Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal System
Structure of the Skeleton
Skull

The skull is divided into the neurocranium (braincase), which houses the Brain, and the splanchnocranium (facial Skeleton), which contains the initial PARTS OF THE respiratory and digestive tracts.

Neurocranium. The neurocranium is formed by unpaired bones: the occipital, sphenoid, frontal, and ethmoid, and paired bones: the parietal and temporal (Atl. Figs. 18-20). Some bones (the sphenoid and ethmoid), located at the boundary between the neurocranium and the facial skeleton, also functionally participate in forming the latter.

The parietal bones (ossa parietalia) are nearly quadrilateral, forming the roof and sides of the skull. Their convex portions are called the parietal eminences.

The Frontal bone (os frontale) adjoins the anterior margin of the parietal bones. It consists of the squamous, orbital, and nasal parts (see Atl.). On its convex squamous part, two frontal eminences project anteriorly; below them lie the superciliary arches, which terminate laterally in the zygomatic processes, and further down are the two supraorbital foramina or notches. On the inferior concave surface of the orbital part, near the zygomatic process, lies the lacrimal fossa, and medially, the trochlear fossa, and sometimes a spine—the attachment site for the cartilaginous trochlea (pulley) through which one of the extraocular Muscles passes. The nasal part, which surrounds the ethmoidal notch, lies between the orbital parts. Within the frontal bone is the frontal sinus, which communicates with the Nasal cavity.

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Fig. 1.37. Skulls:

A — shark; B — trout; 1 — auditory capsule; 2 — Orbit; 3 — olfactory capsule; 4 — labial cartilages; 5 — palatoquadrate and 6 — mandibular cartilages of the I visceral arch; 7 — hyomandibular and 8 — hyoid cartilages of the II visceral arch; I—V — branchial arches; 9 — premaxillary, 10 — maxillary, 11 — quadrate; 12 — articular and 13 — dentary bones

The Occipital bone (os occipitale) participates in The formation of the base and vault of the neurocranium, closing it posteriorly and inferiorly (Fig. 1.40). The bone consists of a concave squamous part, paired lateral parts with jugular processes and condyles (articulating with the atlas), and a basilar part. These four parts surround the foramen magnum. The base of each condyle is pierced by the short hypoglossal canal. The jugular processes project laterally from the condyles. Across the external surface of the squamous part run the rough superior and inferior nuchal lines, and the external occipital protuberance projects. On the cerebral surface of the squamous part rises the internal occipital protuberance, from which the cruciform eminence diverges, featuring wide grooves for the venous sinuses.

The temporal bones (ossa temporalia) adjoin the occipital bone. They participate in forming the lateral wall and Base of the neurocranium, house the Organs of Hearing and Equilibrium, serve as attachment sites for the Muscles of Mastication and Neck Muscles, and articulate with the Mandible. Due to its diverse Functions, the Temporal bone has a complex Structure (Atl. Fig. 21). On its lateral surface is the external acoustic opening, surrounded by: the squamous part superiorly, the mastoid part posteriorly, the tympanic part anteriorly and inferiorly, and the petrous part (pyramid) medially. The squamous part is a slightly concave plate closing the neurocranium laterally. On it projects the anteriorly directed zygomatic process, which articulates with the Zygomatic bone. Beneath its base lie the mandibular fossa and articular tubercle, where articulation with the HEAD of the mandible occurs. The mastoid part is formed by the mastoid process (an attachment site for muscles), which is easily palpable through the Skin behind the auricle. Internally, the process consists of small air-filled cavities called mastoid Cells. Unlike other pneumatized bones, they communicate with the Middle ear cavity. The tympanic part is smaller than the other parts; it bounds the external acoustic meatus.

