Mastoid process

The mastoid process is a protrusion of the temporal bone in the skull used to attached four muscles to the skull. The skull has a mastoid process on the left and right side. The muscles that attach to the mastoid process are the splenius capitis, longissimus capitis, digastric muscle, and sternocleidomastoid.

The mastoid process can easily be felt behind the lower half of the ear. Turning the head 90° away from that side will contract the sternocleidomastoid muscle and its attachment to the mastoid process.

The temporal styloid process is found adjacent to the mastoid process and serves a similar purpose.

The mastoid process contains small air pockets called mastoid cells (or mastoid sinuses - they are air pockets not biological cells) that appear to have several unclear functions.

Word origin
The word mastoid means breast-like, referring to its rounded cone shape.