Petrous part of the temporal bone

The petrous portion of the temporal bone or pyramid is pyramidal and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents for examination a base, an apex, three surfaces, and three angles, and contains, in its interior, the essential parts of the organ of hearing. The petrous portion is among the most basal elements of the skull and forms part of the endocranium.

Base

The base is fused with the internal surfaces of the squama and mastoid portion.

Apex

The apex, rough and uneven, is received into the angular interval between the posterior border of the great wing of the sphenoid and the basilar part of the occipital; it presents the anterior or internal orifice of the carotid canal, and forms the postero-lateral boundary of the foramen lacerum.

Surfaces

Anterior surface

The anterior surface forms the posterior part of the middle fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the petrosquamous suture, remains of which are distinct even at a late period of life. It is marked by depressions for the convolutions of the brain, and presents six points for examination:

1. near the center, the arcuate eminence (eminentia arcuata), which indicates the situation of the superior semicircular canal.
2. in front of and a little lateral to this eminence, a depression indicating the position of the tympanic cavity:......
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