The petrous part (pyramid) encloses the tympanic cavity and the Inner ear cavity. On its posterior surface is the internal acoustic opening, and lateral to it is the slit-like opening of the vestibular aqueduct. On the anterior surface, the flat roof of the tympanic cavity is visible, and medial to it is the arcuate eminence. At the apex of the petrous part lies a small depression for the trigeminal ganglion. On the inferior surface, the styloid process projects, and the external opening of the carotid canal is located. This canal passes through the petrous part and then opens at its apex via the carotid foramen. Between the styloid and mastoid processes lies the stylomastoid foramen. In the angle between the squamous and petrous parts, the musculotubal canal opens, enclosing the auditory tube that leads to the middle ear cavity.

Fig. 1.40. Occipital bone, external view:

1 — lambdoid margin; 2 — squamous part; 3 — foramen magnum; 4 — condyle; 5 — hypoglossal canal; 6 — basilar part; 7 — superior and 8 — inferior nuchal lines; 9 — external occipital protuberance; 10 — external occipital crest; 11 — jugular process

Phylogeny of the skull. In the early Embryonic period, the Skull of the human embryo bears little resemblance to its adult form. This is the primary skull, which is represented mainly by Cartilage. Cartilaginous visceral arches adjoin it on the ventral side. Subsequently, ossification of all cartilaginous elements occurs, and the skull is supplemented by dermal bones.

In cartilaginous Fishes (such as sharks), the cartilaginous skull consists of a braincase, primary jaws, and branchial arches (Fig. 1.37). In more highly organized fishes, such as sturgeons, cartilage is replaced by Bone tissue, but only in the form of separate ossification centers; the bones are separated from each other by remnants of cartilage. This forms the "internal" skull, which is joined by an "external" skull of dermal origin. This fusion of the internal and external skull led to The Development of the complex skull of higher vertebrates; it is well-developed in bony fishes (such as trout). In these, the palatoquadrate cartilage has transformed into the palatine and quadrate bones, which are pushed backward by two dermal bones—the Maxilla and premaxilla (Fig. 1.37, B). The mandibular cartilage of these fishes transformed into the articular bone, which articulates with the quadrate (the latter, in turn, with the skull), forming the jaw joint. Anteriorly, the articular bone is covered by a Sheath of the dentary bone, which, together with the maxilla and premaxilla, forms the beginning of the facial skeleton.

In the larval stage, amphibians possess a skull similar in structure to that of their fish ancestors, while as adults they have a well-developed bony skull, though with remnants of cartilage. Some bones develop in place of cartilage, while others are dermal. The hyomandibular cartilage (the upper segment of the hyoid arch) transforms into the auditory ossicle—the stapes, while the hyoid cartilage and branchial arches support the Tongue and form the Hyoid bone, which surrounds the Pharynx.

In the skull of reptiles, minor remnants of cartilage persist between the bones, but the number of bones is still large. The facial skeleton is firmly fused with the neurocranium, the cavity of which increases in size due to the Development of the brain. A bony hard palate is formed, separating the nasal and oral cavities.

The skull of mammals and humans is entirely bony; remnants of cartilage in adults persist only in the nasal septum. The number of bones is significantly reduced compared to reptiles due to their fusion with one another (Fig. 1.38). The upper jaws are formed only by dermal bones, which already appeared in bony fishes.

The Sphenoid bone (os sphenoidale) lies at the base of the neurocranium and articulates with all of its bones (Atl. Fig. 22A), wedging itself between them, as it were. The bone has a complex structure because many large nerves pass through it; it participates in the Formation of the orbit, temporal, and infratemporal fossae, and serves as an attachment site for the muscles of mastication.

The bone consists of a body containing an air sinus that communicates anteriorly with the nasal cavity (Atl. Fig. 20B). A depression on the superior surface of the body is called the sella turcica, which houses an endocrine gland—the Pituitary Gland. The greater wings extend laterally from both sides of the body; at the base of each, the foramen rotundum, foramen ovale, and foramen spinosum are located in succession. The anterior surface of the greater wings forms the lateral wall of the orbit. Superior to the greater wings, the lesser wings project from the body of the bone, pierced at their base by the optic canal, which houses the cranial nerve of the same name. The lesser wings are separated from the greater wings by the superior orbital fissure and participate in forming the orbit. The pterygoid processes project downward from the body, consisting of two (medial and lateral) plates, with the pterygoid fossa located between them. The base of the processes is pierced by the pterygoid canal. The processes serve as attachment sites for muscles.

