Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

Special Part
Axial skeleton, sceleton axiale - The skull as a whole

The formed human Skull consists of 29 permanent bones, of which 11 are paired and 7 are unpaired. The skull determines the shape of the HEAD, which depends on The Human Body type and individual characteristics. The number of bones forming the skull may increase when the frontal, parietal, or occipital bones are divided by Wormian sutures, or due to accessory bones formed from additional centers of ossification along the sutures (sutural bones, ossa suturarum) and fontanelles (fontanelle bones, ossa fonticulorum). The External Structure of the skull is viewed from several Perspectives known as norma. In anatomy and anthropology, the facial, lateral, vertical, basilar, and occipital norma are described.

The facial norma, norma facialis, allows examination of the anterior part of the cranial vault — the forehead (frons) — and the main PARTS OF THE facial skull: the orbits, the piriform aperture, the upper and lower jaws with alveolar processes and Teeth, and the mental protuberance.

The lateral norma, norma lateralis, provides the clearest understanding of the proportions between the neurocranium and viscerocranium, as well as the cranial vault and base. The lateral view reveals most of the skull bones, the temporal fossa, the zygomatic arch, the mastoid process, and the external acoustic meatus.

The vertical norma, norma verticalis, allows examination of the BONES OF THE cranial vault and their sutures (sagittal, coronal, and lambdoid).

The basilar norma, norma basilaris, is used to study the external base of The Skull and its foramina.

The occipital norma, norma occipitalis, allows examination of the posterior part of the cranial vault and base (lambdoid and occipitomastoid sutures, mastoid processes, external occipital protuberance, and nuchal lines).

Skull shape is determined using cranioscopy (visual inspection of the skull) and craniometry (measurement), which are widely used in anthropology. There are three main skull shapes According to the cranial index, which is the percentage ratio of the transverse diameter (between the parietal eminences) to the longitudinal diameter (the distance from the nasion to the external occipital protuberance):

- short — brachycephalic (cranial index of 80 and above);

- medium — mesocephalic (79–76);

- long — dolichocephalic (below 75).

The skull, cranium, is divided into two parts: the braincase (neurocranium), cranium cerebrale, and the facial skull (viscerocranium), cranium faciale. The skull comprises the base, basis cranii, the roof, calvaria, and the vault, fornix, which are demarcated by an imaginary line passing from the external occipital protuberance along the superior nuchal line, the Base of the mastoid process, the upper margin of the external acoustic meatus, the ROOT of the zygomatic process of the Temporal bone, the infratemporal crest of the greater wing of the Sphenoid bone, the upper margin of the superior orbital fissure, then ascending along the sphenozygomatic suture, passing forward along the frontozygomatic suture, the zygomatic process of the Frontal bone, and along the supraorbital margin of the frontal bone, reaching its nasal part.

The cranial vault, fornix cranii, has a smooth external surface covered by the pericranium. The vault features various types of sutures: coronal, sutura coronalis, sagittal, sutura sagittalis, and lambdoid, sutura lambdoidea. On the lateral surface of the vault lies the temporal fossa, fossa temporalis, separated from the infratemporal fossa by the infratemporal crest, crista infratemporalis.

In the anterior part of the vault, one distinguishes the eminence (frontal tuber, tuber frontale) and the superciliary arch, arcus superciliaris; in the lateral part, the parietal eminences, tubera parietalia; and in the posterior part, the external occipital protuberance, protuberantia occipitalis externa.

On the internal surface of the vault, there are a series of eminences and impressions resulting from the close fit of the cerebral hemispheres, as well as sulci for Blood Vessels and venous sinuses. These include the sulcus of the superior sagittal sinus running along the midline, and the sulci of the transverse sinuses extending from the external occipital protuberance, which continue into the sulci of the sigmoid sinuses. Along the margin of the superior sagittal sinus lie the granular foveolae (pacchionian depressions) lodging arachnoid granulations, as well as two to three nutrient foramina, foramina nutricia, through which blood vessels pass.

The skull base, basis cranii, is divided into two parts: the external base, basis cranii externa, and the internal base, basis cranii interna.

