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Anatomy of the Skull

The skull vault (figure 1a) is formed from the frontal bone anteriorly, a parietal bone, on each side, and the occipital bone posteriorly. At birth there is a diamond shaped defect between the parietal and frontal bones (anterior fontanel – bregma – this closes at 18 months), and a triangular defect between the parietal and occipital bones posteriorly (posterior fontanel – lambda – this closes at 6 months).

Figure 1b,c shows the bones and radiograph of the anterior aspect of the skull. The frontal bone contains the frontal air sinuses on either side of the midline and, anterior to these, forms the superciliary arches (more prominent in the male), before turning backwards to form the thin superior orbital roof. A palpable supraorbital notch, at the junction of the medial third and lateral two-thirds of the superciliary arch, contains the supraorbital vessels and nerve.

Figure 1
a,b. Bones of skull vault and face, and c. anteroposterior skull radiograph.

 

 

1. Frontal
2. Frontoparietal suture
3. Bregma
4. Saggital suture
5. Superior temporal line
6. Parietal
7. Lambda
8. Parietooccipital suture
9. Occipital

1. Frontal
2. Glabella
3. Supraorbital notch
4. Nasion
5. Frontozygomatic suture 6. Nasal bones
7. Zygomatic
8. Infraorbital foramen
9. Maxilla
10. Ramus of mandible
11. Mental foramen
12. Body of mandible

The two maxillary bones form the central part of the face. Each contains a maxillary air sinus. The thin superior surface of the sinus forms the floor of the orbit, within which lies the infraorbital nerve that emerges through the infraorbital foramen. The maxilla forms the medial wall of the nose. This is incomplete and largely covered by the inferior concha bone, and the superior and middle conchae of the ethymoid bone. Posteriorly the maxilla overlaps the pterygoid process of the sphenoid bone.

Laterally the maxilla articulates with the zygomatic bone. This forms the prominence of the cheek; it extends upwards to meet the frontal bone, at the palpable zygomatico-frontal suture in the lateral wall of the orbit. Posteriorly the zygomatic bone articulates with the zygomatic process of the temporal bone to form the zygomatic arch.

The maxilla is thicker and arched inferiorly where its alveolar process carries the upper teeth. It has an inferomedial thin horizontal projection, the palatine process; the palatine processes from each side form the anterior part of the hard palate. The horizontal process of the palatine bone forms the posterior part of the hard palate on each side; its perpendicular plate completes the back of the lateral wall of the nose. The midline nasal septum is formed from the perpendicular plate of the ethmoid, posteriorly by the vomer and anteriorly by septal cartilages; the latter articulate with the nasal spine of the frontal bone and the line of union of the nasal bones.

The skull is formed laterally by the downward extension of the parietal bone, and the occipital, temporal and sphenoid bones (figure 2a-d). The temporal bone extends medially across the skull base, housing the middle and inner ear. The body of the sphenoid bone forms part of the skull base. Laterally it has lesser and greater wings: the lesser wing forms the ridge between the anterior and middle cranial fossae. The greater wing extends onto the lateral surface of the skull, its union with the frontal, parietal and temporal bones is known as the pterion, this overlies the middle meningeal artery. The skull base is completed posteriorly by the occipital bone (figure 2d). This extends anteriorly to the body of the sphenoid, contains the foramen magnum and, inferiorly, gives attachment to powerful neck muscles.

The mandible is a single bone concerned with mastication. It has a body and a superior ramus on each side. Each superior ramus has a condylar process (by which the mandible is attached to the skull at the temporomandibular joint) and a coronoid process (for attachment of the temporalis muscle). The upper border of the horseshoe shaped body carries the lower dentition. The inferior alveolar (dental) nerves pass through the bone on each side, and emerge at the mental foramina. The mental foramen, the infraorbital foramen and the supraorbital notch are in line, approximately 1.5 cm from the midline.

Figure 2
a,b,c,d. Lateral and posterior aspects of head; lateral skull X-rays, note in b. the separation of the frontoparietal sutures associated with trauma and in c. large pituitary fossa associated with pituitary adenoma; compare bones with anatomy shown in figures 1 and 2

a Bones of lateral head
1. Parietal
2. Superior temporal line
3. Frontal
4. Pterion
5. Temporal
6. Occipital
7. Greater wing of sphenoid
8. Nasal
9. Lacrimal
10. Zygomatic
11. Zygomatic arch
12. External acoustic meatus 13. Condylar process
14. Mastoid process
15. Maxilla
16. Styloid process
17. Coronoid process
18. Ramus of mandible
19. Angle of mandible
20. Body of mandible

d Posterior bones of skull
1. Sagittal suture
2. Parietal
3. Lambda
4. Parietooccipital suture
5. Occipital
6. Temporal
7. Superior nuchal line
8. Mastoid process
9. Atlas
10. Styloid process