The Ethmoid bone (os ethmoidale) is surrounded by other bones such that on an intact skull, only its external part—the orbital plate, which participates in forming the medial wall of the orbit—is visible (Atl. Fig. 22B). Another part of the bone—the cribriform plate—fills the ethmoidal notch of the frontal bone and is visible from the cerebral surface of the skull. Rising superiorly from this plate is the longitudinal crista galli; its downward continuation into the nasal cavity is the perpendicular plate, which participates in forming the nasal septum (Atl. Figs. 18, 20B). The large paired portion of the bone—the ethmoidal labyrinths, consisting of bony air cells—hangs down into the nasal cavity.

Fig. 1.38. Derivatives of branchial arches in humans (diagram):

I — first visceral (mandibular) arch; II — second visceral (hyoid) arch; III—VII — branchial arches; 1 — malleus; 2 — incus; 3 — stapes; 4 — styloid process and its continuation — the stylohyoid ligament, transitioning ventrally into the lesser horn of the hyoid bone (5); 6 — thyroid cartilage

The premaxillary (incisive) bones develop independently but soon fuse with the maxillary bones. The mandible articulates directly with the temporal bone rather than through the quadrate bone, which is a defining feature of mammals.

The middle and superior nasal conchae project from the labyrinths toward the perpendicular plate.

Facial region. Unlike the neurocranium, the facial skeleton is dominated by paired bones, which include the maxillae, nasal, lacrimal, zygomatic, and palatine bones, as well as the inferior nasal conchae. There are only three unpaired bones: the Vomer, the mandible, and the hyoid bone (Atl. Figs. 18, 19, 20B).

The maxilla is a large paired bone that occupies a central position in the facial skeleton, consisting of a body and four processes (Atl. Fig. 23). Within the body lies the large, air-filled maxillary (maxillary antrum) sinus, which opens into the nasal cavity. The anterior (facial) surface of the body is concave and features the canine fossa, above which lies the infraorbital foramen of the canal of the same name that traverses the bone. The superior surface of the body forms the floor of the orbit, while the nasal surface forms the lateral wall of the nasal cavity. A small bone, the Inferior nasal concha, attaches to this wall. The posterior surface of the bone faces the infratemporal fossa. Of the four processes extending from the body, the frontal process articulates with the frontal bone, and the zygomatic process with the zygomatic bone. The palatine processes, together with the adjacent posterior palatine bones (ossa palatina), form the hard palate. The alveolar process contains eight sockets housing the upper Teeth.

The nasal bones (ossa nasalia) are located in the region of the bridge of the Nose and bound the upper part of the piriform aperture leading into the nasal cavity. Deep within the cavity, the vomer is visible—a sagittally oriented plate that articulates with the sphenoid, ethmoid, palatine, and maxillary bones.

The lacrimal bones (ossa lacrymalia) are the smallest BONES OF THE facial skeleton. Forming part of the medial wall of the orbit, they articulate with the frontal, ethmoid, and maxillary bones.

The zygomatic bones (ossa zygomatica) each have three processes—the frontal, temporal, and maxillary, named after the bones with which they articulate. The zygomatic bones form the inferolateral margins of the orbits and, together with the zygomatic processes of the temporal bones, form the zygomatic arches.

