Posterior and Lateral Views of the Skull
The posterior and lateral views of the skull show us important bones that maintain the integrity of the skull. The posterior surface protects the region of the brain that contains the occipital lobes and cerebellum. The lateral bones include the temporal and zygomatic bones which encase the brain and provide attachment to the muscles of the face respectively. The zygomatic process for instance is a crucial bony feature which allows for the attachment of the masseter, an essential muscle of mastication. We will also discuss the sutures of the skull which unite individual bones in adult life.
The zygomatic arch is a ring of flat bone formed by the union of the cheekbone and the zygomatic process of the temporal bone. The bone forms the lateral part of the orbital socket and the floor of the orbit as well.
The incisive foramen is a funnel-shaped opening also known as the anterior palatine foramen. It is found in the hard palate directly behind the incisor teeth. That foramen can be sometimes seen from the posterior view of the skull. Blood vessels and nerves pass through it, and the opening is continuous with the incisive canal which contains the sphenopalatine artery that supplies the oral mucosa, and the nasopalatine nerve, which then ascends from the floor of the nasal cavity.
The mandibular foramen sits in the internal superomedial surface of the mandible. That foramen allows the entrance of the inferior alveolar artery and nerve. The nerve is a branch of the trigeminal nerve (CN5) and provides sensation to the teeth. The inferior alveolar artery terminates as the mental nerve and exits via the mental foramen to provide sensation to the chin area. The mandibular division (V3) of the trigeminal nerve provides motor supply to the muscles of mastication including the temporalis, the masseters and the medial and lateral pterygoid. The lateral pterygoid muscle is much smaller than the medial pterygoid and is the weakest of the mastication muscles. As such it is the only muscle that opens the jaw. In terms of Evolution, the strength of closing the jaw is far more important than the ability to open it. This is also seen in other animals such as the crocodile, who can close its jaw with immense strength but a simple rubber band would suffice to prevent it from opening!
The mastoid foramen is a large foramen found on the posterior border of the temporal bone. Its primary function is to transmit the mastoid emissary vein to the sigmoid sinus. The palatine process of the maxilla is a horizontal projection of the bone that forms the anterior three quarters of the hard palate. The horizontal plate of the palatine bones forms the posterior quarter. The palatine bone is located at the posterior end of the nasal cavity between the pterygoid process of the sphenoid and the maxilla. It consists of two main parts - the horizontal and perpendicular plate. The bone also consists of numerous processes including the pyramidal process, that extends posteroinferiorly at the junction between the perpendicular and horizontal plate. It comprises the lesser palatine canals where the corresponding nerves and vessels pass through. The orbital and sphenoidal part arise from the upper most portion of the vertical part and are separated from eachother by the sphenopalatine notch.
The zygomatic process of the temporal bone can be described as a lengthy curved process which extends from the squamous part of the temporal bone and forms a joint with the zygomatic bone. The process is at first directed laterally, then twists on itself so its surfaces face medially and laterally. The superior border of the zygomatic process of the temporal bone is long thin and sharp and serves for the attachment of the temporal fascia. On the other hand, the inferior border is short, thick and arched, and has attached to it some of the fibres of the masseter. The lateral surface is convex and subcutaneous, while the medial surface is concave and provides attachment to the masseter. The anterior portion of the the zygomatic process of the temporal bone is deeply serrated and articulates with the zygomatic bone, while the posterior end is connected to the squama by two roots - the anterior and posterior roots. The posterior root, a prolongation of the upper border, is strongly marked: it runs backward above the external auditory meatus. The anterior root, continuous with the lower border, is short but broad and strong. It is also directed medially and ends in a rounded eminence, the articular tubercle.
The occipital condyles are articulating processes on anterior margin of the foramen magnum. The skull sits on top of the first cervical vertebrae also known as the ‘Atlas’. Atlas was the Greek titan who holds up the skies, so the role of the first cervical vertebrae is exemplified in its name. The atlantooccipital joint allows for 15 degrees of flexion (nodding). Movement of the head occurs almost entirely at the joint the atlas forms with the odontoid peg and the superior surface of the lateral masses of the axis (C2 vertebrae).
