Anterior and Lateral Views of the Skull
The human skull consists of about 22 to 30 single bones which are mostly connected together by ossified joints, so called sutures. The skull is divided into the braincase (cerebral cranium) and the face (visceral cranium). Its main task is the protection of the most important organ in the human body: the brain. The brain is almost entirely enclosed by the cerebral cranium with the exception of the foramen magnum and other foramina at the skull base which serve as entry and exit point for blood vessels and cranial nerves.
In this article we will see the bones of the skull as seen from an anterior and lateral view.
The greater wing of the sphenoid is a structure that extends from the side of the body of the bone to curve backwards and laterally. The most posterior part of this projection is a triangular process that fits neatly into the ridge between the petrous and squamous portion of the temporal bone.
The greater wing of the sphenoid contains the foramina for many structures. These include:
- the foramen rotundum for the maxillary branch (CNV/II) of the trigeminal nerve,
- the foramen ovale which is an oval shaped foramen that allows the mandibular branch (CNV/III) of the trigeminal to pass through together with the accessory meningeal artery.
- the foramen spinosum. If the foramen ovale represents the straight line of an exclamation point, the foramen spinosum represents the dot. The middle meningeal artery (branch of the maxillary artery) enters the cranial cavity through here (foramen spinosum).
- an occasional opening in the greater wing is the foramen petrosum which is a small opening between the spinosum and ovale for the lesser petrosal nerve to exit through.
The lesser wings of the sphenoid are paired wings that extend from the anterior clinoid process laterally to form the anterior border of the middle cranial fossa.
The optic canal travels through the medial portion of the wings, and the sella turcica which contains the pituitary gland sits just behind the clinoid processes. Together with the posterior clinoid process (which arise from the body of the pituitary itself) they make a structure that resembles a ‘four poster bed’ in which the pituitary sits. The origin of the words clinoid and clinical are both ‘klinikē’ which is Greek for ‘bedside’.
Facial Skeleton and Sensory Nerves
The orbital surface of Frontal bone is the anteroinferior most section and gives rise to the roof of the orbital socket. The bone articulates with the zygomatic bone laterally the sphenoid bone deep in the eye socket as well as the ethmoid bone at the deep medial section of the eye socket.
The orbital surface of the Zygomatic bone is the lateral rim and border of the orbital socket. The bone articulates medially with the maxilla, superiorly with the orbital process of the frontal bone, and with the sphenoid bone deep in the eye socket.
The infraorbital foramina are paired openings that can be located on the malar surface of the maxillary bone and transmits the infraorbital nerve. This nerve is a branch of the maxillary division (CNV/II) of the trigeminal nerve and provides sensory innervation to the lower eyelid and cheek.
The mental foramina are paired openings that are located on the anterior most aspect of the mandible and transmit the mental nerve. This provides sensory innervation to the chin area and is a branch of the mandibular division (CNV/III) of the trigeminal nerve.
In humans the Glabella is the region found between both eyebrows and refers to both the overlying skin and the underlying bone which is also indented. The bone is the orbital process of the frontal bone in the midline as it articulates with both paired nasal bones. The supraorbital notch transmits the supraorbital nerve which provides sensory innervation to the eyebrow and upper eyelid.
The orbital plate of the ethmoid is a thin plate of bone and forms part of the medial part of the eye socket. It is part of the ethmoid bone which is a small unpaired bone which separates the nasal cavity from the brain.
The Zygomatic bone forms part of the lateral orbital rim as well as the lateral part of the orbital socket. The bone articulates with the maxilla inferiorly, the frontal bone superiorly and the sphenoid bone within the eye socket.
The mandible is the bone that forms the lower jaw. It consists of various parts including the condyle, which articulates with the temporal bone at the temporomandibular joint, the ramus which forms the posterior border, the body which contains the tooth sockets, and the coronoid process to which the temporalis muscle gains its insertion.
The bone contains the mandibular foramen at the inner surface of the ramus of the mandible which allows the inferior alveolar nerve; a branch of the mandibular nerve which is a branch of the trigeminal nerve. It allows the nerve to enter the bone and innervate the lower teeth. The body of the mandible can be described as the main bulk of its structure. It is the horizontal section of the bone which gives rise to the row of inferior teeth. The orbital surface of maxilla is the superior aspect of the bone which gives rise to the medial part of the floor of the orbital socket.
The ramus of the Mandible is a ridge of bone that connects the body of the bone to the condyle. The condyle articulates at the TMJ or Temporomandibular joint. The joint is complex and allows for not only pivot movement for opening and closing, but also some anterior translation. Ligament control the range of the movement in this joint.
Maxilla and Zygomatic Arches
The maxilla is the bone that forms the upper jaw. The zygomatic process articulates with the zygomatic bone laterally, the frontal process articulates with the nasal bones in the anterior midline, the alveolar process gives attachment to the superior teeth and the palatine process which forms the anterior two-thirds of the hard palate.
The Alveolar process is a thickened ridge of bone that contains the teeth sockets that give rise to the teeth which attach by immovable joints known as gomphosis.
The frontal process of maxilla is the portion of the bone that ascends lateral to the nasal bones. Along with the lacrimal bone and the ethmoid bone, the bone articulates with the orbital surface of the frontal bone superiorly. The frontal process of Zygomatic is the part of the bone that articulates with the frontal bone. The bone extends superiorly and forms the lateral part of the eye socket.
The Ethmoid bone is a small unpaired bone that separates the brain from the nasopharynx. The cribriform plate of the ethmoid bone transmits the fibres of the olfactory nerve. The crista galli is an upward facing ridge of bone that divides the two halves of the plate.
The Frontal bone overlies the frontal lobe of the brain anteriorly and forms the forehead and part of the orbital socket. The bone articulates with the parietal cones posteriorly and the greater wing of the sphenoid bone laterally.
The Nasal bones are two small paired structures that form the bony part of the external nose. They articulate with anteroinferior portion of the frontal bone superiorly and the nasal process of the maxillary bone laterally. This is in addition to the opposing nasal bone and the ethmoid bone within the orbit. The bones give attachment to the quadrangular cartilage of the nasal septum and the lateral cartilages that form the lateral part of the nasal bridge.
The Temporal bone is a complex structure with numerous sections. These include the squamous portion which is similar in structure to other bones of the skull and forms the anterosuperior part of the bone. The outer surface of this portion is convex to allow for the attachment of the temporalis muscle. The mastoid is a significant pyramidal protrusion from the underside of the bone and gives rise to the sternocleidomastoid and splenius capitis and the longissimus capitis. The petrous portion of the bone is a hard pyramidal shaped section of the bone that has a complex structure but essentially contains the organs of hearing. The name ‘petrous’ comes from the latin word ‘petrosus’ which means ‘stone-like’, the tympanic section of the bone surrounds the external auditory meatus and the styloid process.
The bones of the skull that are visible from an anterior and a lateral view are the following:
- the sphenoid bone (with the greater and the lesser wings)
- the frontal bone (especially the orbital surface)
- the zygomatic bone
- the maxilla
- the mandible
- the nasal bones
- the ethmoid bones
- the parietal bone and
- the temporal bone
All these bones have several openings (foramina) through which the various nerves and vessels pass.