Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal system
Skull
Bones of the neurocranium

The Skull, formed by paired and unpaired bones, protects the Brain and Sensory Organs from external impacts, Supports the initial segments of the digestive and respiratory systems, and houses the cavities for the sensory organs.

The skull is conventionally subdivided into the neurocranium and the viscerocranium (Figs. 21 and 22). The neurocranium encloses and protects the brain. Inextricably linked with it is the viscerocranium, which serves as the bony framework for the face and the initial segments of the digestive and respiratory systems.

In the adult human, the neurocranium consists of four unpaired bones—the frontal, occipital, sphenoid, and ethmoid—and two paired bones—the parietal and temporal.

The Frontal bone forms the anterior wall of the cranial vault, the walls of the anterior cranial fossa, and the superior wall of the orbits. It comprises the vertical frontal squama, the horizontal orbital parts, and the nasal part situated between them. The anterior surface of the frontal bone features the supraorbital margins, above which lie the superciliary arches, separated by a smooth area known as the glabella. Laterally, each supraorbital margin continues into the zygomatic process, which articulates with the Zygomatic bone. The internal surface of the frontal bone is concave, transitioning inferiorly into the orbital parts. A groove for the superior sagittal (venous) sinus runs along the internal surface of the frontal bone. Inferiorly, the frontal squama transitions into the orbital parts.

The orbital parts (right and left) face the Orbit with their inferior surfaces and the cranial cavity with their superior surfaces. These plates are separated from each other by the ethmoidal notch, which in the intact skull is occupied by the cribriform plate of the Ethmoid bone. Inferiorly, the nasal part features apertures leading into the frontal sinus—an air-filled cavity located within the frontal bone at the level of the glabella and superciliary arches. The inferior margin of the nasal part articulates with the nasal bones.

The Occipital bone forms the posterior wall and Base of the neurocranium. It consists of four parts surrounding the foramen magnum: the basilar part, located anterior to the foramen; two lateral parts; and the squama, which occupies the posterosuperior position.

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Fig. 21. Human skull. Anterior view:

1 — coronal suture; 2 — Parietal bone; 3 — orbital part of frontal bone; 4 — orbital surface of greater wing of Sphenoid bone; 5 — zygomatic bone; 6 — Inferior nasal concha; 7 — Maxilla; 8 — mental protuberance of Mandible; 9 — Nasal cavity; 10 — Vomer; 11 — perpendicular plate of ethmoid bone; 12 — orbital surface of maxilla; 13 — inferior orbital fissure; 14 — Lacrimal bone; 15 — orbital plate of ethmoid bone; 16 — superior orbital fissure; 17 — squamous part of Temporal bone; 18 — zygomatic process of frontal bone; 19 — optic canal; 20 — Nasal bone; 21 — frontal eminence

In the intact skull, the basilar part of the occipital bone is directed anteriorly and superiorly, where it fuses with the body of the sphenoid bone. As a result of this fusion, an inclined surface called the clivus is formed on its superior aspect, which supports the Medulla Oblongata and the Pons. The Inferior surface of the basilar part bears the pharyngeal tubercle, to which the posterior wall of the Pharynx is attached.

Fig. 22. Human skull. Lateral view:

1 — parietal bone; 2 — coronal suture; 3 — frontal eminence; 4 — temporal surface of greater wing of sphenoid bone; 5 — orbital plate of ethmoid bone; 6 — lacrimal bone; 7 — nasal bone; 8 — temporal fossa; 9 — anterior nasal spine; 10 — body of maxilla; 11 — mandible; 12 — zygomatic bone; 13 — zygomatic arch; 14 — styloid process; 15 — condylar process of mandible; 16 — mastoid process; 17 — external acoustic meatus; 18 — lambdoid suture; 19 — squamous part of occipital bone; 20 — superior temporal line; 21 — squamous part of temporal bone

The lateral PARTS OF THE occipital bone are located on either side of the foramen magnum. The inferior surface of these lateral parts bears the oval occipital condyles, which articulate with the atlas (the first cervical vertebra). Posterior to each condyle lies the condylar fossa, at the bottom of which opens the condylar canal containing a venous vessel. The hypoglossal canal for the Hypoglossal nerve passes through the lateral part on each side, superior to the condyle. The lateral margins of the bone exhibit jugular notches that, together with the corresponding notch of the temporal bone, form the jugular foramen, through which pass the Internal jugular vein, glossopharyngeal, vagus, and accessory nerves.

The squama of the occipital bone forms the posterior boundary of the foramen magnum and ascends steeply as a broad, convex-concave plate. Its external surface features the external occipital protuberance, to which the nuchal ligament attaches. Extending laterally from this prominence is the rough superior nuchal line, which serves as the attachment site for the trapezius Muscles. A low median external occipital crest runs downward from the external occipital protuberance toward the foramen magnum, flanked on either side by the inferior nuchal line. The internal surface of the squama displays four large fossae: the inferior fossae accommodate the posterior surfaces of the cerebellar hemispheres, while the superior fossae lodge the occipital lobes of the cerebral hemispheres. These fossae are separated by a cruciate eminence, at the center of which is the internal occipital protuberance. Inferiorly, the protuberance transitions into the internal occipital crest; superiorly, the groove for the superior sagittal (venous) sinus extends from it; and laterally, the transverse sinus groove (also venous) diverges to the right and left.