The temporal fossa is a shallow depression found on the external surface of the temporal bone. It is bound anteriorly by the posterior surface of the zygomatic bone. The inferior border is the zygomatic process laterally and the greater wing of the sphenoid medially. The superior border is demarcated by the two temporal lines that arch across the skull from the zygomatic process of the frontal bone to the supramastoid crest of the temporal bone.
The inferior nuchal line is found on the occipital bone and demarcates the site of insertion of the obliquus capitis superior muscle, the rectus capitis posterior major and minor muscles. The borders of the suboccipital triangle are marked by the obliquus capitis inferior, the obliquus capitis superior and the rectus capitis posterior major. That triangle contains the vertebral artery just prior to it piercing the posterior atlantooccipital membrane. The superior nuchal line is found on the occipital bone and demarcates the site of origin of the occipitalis muscle and the trapezius muscle. The highest nuchal line is also found on the occipital bone and demarcates the site of origin of the galea aponeurotica, that can be described as a tough fibrous sheet of tissue that covers the upper part of the skull and is attached between the occipitalis, the external occipital protuberance and the highest nuchal line of the occipital bone.
The external acoustic meatus is a bony ring that forms around the opening of the auditory canal and is a part of the temporal bone. The external auditory canal adheres closely to the bony surface of the temporal auditory canal. The occipital bone overlies the occipital lobe of the brain (which contains the primary visual cortex), and forms joints with the parietal bones superiorly and the temporal bones laterally. The bone forms two condyles at its inferior surface for articulating with the Atlas vertebrae.
The temporal bone has a ‘petrous portion’ which means hard or stone-like in latin. That section of the temporal bone can be described as a pyramidal structure which lodges between the sphenoid and occipital bones. This region of the bone has a complex structure but can basically be described as containing the organs of hearing. The 'petrous portion' of the temporal bone has another subsection known as the tympanic part. That part of the bone surrounds the styloid process and external auditory meatus. The squamous portion of the temporal bone is quite similar in structure to other bones of the skull. It is located in the anterosuperior section of the temporal bone. This region of the bone is externally convex, and allows the temporalis muscle to attach.
The mastoid processes are bilateral bony lumps found inferoposteriorly to the external auditory meatus. Those processes allow for insertion of several muscles including longissimus capitis, the sternocleidomastoid and the splenius capitis. The pterygoid processes of the sphenoid bone are paired processes that descend inferiorly, and perpendicularly from the regions where the body and great wings unite. Each process consists of a medial and lateral pterygoid plate. The styloid processes are bony points that act as sites of origin for the styloglossus muscle. They arise anteromedially to the mastoid process. The stylomastoid foramen lies between the styloid and mastoid processes, and transmits the facial nerve and stylomastoid artery.
The external occipital protuberance is a raised bump from the posterior most part of the occipital bone. Extending laterally from it on either side are the superior nuchal lines. The inion is the highest part of that protuberance.
Sutures of the Skull
The lambdoid suture which can be described a non-movable fibrous joint which lies between the parietal and occipital bones. Its name derives from the Greek letter lambda which it is similarly shaped to. Lambda is the junction between the sagittal and lambdoid sutures and corresponds to the posterior fontanelle during infancy. The posterior fontanelle is triangular, is 5 inches at its widest part and closes at 2 months. The sagittal suture lies in the midsagittal plane and divides the two parietal bones superiorly. This joint line is continued anteriorly as the metopic suture, which fuses at age 1. The bregma, which resembles the letter of the Greek alphabet, is the junction between the sagittal and coronal sutures and corresponds to the anterior fontanelles during infancy.
The anterior fontanelle is diamond shaped and is approximately two inches in its widest part and normally closes at 12-18 months. They lie at the junction of the frontal bones, with the parietal bones. The sphenoparietal suture lies between the two bones that it is named after and forms the horizontal bar of the H that is known as the anterior pterion. The squamous suture divides the temporal bone from the parietal bone. It curls posteriorly starting at the anterior pterion and joins the inferior border of the parietal bone to the squamous portion of the temporal bone.