The skull as a whole

The skull has a complex relief on both the internal and external surfaces, which is associated with the location of the brain, sensory organs, and the presence of numerous openings and channels for the passage of blood vessels and nerves in its bony receptacles.

All the bones of the skull, except for the mandible and hyoid bone, are immobilely and firmly connected to each other by means of serrated, flat, squamous sutures in the area of the cranial vault and face, as well as permanent and temporary cartilaginous connections (synchondroses) at the base of the skull. The names of the sutures and synchondroses come from the names of the connecting bones (for example, the sphenoid-frontal suture, the petro-occipital suture). Some sutures are named according to their location, shape or direction (sagittal suture, lambdoid suture).

When examining the skull from above (norma verticalis), the vault, or roof, of the skull is visible; from below (norma basilaris), the base of the skull; from the front (norma facialis), the facial skull; from the back (norma occipitalis), the occipital part; from the sides (norma lateralis), a series of depressions (pits), limited by various bones.

Cerebral section of the skull

The upper part of the cranium is called the vault or roof of the skull due to its shape. The lower part of the skull serves as the base. The boundary between the vault and the base on the outer surface of the skull is an imaginary line that passes through the external occipital protuberance, then along the superior nuchal line to the base of the mastoid process, above the external auditory opening, along the base of the zygomatic process of the temporal bone and along the infratemporal crest of the greater wing of the sphenoid bone. This line rises upward to the zygomatic process of the frontal bone and along the supraorbital edge reaches the nasofrontal suture. The boundary between the vault and the base is not defined on the inner surface of the skull. Only in its posterior part can this boundary be drawn along the groove of the transverse sinus, corresponding to the superior nuchal line on the outer side of the occipital bone.

The vault (roof) of the skull (calvaria) is formed by the squama of the frontal bone, the parietal bones, the squama of the occipital and temporal bones, and the lateral parts of the greater wings of the sphenoid bone. On the outer surface of the vault of the skull along the midline is the sagittal suture (sutura sagittalis), formed by the junction of the sagittal edges of the parietal bones. Perpendicular to it, on the border of the frontal squama with the parietal bones in the frontal plane, is the coronal suture (sutura coronalis). Between the parietal bones and the occipital squama is the lambdoid suture (sutura lambdoidea), similar in shape to the Greek letter "lambda". On the lateral surface of the cranial vault on each side between the squama of the temporal and parietal bones there is a squamous suture (sutura squamosa), as well as serrated sutures (suturae serratae) between other adjacent bones.

In the anterior parts of the cranial vault there is a convex part - the forehead (frons), formed by the scales of the frontal bone. On the sides are visible frontal tubercles, above the eye sockets - superciliary arches, and in the middle - a small platform - glabella. On the upper lateral sides of the cranial vault protrude parietal tubercles. Below each tubercle passes an arched upper temporal line (linea temporalis superior) - the place of attachment of the temporal fascia. Below this line is visible a more clearly expressed lower temporal line (linea temporalis inferior) - the place of the beginning of the temporal muscle. On the anterolateral side of the skull there are two fossae - temporal and infratemporal.

The temporal fossa (fossa temporalis) is limited at the top by the inferior temporal line, and at the bottom by the infratemporal crest of the greater wing of the sphenoid bone. Laterally, the temporal fossa is limited by the zygomatic arch (arcus zygomaticus), and anteriorly by the temporal surface of the zygomatic bone. The infratemporal crest separates the temporal fossa from the infratemporal.

The infratemporal fossa (fossa infratemporalis) is clearly visible when examining the skull from the side. The upper wall of the infratemporal fossa is the lower surface of the greater wing of the sphenoid bone. The medial wall is formed by the lateral plate of the pterygoid process of this bone. The anterior wall is limited by the tubercle of the maxilla and partly by the zygomatic bone. The infratemporal fossa has no lateral or lower walls. In front, this fossa communicates with the orbit through the inferior orbital fissure (fissura orbitalis inferior), medially - through the pterygomandibular fissure with the pterygopalatine fossa. The entrance to the pterygopalatine fossa is located in the anterosuperior parts of the infratemporal fossa.

The pterygopalatine fossa (fossa pterygopalatina) is bounded anteriorly by the tubercle of the maxilla, posteriorly by the base of the pterygoid process of the sphenoid bone, and the medial-perpendicular plate of the palatine bone. The pterygopalatine fossa has no lateral wall; on this side it communicates with the infratemporal fossa. Five openings open into the pterygopalatine fossa. This fossa communicates medially with the nasal cavity through the sphenopalatine foramen (foramen sphenopalatinum), with the middle cranial fossa above and posteriorly by means of the round opening. Posteriorly, the pterygopalatine fossa communicates with the region of the foramen lacerum of the skull by means of the pterygoid canal. The fossa communicates with the orbit through the inferior orbital fissure, and with the oral cavity through the greater palatine canal. Blood vessels, cranial nerves and their branches pass through these and other openings.

On the inner (cerebral) surface of the cranial vault, sutures (sagittal, coronal, lambdoid, squamous), finger-like impressions - imprints of the convolutions of the cerebrum, as well as narrow arterial and venous grooves (sulci arteriosi et venosi) - places where arteries and veins adjoin are visible.

Near the sagittal suture are granulation pits (foveolae granulares), formed by the protrusion of the arachnoid membrane of the brain.

The base of the skull can also be examined from two positions: from the outside (from below) - the outer base of the skull and from the inside (after a horizontal cut has been made at the level of the border with the vault) - the inner base.

