The skull is composed of multiple small bones held together by a fibrous joints. Its inferior surface gives rise to a number of projections, and these allow for the attachment of many structures of the neck and face. In this article we will discuss the gross and functional anatomy, together with the clinical relevance of one of the projections, the mastoid process.
Borders and relations
It might be a good idea to learn the full anatomy of the skull before zoning in on specific structures like the mastoid practice. Our skull bone quizzes and diagrams are ready and waiting for you!
The posterior border articulates with the occipital bone, and the anterior border is merged with the descending portion of the squamous section of the temporal bone. The petrosquamous suture runs vertically from the superior border of the mastoid process.
The mastoid part has a rough outer surface that gives rise to the occipital belly of occipitofrontalis muscle, which covers the skull from the superior nuchal line to the mastoid process. This muscle is innervated by the posterior auricular branch of the facial nerve (cranial nerve VII).
It also gives rise to auricularis posterior muscle that attaches to the lower part of the cranial surface of the concha (outer ear).
The mastoid process itself (the pointed projection) gives rise to:
- the sternocleidomastoid muscle (rotates the head to the contralateral side)
- the splenius capitis muscle (extends rotates and laterally flexes the head)
- the posterior belly of the digastric muscle (opens the jaw when the masseter and temporalis muscles are relaxed)
Fossae, grooves and features
The medial surface of the mastoid portion has a deep groove called the digastric fossa, which allows the digastric muscle to attach. Further medial to this groove is the occipital groove, which the occipital artery passes over. The sigmoid sulcus also lies on the inner portion of the mastoid bone and is there the transverse venous sinus of the brain lodges.
The styloid process lies anterior and medial to the mastoid process, and in between them is the stylomastoid foramen. This foramen allows the muscular branch of the facial nerve to leave the skull and go onto innervate the muscles of facial expression. The mastoid bone is normally pneumatised or air filled by the mastoid air cells.
This is a condition caused by infection of the mastoid air cells. Symptoms include tenderness over the area, fever and swelling. The area may be red, and the patient may have earaches. It is commonly caused by untreated otitis media, where the infection tracks from the middle ear into the mastoid section of the temporal bone. The mastoid process is underdeveloped at birth which leaves the posterior auricular branch of the facial nerve (which ascends anterior to the mastoid process) superficial and unprotected.