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 of one of the projections, the mastoid process. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The mastoid process is a pyramidal bony projection from the posterior section of the temporal bone. The superior border of the mastoid portion of the temporal bone articulates with the parietal bone. 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, innervated by the posterior auricular branch of the facial nerve aka 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), and the longissimus capitis muscle (laterally flexes and rotates the head and neck if one side alone contracts, extends the head if both contract). 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 (cranial nerve VII) 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.