The external base of the skull (basis cranu externa) is closed by the facial bones in the anterior section. The posterior section of the base of the skull is formed by the outer surfaces of the occipital, temporal and sphenoid bones. Numerous openings through which arteries, veins and nerves pass in a living person are visible here. Almost in the center of the posterior section there is a large (occipital) opening, and on its sides are the occipital condyles. Behind each condyle is a condylar fossa with an inconstant opening - the condylar canal. The hypoglossal canal passes through the base of each condyle. The posterior section of the base of the skull is limited in front by the external occipital protuberance with the superior nuchal line extending from it to the right and left. In front of the large (occipital) opening is the basilar part of the occipital bone with the pharyngeal tubercle, it passes into the body of the sphenoid bone. On each side of the occipital bone, the inferior surface of the pyramid of the temporal bone is visible, on which are located the external opening of the carotid canal, the musculotubular canal, the jugular fossa and the jugular notch. The latter, together with the jugular notch of the occipital bone, forms the jugular foramen, the styloid process, the mammillary process and the stylomastoid opening between them. Adjacent to the pyramid of the temporal bone on the lateral side is the tympanic part of the temporal bone, surrounding the external auditory opening. Behind, the tympanic part is separated from the mammillary process by the tympanomastoid fissure. On the posteromedial side of the mammillary process are the mammillary notch and the groove of the occipital artery.

On the lower part of the squamous part of the temporal bone, the mandibular fossa is visible, forming the temporomandibular joint with the condylar process of the lower jaw. Anterior to this fossa is the articular tubercle. Between the petrous and squamous parts of the temporal bone, the posterior part of the greater wing of the sphenoid bone enters. The spinous and oval openings are clearly visible here. The pyramid of the temporal bone is separated from the occipital bone by the petrooccipital fissure (fissura petrooccipitalis), and from the greater wing of the sphenoid bone by the sphenopetrosal fissure (fissura sphenopetrosa). On the lower surface of the outer base of the skull, an opening with jagged edges is visible - the lacerated opening (foramen lacerum), which is located between the apex of the pyramid, the body of the occipital bone and the greater wing of the sphenoid bone.

The internal base of the skull (basis cranii interna) has a concave, uneven surface, reflecting the complex relief of the lower surface of the brain. Three cranial fossae are distinguished at the internal base of the skull: anterior, middle, and posterior. The anterior cranial fossa is separated from the middle by the posterior edge of the lesser wings and the tubercle of the sella turcica of the sphenoid bone. The boundaries between the middle and posterior fossae are the upper edge of the pyramids of the temporal bones and the back of the sella turcica of the sphenoid bone. When examining the internal base of the skull, numerous openings for the passage of arteries, veins, and nerves are visible.

The anterior cranial fossa (fossa cranii anterior) is formed by the orbital parts of the frontal bones, as well as the cribriform plate of the ethmoid bone, through the openings of which the fibers of the olfactory nerves (I pair) pass. In the middle of the cribriform plate rises the cock's comb, in front of which is the blind opening.

The middle cranial fossa (fossa cranii media) is significantly deeper than the anterior one. It is formed by the body and greater wings of the sphenoid bone, the anterior surface of the pyramids and the squamous parts of the temporal bones. The central part of the fossa is occupied by the sella turcica. The pituitary fossa is distinguished in it, in front of which is the pre-cross groove (sulcus prehiasmatis), leading to the right and left optic canals, through which the optic nerves (II pair) pass. On the lateral surface of the body of the sphenoid bone, the carotid groove is visible, and near the apex of the pyramid there is an irregularly shaped lacerated foramen. Between the lesser wing, the greater wing and the body of the sphenoid bone is the superior orbital fissure (fissura orbitalis superior), through which the oculomotor (III pair), trochlear (IV pair), abducens (VI pair) nerves and the ophthalmic nerve (first branch of the V pair) pass into the orbit. Behind the superior orbital fissure is a round opening for the maxillary nerve (the second branch of the 5th pair), then an oval opening for the mandibular nerve (the third branch of the 5th pair). At the posterior edge of the greater wing is a spinous opening for the middle meningeal artery to enter the skull. On the anterior surface of the pyramid of the temporal bone are the trigeminal impression, the cleft of the canal of the greater petrosal nerve, the groove of the greater petrosal nerve, the cleft of the canal of the lesser petrosal nerve, the roof of the tympanic cavity and the arcuate eminence.

The posterior cranial fossa (fossa cranii posterior) is the deepest. It is formed by the occipital bone, the posterior surfaces of the pyramids and the inner surface of the mammillary processes of the right and left temporal bones. The fossa is completed by the body of the sphenoid bone (in front) and the posteroinferior angles of the parietal bones (from the sides). In the center of the fossa there is a large (occipital) opening, in front of it there is a slope (clivus), formed by the fused bodies of the sphenoid and occipital bones in adults, on which lie the pons (brain) and medulla oblongata. Behind the large (occipital) opening along the midline is the internal occipital crest. The internal auditory opening (right and left) opens into the posterior cranial fossa on each side, leading to the internal auditory canal. In the depth of this opening, the facial canal for the passage of the facial nerve (VII pair) begins. The vestibulocochlear nerve (VIII pair) emerges from the internal auditory opening.

In the depths of the posterior cranial fossa, two paired large formations are visible: the jugular foramen, through which the glossopharyngeal (IX pair), vagus (X) and accessory (XI pair) nerves pass, and the hypoglossal canal for the nerve of the same name (XII pair).

The internal jugular vein exits the cranial cavity through the jugular foramen, into which the sigmoid sinus passes, lying in the groove of the same name.

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