Fig. 1.39. Sagittal section and general view of the skull:

A — sagittal section of the skull: 1 — dog; 2 — chimpanzee; 3 — human. The bones of the neurocranium are blackened, and those of the facial skeleton are hatched. Arrows indicate THE POSITION OF the foramen magnum; B — ratio of the cranial to facial Regions of the skull in humans (1) and baboons (2)

The mandible (mandibula) is an unpaired bone consisting of a body and two rami (Fig. 1.41). Anteriorly, the mental protuberance projects from the body, flanked by the mental tubercles. On the midline of the inner surface of the body is the mental spine, from which two prominent lines extend laterally. The upper margin of the body contains 16 dental alveoli. The rami extending from the body form the mandibular angle, which features rough areas on both its inner and outer surfaces for the attachment of masticatory muscles. The rami terminate in two processes: the anterior coronoid process, which serves as an attachment site for the temporalis Muscle, and the posterior condylar process (consisting of a Head and Neck), which articulates with the temporal bone. On the inner surface of the ramus is the mandibular foramen, leading into the mandibular canal, which runs along the roots of the teeth and opens on the outer surface of the body as the mental foramen.

The hyoid bone (os hyoideum) is a small, curved bone suspended from the styloid process of the temporal bone by a long ligament (Fig. 1.42). It consists of a body, lesser horns, and greater horns. This bone is easily palpated in the neck just above the Larynx.

Articulations of the skull bones. Almost all bones of the skull are immovably joined by sutures. This is a type of syndesmosis, where the layer of Fibrous Connective Tissue linking adjacent bones is barely visible (see Atl. pp. 27, 28). In adults, and especially in the elderly, most sutures ossify, but a synchondrosis persists between the petrous part of the temporal bone and its neighbors. The only mobile bone of the skull (besides the hyoid) is the mandible.

Fig. 1.41. The mandible:

A — external view; B — internal view; 1 — body; 2 — ramus; 3 — mental protuberance; 4 — digastric fossa; 5 — angle; 6 — masseteric tuberosity; 7 — pterygoid tuberosity; 8 — mylohyoid line; 9 — coronoid process; 10 — condylar process; 11 — mandibular foramen; 12 — mental foramen

The quadrate bone is transformed into an auditory ossicle—the incus, while the remnant of the mandibular cartilage (the articular bone in fish) becomes the malleus. As early as in amphibians, the upper portion of the hyoid arch forms the stapes. Thus, all three auditory ossicles—the stapes, incus, and malleus—originate from the remnants of the visceral (mandibular and hyoid) arches. The middle portion of the hyoid arch becomes the styloid process of the temporal bone, which is connected by a ligament to the lower portion of this arch; the latter, together with the remnant of the first branchial arch, is transformed into the hyoid bone. The second and third branchial arches form the thyroid cartilage of the larynx, the fourth arch forms the arch of the cricoid cartilage of the larynx, and the fifth arch is completely lost.

The final stage of skull development involves the fusion of individual bones into single structures, such as the occipital and temporal bones.

In mammals, the neurocranium is typically much smaller than the facial skeleton. In humans, the neurocranium is progressively developed and strongly predominates over the facial skeleton (Fig. 1.39).

Fig. 1.42. The hyoid bone:

A — position relative to The Skull and Vertebral Column; B — superior view; 1 — body; 2 — lesser horn and 3 — greater horn

The temporomandibular joint (articulatio temporomandibularis) is a paired, combined joint formed by the ellipsoidal head of the mandible, and the mandibular fossa and articular tubercle of the temporal bone. A biconcave fibrous articular disc is situated between the articulating bones, dividing the joint cavity into upper and lower compartments. When the jaw protrudes, its heads, along with the discs, slide onto the articular tubercles. Consequently, the joint permits depression and elevation of the jaw (around the transverse axis), lateral deviation (around the vertical axis), protrusion, and retraction, which are essential for mastication and movements associated with articulate speech.

The skull is articulated with THE VERTEBRAL COLUMN via the atlanto-occipital joint.