The external base of the skull, basis cranii externa, has a complex configuration with prominences and foramina for the passage of blood Vessels and nerves (Fig. 49). In the posterior part lies the external occipital protuberance, protuberantia occipitalis externa, from which the median occipital crest, crista occipitalis externa, descends, flanked by the nuchal lines. Slightly anterior to the squamous part is the foramen magnum, bounded by the lateral masses of the Occipital bone and anteriorly by the body of the sphenoid bone. Posterolateral to the occipital condyles runs the hypoglossal canal, canalis condylaris, while posterior to them lies the condylar fossa, fossa condylaris, which may lead to an inconstant condylar canal serving as a venous emissary. Anterior to the foramen magnum on the body of the occipital bone is the pharyngeal tubercle, tuberculum pharyngeum. Medial to the mastoid process are the mastoid notch, incisura mastoidea, the sulcus for the occipital artery, sulcus arteriae occipitalis, and the mastoid foramen, foramen mastoideum, which acts as a venous emissary. Closer to the anterior margin near the mastoid process are the styloid process and the stylomastoid foramen, foramen stylomastoideum, through which the Facial Nerve exits.

On the Inferior surface of the petrous part are the jugular fossa, fossa jugularis, and the jugular foramen, foramen jugulare, through which Cranial Nerves IX–XI pass, and where the External Jugular Vein originates. More anteriorly lies the external opening of the carotid canal, foramen caroticum externum, and near the apex of the petrous part is the lacerum foramen, foramen lacerum.

¹ G. Vidius (Guido Guidi, G. Vidianus), 1500–1569, an Italian anatomist.

Closer still to the anterior part, near the root of the pterygoid processes, runs the pterygoid canal, canalis pterygoideus (Vidius¹), which opens into the pterygopalatine fossa. On the greater wings of the sphenoid bone are the oval foramen, foramen ovale, and the spinous foramen, foramen spinosum. Anterior to the lacerum foramen, between the medial plates of the pterygoid processes and the Vomer, lie the apertures of the Nasal cavity — the choanae, choanae. Lateral to the pterygoid process, between its lateral lamina and the inferior surface of the greater wing of the sphenoid bone, is the infratemporal fossa, fossa infratemporalis. In the posterior parts of the hard palate, on each side, are the greater and lesser palatine foramina, foramen palatinum majus et foramina palatina minora, which lead into the respective canals, while posterior to the incisor alveoli lies the incisive foramen, foramen incisivum.

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Fig. 49. Skull, cranium: external base, inferior view

(after R. D. Sinelnikov)

The internal skull base, basis cranii interna (Fig. 50), presents an uneven surface forming three depressions: the anterior, middle, and posterior cranial fossae, fossae cranii [craniales]. The depressions within these cranial fossae are "weak spots" susceptible to typical fractures.

Fig. 50. Skull, cranium: internal base, internal view (after R. D. Sinelnikov)

The anterior cranial fossa, fossa cranii anterior, is located in the region of the frontal and ethmoid bones, housing the frontal lobe of the cerebrum. The anterior cranial fossa is formed by the orbital surfaces of the frontal bone, the lesser wings of the sphenoid bone, and the cribriform plate; it is separated from the middle fossa by the posterior margin of the lesser wings and the limbus sphenoidalis. On the orbital surfaces of the frontal bone, cerebral elevations and digital impressions are visible, which represent the imprints of the sulci and gyri of the cerebral hemispheres.

The anterior cranial fossa communicates with the nasal cavity via the openings in the cribriform plate. The olfactory filaments of the first pair of cranial nerves pass through these openings. The cribriform plate is a vulnerable area of the anterior cranial fossa and can be injured in Direct and Indirect trauma. In the center of the fossa lies the crista galli, and anterior to it is the foramen caecum, to which the dura mater attaches.

