The periotic is the most important of them, as it replaces the cartilaginous auditory capsule of the embryo, and encloses the essential organ of hearing. It commences to ossify from three centres corresponding to the pro-otic, epi-otic and opisthotic of lower skulls, such as those of the Turtle and Crocodile.

These ossifications however very early combine to form a single bone, the periotic, which nevertheless consists of two portions, the petrous and the mastoid, differing considerably from one another.

Fig. 74. Diagram of the mammalian tympanic cavity and associated

parts (modified from Lloyd Morgan).

1. external auditory meatus.7. fenestra ovalis.
2. tympanic membrane.8. fenestra rotunda.
3. malleus.9. Eustachian tube.
4. incus.10. cavity occupied by the cochlea.
5. lenticular.11. cavity occupied by the
6. stapes.membranous labyrinth.

The petrous portion lies dorsally and anteriorly, and is much the more important of the two, as it encloses the essential part of the auditory organ. It forms an irregular mass of hard dense bone, projecting into the cranial cavity, and does not appear on the external surface at all. The mastoid portion lies ventrally and posteriorly, is smaller, and formed of less dense bone than is the petrous portion, from which it differs also in the fact that it appears on the surface of the skull, just external to the exoccipital. The petrous portion bears a ridge, which together with a ridge on the supra-occipital, and the tentorium (fig. 72, 21), a transverse fold of the dura mater[142], separates the large cerebral fossa from the cerebellar fossa, which is much smaller than the cerebral fossa and lies behind and partly beneath it. The plane of the tentorium is called the tentorial plane, and the angles that it makes with the basicranial axis and with the occipital and ethmoidal planes vary much in different mammals.

The periotic has its inner surface marked by important depressions, while both inner and outer surfaces are pierced by foramina. At about the middle of its inner surface are seen two deep pits, one lying immediately above the other. Of these the more ventral is a foramen, the internal auditory meatus (fig. 72, 20), through which the VIIth (facial) and VIIIth (auditory) nerves leave the cranial cavity, the facial nerve passing through the bone and afterwards leaving the skull by the stylomastoid foramen (fig. 75, VII), while the auditory passes to the inner ear. The more dorsal of the two pits is not a foramen but the floccular fossa (fig. 72, 23) which lodges the floccular lobe of the cerebellum. In some skulls another wide and shallow but fairly prominent depression is seen dorsal to and slightly behind the floccular fossa, this also lodges part of the cerebellum. Behind the internal auditory meatus, between the periotic and exoccipital is seen the internal opening of the foramen lacerum posterius (fig. 72, 22). The shape of this opening varies. The ventro-anterior border of the periotic is marked by a deep notch, the sides of which sometimes unite, converting it into a foramen.

On the outer side of the periotic, and clearly seen only after the removal of the tympanic, are two holes, the fenestra ovalis and the fenestra rotunda.

The tympanic (figs. 72, 15 and 75, 4) is a greatly expanded boat-shaped bone, which forms the auditory bulla and lies immediately ventral to the periotic; it is separated from the periotic by the tympanic cavity into which the fenestra rotunda and the fenestra ovalis open.