The mastoid part of the temporal bone is its posterior component. The inferior conical projection of the mastoid part is called the mastoid process.

Gross anatomy

An irregular cavity atwithin the anterosuperior aspect of the bone is called the mastoid (or tympanic¹) antrum, which communicates with the attic of the tympanic cavity¹. Starting around birth and continuing through childhood, pneumatization develops outwards from the mastoid antrum, resulting in the mastoid air cells.

The roof of the mastoid antrum is called the tegmen mastoideum, which is a posterior extension of the tegmen tympani. The tegmen mastoideum forms part of the floor of the middle cranial fossa, separating the mastoid from the cranial cavity.

The lateral aspectsurface of the mastoid process is sometimes called the mastoid cortex.

The medial aspectsurface of the mastoid process contains a deep groove called the digastric fossa (mastoid notch), which originates the digastric muscle. Medial to that is the occipital groove, which is traversed by the occipital artery.

At the inner posterior surface of the mastoid, the sigmoid sulcus lodges the sigmoid sinus and partially the transverse sinus².

The posterior aspectsigmoid plate is the thin lamina of bone that separates mastoid air cells from the bone perforated bydural venous sinuses. Around this area is the inconstant mastoid foramen, which carries small vesselstransmits an emissary vein and occipital artery branch ^1,2.

Articulations

The mastoid part of the temporal bone articulates with the following bones ^1,2: 

Attachments

Lateral attachments (from superior to inferior)¹:

• occipitalis muscle (anterior) 
• occipital(occipital belly of occipitofrontalisoccipito-frontalis ²)
• posterior auricular muscle (posterior)
• sternocleidomastoid muscle
• splenius capitis muscle
• longissimus capitis muscle

Medial attachments¹:

• posteriordigastric muscle (posterior belly)

Relations and/or Boundaries

The mastoid is derived from the squamous and petrous parts of the digastric muscle

Articulations

Mastoidtemporal portion. The anterior portion belongs to the squamous part of temporal bone articulatesand the posterior portion belongs to the petrous part ³. Therefore, the mastoid part is fused anterosuperiorly with the following bones: 

• superiorly: mastoid angle of the parietal bone
• posteriorly: inferior border of the occipital bone
• anteriorly: descending process of the squamous temporal bone inferior to and anteromedially with the supramastoid crest

Anteroinferiorly, the mastoid forms the posterior bony wall of the external auditory canal and tympanic cavity (retrotympanum) ¹.

Variant anatomy

The degree of mastoid pneumatization is variable, categorized as pneumatic (full air cell development), sclerotic (solid bone), or mixed (air cells and marrow) ².