The middle cranial fossa, fossa cranii media, is formed by the sphenoid and temporal bones and contains the temporal lobes of the cerebral hemispheres. It is separated from the posterior cranial fossa by the dorsum sellae and the superior border of the petrous part of the temporal bone. In the center of the middle cranial fossa lies the sella turcica with the hypophyseal fossa for the Pituitary Gland. Anterior to the sella turcica is the tuberculum sellae, across which runs the chiasmatic sulcus, sulcus chiasmatis, continuing into the optic canal, where the Optic nerve and ophthalmic artery pass. The sulcus of the Internal Carotid Artery runs along the lateral surface of the body. Posteriorly to it lies the internal opening of the carotid canal, and near the apex of the petrous part is the foramen lacerum. On the greater wings of the sphenoid bone, the foramen rotundum, foramen ovale, and foramen spinosum are distinguished. The foramen rotundum leads into the pterygopalatine fossa and transmits the maxillary nerve; the foramen ovale transmits the mandibular nerve; and the foramen spinosum transmits the middle meningeal artery, arteria meningea media. The middle cranial fossa communicates with the Orbit via the optic canal, canalis opticus, and the superior orbital fissure, fissura orbitalis superior. The optic nerve and ophthalmic artery pass through the optic canal.

The posterior cranial fossa, fossa cranii posterior, is situated in the occipital region and contains the occipital lobes of the cerebral hemispheres, the Medulla Oblongata, the Pons, and the Cerebellum. The posterior cranial fossa is formed by the occipital bone, the posterior surface of the petrous part of the temporal bone, the body of the sphenoid bone, and partially by the Parietal bone. In the center of the fossa is the foramen magnum, which connects it to the vertebral canal. Anterior to the foramen magnum is the clivus, and posterior to it is the internal occipital protuberance, protuberantia occipitalis interna, from which diverge the sulci of the transverse sinus, sulcus sinus transversi, transitioning into the sigmoid sinuses, sulcus sinus sigmoidei, which ultimately terminate at the jugular foramen, foramen jugulare.

On the posterior surface of the petrous part, we distinguish the internal acoustic meatus, porus acusticus internus, which serves as the exit site for the Vestibulocochlear nerve and the entry site for the facial nerve. In the region of the body of the occipital bone and the medial margin of the petrous part runs the sulcus for the inferior petrosal sinus, sulcus sinus petrosi inferior.

On the external lateral surface of the skull, norma lateralis, one distinguishes: the temporal fossa, fossa temporalis, the infratemporal fossa, fossa infratemporalis, and the pterygopalatine fossa, fossa pterygopalatina (Fig. 51).

The temporal fossa, fossa temporalis, is bounded superiorly from the cranial vault by the superior temporal line. The medial wall of the temporal fossa is formed by the inferior part of the parietal bone, the squamous part of the temporal bone, and the external surface of the greater wing of the sphenoid bone. The anterior wall is formed by the Zygomatic bone, and the lateral wall by the zygomatic arch. The temporal fossa is filled by the temporalis Muscle, musculus temporalis.

The infratemporal fossa, fossa infratemporalis, is bounded: superiorly by the greater wing of the sphenoid bone and the squamous part of the temporal bone; medially by the lateral plate of the pterygoid process; anteriorly by the infratemporal surface of the Maxilla and partially by The surface of the zygomatic bone; and laterally by the ramus of the Mandible. The infratemporal fossa is separated from the temporal fossa by the infratemporal crest, crista infratemporalis. It communicates with the orbit through the inferior orbital fissure and with the pterygopalatine fossa through the pterygomaxillary fissure. The fossa is filled with the pterygoid Muscles, a segment of the maxillary artery, arteria maxillaris, the pterygoid venous plexus, plexus pterygoideus, the lingual nerve, and the inferior alveolar nerve.

Fig. 51. External lateral surface of the skull, norma lateralis

The pterygopalatine fossa, fossa pterygopalatina, is bounded: anteriorly by the maxillary tuberosity, tuber maxillae; posteriorly by the pterygoid process of the sphenoid bone; medially by the perpendicular plate of the Palatine bone; and superiorly by the greater wing of the sphenoid bone. The pterygopalatine fossa communicates:

- with the temporal fossa via the pterygomaxillary fissure, fissura pterygomaxillaris;

- with the foramen lacerum via the pterygoid canal, canalis pterygoideus;

- with the middle cranial fossa via the foramen rotundum;

- with the nasal cavity via the sphenopalatine foramen, foramen sphenopalatinum;

- with the orbit via the inferior orbital fissure, fissura orbitalis superior;

- with the Oral Cavity via the greater palatine canal, canalis palatinus major.

The fossa contains adipose tissue, the maxillary nerve, the terminal segment of the maxillary artery, arteria maxillaris, Veins that drain into the pterygoid plexus, and the pterygopalatine ganglion, ganglion pterygopalatinum.