Fig. 1.43. Skull development in the embryo:

A — cartilages of the skull base (9 weeks); B — LATERAL VIEW OF the head region of a 9-week embryo; C — 2.5-month embryo. Skull bones: 1 — frontal; 2 — parietal; 3 — occipital; 4 — sphenoid; 5 — temporal; 6 — maxilla; 7 — zygomatic; 8 — mandible; 9 — laryngeal cartilages

Ontogeny of the skull. In the human skull, some bones develop by replacing cartilage, while others develop as membrane bones. The formation of membrane bones begins as early as the middle of the second month of intrauterine life, whereas the replacement of cartilage by bone starts later — from the end of the 2nd or even from the 3rd month (Fig. 1.43). All ossification centers appear in a specific order, and initially, their number is large; for example, in the sphenoid bone, it reaches ten. The fusion of these centers occurs in utero and continues after birth.

Topography of the skull. The volume of the neurocranium averages 1450 cm³ in men; in women, it is smaller and averages 1300 cm³.

The neurocranium is divided into the calvaria and the base (Atl. Figs. 18-20). The calvaria is formed by the parietal bones, the squamous parts of the frontal, occipital, and temporal bones, and a portion of the greater wings of the sphenoid bone. The remaining parts of these bones and the ethmoid bone form the base. The bones of the calvaria are flat. On their concave internal surface, arterial grooves, inconstant small foramina through which Veins pass, impressions of venous sinuses, and a series of flattened depressions and elevations corresponding to the brain's relief are visible. The internal surface of the base also mirrors the relief of the Inferior surface of the brain. Here, the anterior, middle, and posterior cranial fossae are distinguished (Atl. Fig. 20A).

The anterior cranial fossa (fossa cranii anterior) is formed by the frontal bone and the cribriform plate of the ethmoid bone; its posterior boundary is formed by the margins of the lesser wings and the body of the sphenoid bone.

Fig. 1.44. SKULL OF A newborn:

A — lateral view; B — superior view; C — inferior view; 1 — anterior (frontal), 2 — posterior (occipital); 3 — posterolateral and 4 — anterolateral fontanelles; 5 — deciduous teeth (unerupted); 6 — sphenoid bone; 7 — basilar part; 8 — lateral parts and 9 — squamous part of the occipital bone

As the membrane bones of the fetal calvaria grow, they approach each other, but even in the newborn, membranous connective tissue areas called fontanelles remain between them (Fig. 1.44). They are particularly large where several membrane bones meet. Fontanelles are located at the corners of both parietal bones, resulting in the unpaired anterior (frontal) and posterior (occipital) fontanelles, and the paired anterolateral (sphenoidal) and posterolateral (mastoid) fontanelles (Fig. 1.44). Due to this connection, the bones of the calvaria can overlap at their margins. Therefore, the skull of a mature fetus can change its shape and adapt to the diameter of the lesser pelvis when passing through the birth canal. In infants, the posterior fontanelle closes during the second month of life; the largest one — the anterior fontanelle — is easily palpable and, in a normally developing child, closes completely only by one and a half years of age. In addition to fontanelles, cartilage is also preserved between some bones of the skull base in newborns.

In a newborn, the skull is much larger relative to the size of the entire body than in adults (Fig. 1.45). The head has the largest circumference of any part of the body, and therefore, even in a normal fetal presentation, it is the most difficult part to pass through the birth canal. The height of the head from the crown to the chin in a newborn fits into the body length four times. With age, this ratio changes because the growth of the skull (head) lags behind the growth of the entire body. In an adult, the height of the head is only 1/8 of the body length.

A newborn has no erupted teeth, so the jaws are poorly developed. Due to the rapid development of the brain and Sensory Organs, the neurocranium is relatively large and sharply predominates over the viscerocranium (5:1). Its capacity is 385–450 cm³. The bones are thin, smooth, and flexible in the vault, and the parietal eminences are prominent, but the attachment sites for muscles and ligaments are not pronounced, and the mastoid process of the temporal bone is barely noticeable (similar to anthropoids). The mandible consists of two halves (Fig. 1.46) that fuse during the second year of life. The frontal bone also consists of two parts (fusing completely only by six years of age), and the occipital and temporal bones consist of as many as four parts (Fig. 1.47).

