It is perforated by numerous foramina (holes); for example, the mastoid foramen is situated near the posterior border and transmits a vein to the transverse sinus and a small branch of the occipital artery to the dura mater.

The groove for the transverse sinus is separated from the innermost of the mastoid cells by a very thin lamina of bone, and even this may be partly deficient.

Anteriorly, the mastoid portion is fused with the descending process of the squama above; below, it enters into the formation of the ear canal and the tympanic cavity.

In addition to these a large irregular cavity is situated at the upper and front part of the bone.

Like the mastoid cells, it is filled with air and lined by a prolongation of the mucous membrane of the tympanic cavity, with which it communicates.

The tympanic antrum is bounded above by a thin plate of bone, the tegmen tympani, which separates it from the middle fossa of the base of the skull, below by the mastoid process, laterally by the squama just below the temporal line, and medially by the lateral semicircular canal of the internal ear, which projects into its cavity.

[3] Because of the late postnatal development of the mastoid process, antenatal injuries to the region often recover spontaneously.