The sphenoid bone contributes to The formation of the base and lateral walls of the neurocranium. It consists of a central body and three pairs of processes: the greater and lesser wings, which project laterally, and the pterygoid processes, which project inferiorly. The superior surface of the sphenoid body features a depression known as the sella turcica. At the center of the sella lies the hypophyseal fossa, which houses the Pituitary Gland, one of the most important Endocrine glands. Inside the body of the sphenoid bone is an air-filled cavity, the sphenoid sinus, which communicates with the nasal cavity.

Two lesser wings project laterally from the anterosuperior surface of the sphenoid body, separating the anterior cranial fossa from the middle cranial fossa. At the base of each lesser wing is the optic canal, transmitting the Optic nerve. The greater wings, extending laterally from the body, form the walls of the middle cranial fossa, the orbit, as well as the temporal and infratemporal fossae. Between the lesser and greater wings, lateral to the body of the bone, lies the broad superior orbital fissure, leading from the cranial cavity into the orbit. The base of the greater wing contains the foramen rotundum, through which the maxillary nerve passes. Lateral and posterior to the foramen rotundum lies the foramen ovale (transmitting the mandibular nerve), and further laterally is the foramen spinosum for the middle meningeal artery. Extending inferiorly from the base of the greater wing on each side is the pterygoid process, at the base of which the pterygoid canal runs anteroposteriorly, transmitting the nerve of the same name. Each pterygoid process consists of two plates—medial and lateral—between which, posteriorly, lies the pterygoid fossa.

The ethmoid bone is situated anterior to the body of the sphenoid bone. It contributes to the Formation of the floor of the anterior cranial fossa, the orbit, and the nasal cavity. The ethmoid bone is composed of a sagittally oriented perpendicular plate, a cribriform plate, and ethmoidal labyrinths. The upward continuation of the perpendicular plate is the crista galli, which projects into the cranial cavity. The horizontally disposed cribriform plate forms the middle part of the floor of the anterior cranial fossa and the roof of the nasal cavity. The olfactory nerves pass through the foramina of the cribriform plate. Suspended from the lateral margins of the cribriform plate are the labyrinths of the ethmoid bone.

The ethmoid labyrinth is constructed from numerous air Cells that communicate with one another and open into the nasal cavity. The lateral wall of the labyrinth is formed by the orbital plate, which constitutes the medial wall of the orbit. Projecting from the Medial surface of the labyrinth are two thin, inferiorly curved plates—the superior and middle nasal conchae—which hang freely within the nasal cavity.

The temporal bone forms part of the lateral wall and base of the skull. It comprises the petrous part (pyramid), the tympanic part, and the squamous part. The temporal bone houses the organs of Hearing and Balance, which are enclosed within its pyramid. Several canals containing nerves and Blood Vessels traverse the pyramid (Table 6).

The petrous part has the shape of a three-sided pyramid, with its apex directed anteriorly and medially, and its base passing posteriorly into the mastoid process. The pyramid presents anterior and posterior surfaces facing the cranial cavity, as well as an inferior surface that contributes to the external cranial base. Near the apex on the anterior surface of the pyramid is the trigeminal impression, which lodges the trigeminal ganglion. Posterior to this impression lies the arcuate eminence, formed by the anterior semicircular canal of the bony labyrinth located within the pyramid. Laterally from the eminence are the flat roof of the tympanic cavity (tegmen tympani) and, anteriorly, two small openings for the canals of the greater and lesser petrosal nerves. The groove for the superior petrosal (venous) sinus runs along the superior border of the pyramid, which separates its anterior and posterior surfaces.

Table 6 Canals of the temporal bone

The posterior surface of the pyramid features the internal acoustic opening, which leads into the internal acoustic meatus, transmitting the facial and vestibulocochlear nerves as well as blood vessels. Also located on the posterior surface are the external aperture of the vestibular aqueduct, while the cochlear canaliculus opens on the inferior margin. The aqueduct and canaliculus lead into the labyrinth of the Vestibulocochlear Organ. Closer to the mastoid process runs the groove for the sigmoid (venous) sinus.

The inferior surface of the pyramid bears the jugular fossa. Lateral to it is the elongated styloid process, posterior to which lies the stylomastoid foramen—the exit of the facial canal. Anterior to the jugular fossa is the large opening leading into the carotid canal, which terminates at the apex of the pyramid. On the ridge between the jugular fossa and the opening of the carotid canal lies the petrosal fossa, which leads into the tympanic canaliculus terminating on the anterior surface of the pyramid. The air Cells of the mastoid process communicate with the tympanic cavity situated within the pyramid. At the apex of the pyramid, lateral to the carotid canal opening, the musculotubal canal opens into the tympanic cavity. Its upper semicanal transmits the tensor tympani muscle, while the lower semicanal forms the bony part of the auditory tube, connecting the tympanic cavity to the pharynx.

The tympanic part of the temporal bone is a curved plate that forms the inferior, anterior, and posterior BOUNDARIES OF THE external acoustic meatus.

The squamous part (squama) of the temporal bone forms part of the lateral wall of the cranium. Arising from the external surface of the squama is the zygomatic process, which projects anteriorly and joins the temporal process of the zygomatic bone to form the zygomatic arch. Inferior to the base of the zygomatic process lie the articular tubercle and the mandibular fossa, which accommodates the condyle of the mandible to form the temporomandibular joint.

The parietal bone, together with its counterpart on the opposite side, forms the roof (calvaria) of the cranium. It is a convex-concave quadrilateral plate marked on its external surface by the parietal eminence. By its borders, the parietal bone articulates (forming sutures) with the frontal, temporal, and occipital bones, the greater wing of the sphenoid bone, and the contralateral parietal bone. The parietal bone features four angles.