cavity. The anteromedial part of the cribriform plate exhibits a midsagittal elevation called the crista galli (kris'tă = crest; gal'le = of a rooster), to which the cranial dural septa of the brain attach (see section 13.2a). The relatively large sphenoid is located posterior to the frontal bone. It is often referred to as a "bridging bone" because it unites the cranial and facial bones. The lateral expansions of the sphenoid bone are called the greater wings and the lesser wings of the sphenoid. The pituitary gland (see section 17.7a) is suspended inferiorly from the brain into a prominent midline depression between the greater and lesser wings. This depression is termed the hypophyseal fossa, and the bony enclosure around the hypophyseal fossa is called the sella turcica (sel'ă = saddle; tur'si-ka = Turkish). Anterior to the sella turcica are the optic canals through which the optic nerves (CN II) extend from the eyes in the orbits to the brain (figure 8.8). The lateral regions of the cranial base are formed by the petrous (pet'rŭs; petra = a rock) part of each temporal bone. The internal acoustic meatus (also called the internal auditory canal) opens in the more medial portion of the temporal bone and contains the facial nerve (CN VII) and the vestibulocochlear nerve (CN VIII; see section 13.9). The posterior region of the cranial base is formed by the occipital bone. An internal landmark of the occipital bone is the internal occipital protuberance. The internal occipital crest extends from the protuberance to the posterior border of the foramen magnum. Large grooves along the internal aspect of the cranium are formed from impressions from the dural venous sinuses of the brain that lie within them (see section 13.2a). Each bone of the cranium has specific surface features, and each specific bone is shown and summarized in table 8.2. The facial bones are summarized in table 8.3. Articulations of selected cranial and facial bones that are obscured by more superficial bones are shown in figure 8.9. ### Figure 8.8 Internal View of the Cranial Base In this transverse section, the internal portions of the frontal, ethmoid, sphenoid, and occipital bones are prominent. Labels for bones are in bold, whereas labels for bone markings are in regular font. ### Chapter Eight Skeletal System: Axial and Appendicular