Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Cranium

Temporal Bone

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

Absence of the superior part of the squama temporalis has been reported.

A frontosquamous suture frequently occurs, which is normally found in the skulls of the chimpanzee, gorilla, and gibbon.

The petrosquamous suture of the temporal bone sometimes persists. The mastoid cells vary in extent and may invade the squamous part of the bone; the air cells may not be symmetrical on the two sides.

The squama in some cases is divided into two or four parts.

A bar of bone in the dura over the trigeminal nerve has been interpreted as a vestige of the primitive cranial wall as present in reptilia.

The styloid processes vary markedly in length, and the stylohyoid ligament may calcify to make a rigid connection with the hyoid bone. A 75 mm long styloid process has been reported. The styloid process may be composed of two to five osselets, articulating by synchrondoses.

The greater horn of the hyoid may be joined to the body of the hyoid by an articulation. The zygomatic arch may be replaced by a fibrous band. In some cases, the zygomatic process of the temporal bone is absent.

The mastoid process may possess a double apex, with the medial portion divided from the lateral by a fissure. Usually, the medial lip of the groove forms a distinct ridge and provides the point of attachment of the digastric muscle; an enlarged paramastoid process may result from the development of air cells within the medial portion of the process. Air cells, usually confined to the mastoid process, may invade the horizontal or vertical part of the squama, or even the pars petrosa.

The mastoid cells vary greatly in extent and the pneumatization may be unsymmetric on the two sides.

The pars petrosa is very rarely variable. The subarcuate fossa, however, may persist, in part, as a bony canal. In cases of absence of the internal carotid artery, the carotid canal may also be absent. In two cases this was found on the left side; in a third case the side was not specified. The carotid canal may take one of several forms, including that of a sulcus. The squamous part of the temporal bone may exhibit a sizable, extra process just anterior to the external acoustic meatus, called the postglenoid process.

A canal or foramen (of Hüschke) is found in the floor of the bony part of the external auditory meatus, near the tympanic ring during the first five years of life. It may persist throughout life.

The suprameatal spine, which overlies the floor of the mastoid antrum, is absent in 20% of skulls. A bony process may project from the anterior edge of the squama that runs between the sphenoid and parietal bones to the lateral edge of the frontal bone. This process effectively intervenes between the parietal and sphenoid bones. The squama may also exhibit a foramen jugulare spurium by which intracranial venous blood may communicate with the external jugular system. The fetal suture between the squama and the mastoid process sometimes persists in the adult.

A foramen may be present in the petrobasilar fissure in front of the jugular foramen through which runs the inferior petrosal sinus. The jugular foramen, a passage between the petrous portion of the temporal bone and the jugular process of the occipital bone, may be divided into two parts by intrajugular processes.

The upper edge of the internal acoustic meatus may form a tubercle, the crista tentori, that roofs the superior petrosal sinus.

The stapedius muscle may be absent. A stapes with two (doubled) end plates has been reported. One endplate was located in the oval window (as usual), the second was in the round window (Murphy).

The mallens and incus were first described by Berengaria da Carpi in 1521. The stapes was first describe by Ingrassia in 1546.

The malleus was studied in 22 specimens:

The Incus was studied in 22 specimens:

The styloid process of the temporal bone was absent in 58/150 cranial halves; hence it is undependable as a surgical marker for the stylomastoid foramen.

The length of the styloid process is variable in length. The variation in size found by Lang (1983), 0.1 - 0.9 cm. in 18 cases; 1 - 1.9 cm in 40 cases; 2.0 - 2.9 cm in 19 cm; 3.0 - 3.9 cm in 14 cases; and 4.0 - 4.2 in 1. The maximum size found in this study was a 4.2 cm styloid process. The styloid processes are typically bilaterally dissimilar in length. In 18 skulls, the process was absence unilaterally" in 20 skulls, the styloid process was absent bilaterally.

The stylomastoid foramen transmits the facial nerve, stylomastoid artery, and the auricular branch of the vagus nerve.

Styloid Process

The Hyoid Apparatus

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