Skull anatomy

1.        Introduction

The skull is a very complex organ consisting of about 50 bones (Figs 4-G & 4-H).  The bones that form the skull can be divided into two groups:

(1)       Cranial bones (neurocranium) enclose the brain and the organs of hearing.

(2)       Facial bones form the skeleton of the oral and nasal cavities.

Only two movable joints persist in the skull, the mandible and the hyoid apparatus articulate with the temporal bone.  The other joints (sutures) between the bones ossify with increasing age.

The skull has numerous foramina (singular foramen) through which nerves and blood vessels pass.  These will be described in the neuroanatomy course.

2.        Facial Bones

The maxillae are the principal bones of the upper jaw, carrying the cheek teeth in large cavities (dental alveoli). 

The incisives (pre-maxilla) form the rostral part of the skull and bear the incisors.  They have nasal and palatine processes.

The paired nasal bones meet at the dorsal mid-line. 

The paired palatines form the caudal part of the hard palate.

The lacrimal bones are small and form part of the rostral border of the orbit.  They each have a funnel-like fossa leading to the lacrimal canal. 

The zygomatic (malar) bones are placed between the lacrimal and maxilla and contribute to the rostral part of the zygomatic arch. 

3.        Cranial Bones

The occipital is at the caudal end of the skull.  The foramen magnum lies ventrally and conveys the spinal cord.  Two occipital condyles articulate with the atlas.  The nuchal crest provides attachment for the nuchal ligaments and muscles.

The sphenoid lies at the base of the cranium (basisphenoid) and extends laterally into the orbit containing the optic and orbital foramina.  It also extends rostrally between the palatines as the presphenoid.  The pterygoid crest, for the attachment of pterygoid jaw muscles, is formed from the pterygoid and the palatine.

The paired frontals lie dorsal and rostral to the brain.  The two plates of bone separate rostrally forming part of the frontal sinus.  The paired parietals, with the frontals, form the dorsum of the cranium.  Caudally the interparietal (which fuses with the occipital) separates the parietals and forms the sagittal crest for muscle attachment.

The paired temporal bones form most of the lateral surface of the cranium.  They are divided into three parts:

(a)            Squamous temporal forms the ventral part of the temporal fossa and extends rostrally into the zygomatic process of the caudal zygomatic arch.

(b)           Tympanic temporal has the tympanic bulla which encloses the middle ear cavity and opens laterally via the external acoustic meatus

(c)            Petrosal temporal contains the cochlea, vestibule and semi-circular canals of the inner ear.

On the ventral surface of the temporal there is a glenoid cavity for articulation with the mandible.

4.      Mandible

The mandibles (lower jaw) carry the lower teeth and join rostrally at a symphysis soon after birth.  Medially, the mandibular foramen carries vessels and the mandibular alveolar nerve to the rostral mental foramina.  The condyloid process lies caudal to the mandibular notch and articulates with the temporal bone (Fig. 4-I).