Foramina of the skull.
The foramina, or apertures perforating the walls of the skull, are very numerous, and may either be due to holes actually penetrating the bone, or may be small vacuities between the margins of two elsewhere contiguous bones.
They may be divided into two groups, the first including
I. The holes through which the twelve cranial nerves leave the cranial cavity.
a. The most anterior of these nerves, the olfactory, leaves the skull by a number of small holes piercing the cribriform plate (fig. 72, 5).
b. The second, or optic, passes out by a large hole, the optic foramen (fig. 75, II) piercing the orbitosphenoid. The optic foramen is the most anterior of the three prominent holes seen within and immediately behind the orbit.
c. The third, fourth, and sixth nerves, i.e. those supplying the eye muscles, and with them the first or ophthalmic branch of the large fifth or trigeminal nerve, pass out by a large hole, the foramen lacerum anterius (fig. 75, III, IV, V1, VI), which, as has been already mentioned, lies between the orbitosphenoid and alisphenoid.
d. Immediately behind the foramen lacerum anterius, the alisphenoid is perforated by a prominent round hole, the foramen rotundum (fig. 75, V2), through which the second branch of the trigeminal nerve passes out.
e. A quarter of an inch further back there is another prominent hole, the foramen ovale (fig. 75, V3), through which the third branch of the trigeminal nerve leaves the cranium.
f. The seventh or facial nerve, as already mentioned, leaves the cranial cavity and enters the auditory capsule, through an opening in the periotic called the internal auditory meatus, while it finally leaves the skull by the stylomastoid foramen (fig. 75, VII), which lies between the tympanic bulla, the paroccipital process, and the mastoid portion of the periotic.