Age-related Changes in the skull of children involve the growth and fusion of individual ossification centers. After the eruption of teeth, the facial skeleton begins to grow faster than the neurocranium, leading to noticeable changes in the Proportions of the skull parts. Although skull growth finishes by 23–25 years of age, its modification continues into old age. After 30 years, the sutures begin to fuse. In old age, the bones become thinner and lighter, and their spongy bone is resorbed. Due to tooth loss, the alveolar margins of the jaws are resorbed (Fig. 1.46, C), the height of the facial skeleton decreases significantly, and the neurocranium once again sharply predominates over it.

Skull shape. Individual variations in skull shape are quite significant. Its shape is determined by The ratio of the cranial breadth to its length (cranial index) (Fig. 1.48). If this index is below 75, the skull is dolichocranic; 75–79.9 is mesocranic; and above 80 is brachycranic. This index is higher in women than in men, and higher in children than in adults.

In the past, some peoples artificially deformed children's heads through tight binding, As a result of which the skull gradually acquired bizarre shapes — conical, tower-like, etc. (Fig. 1.49). The male skull is on average somewhat larger than the female skull, with more pronounced muscle attachment sites and supraorbital ridges. There are population-specific features in The structure of the maxilla, as well as facial and palatal width.

The middle cranial fossa (fossa cranii media) is separated from the posterior fossa by the superior border of the petrous part of the temporal bone and the dorsum sellae; it is formed by the greater wings of the sphenoid bone, the anterior surface of the petrous parts, and the squamous part of the temporal bone. The lower lateral parts of the fossa are pierced by A large number of foramina. Anterior to the sella turcica lie the optic canals, lateral to them are the superior orbital fissures (both leading into the orbit), and posterior to them are the foramina rotunda, followed by the foramina ovalia, and behind them, the small foramina spinosa. Between the posterior border of the sphenoid bone (lateral to its body) and the petrous part, the foramen lacerum is visible, in the area of which the internal opening of the carotid canal is located.

Fig. 1.45. Body proportions in children and adults: 1 — in a newborn; 2 — in a 2-year-old child; 3 — 4.5 years; 4 — 12 years; 5 — in an adult (shown at different scales)

The posterior cranial fossa (fossa cranii posterior) is almost entirely formed by the occipital bone; only small anterolateral parts of the fossa are formed by the posterior surface of the petrous parts and the mastoid part of the temporal bones. The center of the fossa is occupied by the foramen magnum, which connects the cranial cavity with the vertebral canal. Lateral to the foramen magnum runs the hypoglossal canal, and anteriorly lies the clivus, formed by the fusion of the bodies of the sphenoid and occipital bones. The Medulla Oblongata, a vital part of the brain, lies on the clivus. The jugular foramen is formed between the occipital bone and the petrous part of the temporal bone, and anterior to it, on the posterior surface of the petrous part, the internal acoustic meatus opens.

On the external surface of the skull, two orbits, the entrance to the nasal cavity, and the paired temporal and infratemporal fossae are distinguished.

The orbits (orbitae) are cavities bounded by four walls. Their superior wall is formed by the frontal bone and the lesser wings of the sphenoid, the medial wall by the lacrimal and ethmoid bones, the inferior wall by the maxilla and partly the zygomatic bone, and the lateral wall by the greater wings of the sphenoid bone, and partly the frontal and zygomatic bones. The inferior wall is separated from the lateral wall by the inferior orbital fissure, through which the orbit communicates with the infratemporal fossa and the pterygopalatine fossa. Deep within the orbit lie the superior orbital fissure and the optic canal, leading into the cranial cavity. At the junction of the frontal process of the maxilla with the Lacrimal bone, the nasolacrimal canal begins, leading into the nasal cavity.

