Mastoid foramen

The outer surface of the temporal bone is perforated by numerous foramina; one of these, of large size, situated near the posterior border, is termed the mastoid foramen

It transmits a vein to the transverse sinus and a small branch of the occipital artery to the dura mater.

Variations
The position and size of this foramen are very variable; it is not always present; sometimes it is situated in the occipital bone, or in the suture between the temporal and the occipital.