Fig. 1.46. Mandible:

A — of a 1-year-old child; B — of an elderly person; C — age-related changes in the mandible: 1 — newborn; 2 — 3 years; 3 — 6 years; 4 — adult; 5 — elderly person

The nasal cavity (cavum nasi) opens anteriorly through the piriform aperture, and posteriorly through the two choanae. Deep within it, the bony nasal septum is clearly visible, consisting of the vomer and the perpendicular plate of the ethmoid bone (Atl. Figs. 18, 20B).

The cavity is bounded inferiorly by the maxillary and palatine bones; laterally, additionally by the lacrimal and ethmoid bones, and the pterygoid processes of the sphenoid bone; and superiorly by the nasal, frontal, and ethmoid bones, and the body of the sphenoid bone. Three nasal conchae project into the nasal cavity (the superior and middle are projections of the ethmoidal labyrinths, while the inferior is an independent bone), beneath which the superior, middle, and inferior nasal meatuses are formed. The sphenoidal sinus opens into the superior meatus, the frontal and maxillary sinuses open into the middle meatus, and the nasolacrimal duct opens into the inferior meatus. In addition, the ethmoidal air cells open into the middle and superior meatuses. At the posterior end of the superior concha, the nasal cavity communicates with the pterygopalatine fossa via the sphenopalatine foramen.

Fig. 1.47. Occipital (A) and frontal (B) bones of a newborn:

1 — squamous part (upper portion); 2 — squamous part (lower portion); 3 — lateral and 4 — basilar parts; 5 — frontal suture; 6 — frontal eminence

The temporal fossa (fossa temporalis) is formed by the lateral aspects of the skull and is bounded externally by the zygomatic arch, below which it transitions into the infratemporal fossa.

The infratemporal fossa is covered externally by the ramus of the mandible and communicates with the middle cranial fossa through the foramen ovale and foramen spinosum. Deep within the infratemporal fossa, the funnel-shaped pterygopalatine fossa opens. The foramen rotundum leads into the latter from the middle cranial fossa, the sphenopalatine foramen from the nasal cavity, and the inferior orbital fissure from the orbit. Inferiorly, the pterygopalatine fossa transitions into the narrow pterygopalatine canal, which opens onto the hard palate.

In areas where the skull experiences pressure during mastication, adaptations arise in the form of smoothly curved thickenings of compact bone projecting above the bone surface—buttresses. Two buttresses are particularly important: the frontonasal and the zygomatic. Both abut against the alveolar process of the maxilla; the former ascends from the canine tooth through its frontal process to the nasal part of the frontal bone, while the latter runs from the molars through the zygomatic bone to the zygomatic processes of the frontal and temporal bones. These buttresses ensure the transmission of pressure from the mandible to the maxilla and provide uniform support of the latter against the neurocranium.

Fig. 1.48. European boys with different head shapes

The sinuses located within the maxillary, temporal, frontal, and sphenoid bones increase the volume of the skull while significantly reducing its weight without any major compromise to its strength or other mechanical properties. In newborns, these sinuses are poorly developed. For example, the largest of them—the maxillary sinus—is merely a small invagination of the lateral wall of the nasal cavity in an infant. Developing gradually, the sinuses reach their permanent size only

Fig. 1.49. Deformed skulls:

1 — cone-shaped (from excavations near Kerch); 2 — tower-shaped (belonging to an adolescent from the graves of Ancient Peru)

upon the completion of skull growth. All these sinuses contain air, communicate with the nasal cavity (except for the mastoid air cells), and are called pneumatic cavities, while the bones containing them are termed pneumatized bones.

Review Questions

1. What are the Different types of bones?

2. Describe the Types of bone joints.

3. What divisions make up the human skeleton?

4. Describe the Axial Skeleton and the rib cage.

5. How are the bones of the axial skeleton joined together?

6. What are the Features of human skeletal development in ontogeny?

7. Describe the bones of the neurocranium and viscerocranium. How are they joined together?

8. Describe the topography of the skull.

9. Characterize the development of the skull in ontogeny.