1.1 Skull, Lateral View
A Lateral view of the skull (cranium)
Left lateral view. This view was selected as an introduction to the skull because it displays the greatest number of cranial bones (indicated by different colors in B). The individual bones and their salient features as well as the cranial sutures and apertures are described in the units that follow. This unit reviews the principal structures of the lateral aspect of the skull. The chapter as a whole is intended to familiarize the reader with the names of the cranial bones before proceeding to finer anatomical details and the relationships of the bones to one another. The teeth are described in a separate unit (see p. 36ff).
В Lateral view of the cranial bones
Left lateral view. The bones are shown in different colors to demonstrate more clearly their extents and boundaries.
C Bones of the neurocranium (gray) and viscerocranium (orange)
Left lateral view. The skull forms a bony capsule that encloses the brain, sensory organs, and viscera of the head. The greater size of the neurocranium (cranial vault) relative to the viscerocranium (facial skeleton) is a typical primate feature directly correlated with the larger primate brain.
D Ossification of the cranial bones
Left lateral view. The bones of the skull either develop directly from mesenchymal connective tissue (intramembranous ossification, gray) or form indirectly by the ossification of a cartilaginous model (enchon- dral ossification, blue). Elements derived from intramembranous and endochondral ossification (desmocranium, chondrocranium) may fuse together to form a single bone (e.g., the occipital bone, temporal bone, and sphenoid bone).
The clavicle is the only tubular bone that undergoes membranous ossification. This explains why congenital defects of intramembranous ossification affect both the skull and clavicle (cleidocranial dysostosis).
E Bones of the neurocranium and viscerocranium
F Bones of the desmocranium and chondrocranium
1.2 Skull, Anterior View
A Anterior view of the skull
The boundaries of the facial skeleton (viscerocranium) can be clearly appreciated in this view (the individual bones are shown in B). The bony margins of the anterior nasal aperture mark the start of the respiratory tract in the skull. The nasal cavity, like the orbits, contains a sensory organ (the olfactory mucosa). The paranasal sinuses are shown schematically in C. The anterior view of the skull also displays the three clinically important openings through which sensory nerves pass to supply the face: the supraorbital foramen, infraorbital foramen, and mental foramen (see pp.77 and 93).
В Cranial bones, anterior view
C Paranasal sinuses: pneumatization lightens the bone
Anterior view. Some of the bones of the facial skeleton are pneumatized, i.e., they contain air-filled cavities that reduce the total weight of the bone. These cavities, called the paranasal sinuses, communicate with the nasal cavity and, like it, are lined by ciliated respiratory epithelium. Inflammations of the paranasal sinuses (sinusitis) and associated complaints are very common. Because some of the pain of sinusitis is projected to the skin overlying the sinuses, it is helpful to know the projections of the sinuses onto the surface of the skull.
D Principal lines offeree (blue) in the facial skeleton
a Anterior view, b lateral view. The pneumatized paranasal sinuses (C) have a mechanical counterpart in the thickened bony “pillars” of the facial skeleton, which partially bound the sinuses. These pillars develop along the principal lines of force in response to local mechanical stresses (e.g., masticatory pressures). In visual terms, the frame-like construction of the facial skeleton may be likened to that of a frame house: The paranasal sinuses represent the rooms while the pillars (placed along major lines of force) represent the supporting columns.
E LeFort classification of midfacial fractures
The frame-like construction of the facial skeleton leads to characteristic patterns of fracture lines in the midfacial region (LeFort I, II, and III).
LeFortI: This fracture line runs across the maxilla and above the hard palate. The maxilla is separated from the upper facial skeleton, disrupting the integrity of the maxillary sinus (low transverse fracture).
LeFortII: The fracture line passes across the nasal root, ethmoid bone, maxilla, and zygomatic bone, creating a pyramid fracture that disrupts the integrity of the orbit LeFort III: The facial skeleton is separated from the base of the skull. The main fracture line passes through the orbits, and the fracture may additionally involve the ethmoid bones, frontal sinuses, sphenoid sinuses, and zygomatic bones.
1.3 Skull, Posterior View and Cranial Sutures
A Posterior view of the skull
The occipital bone, which is dominant in this view, articulates with the parietal bones, to which it is connected by the lambdoid suture. The cranial sutures are a special type of syndesmosis (= ligamentous attach ments that ossify with age, see F). The outer surface of the occipital bone is contoured by muscular origins and insertions: the inferior, superior, and supreme nuchal lines.
В Posterior view of the cranial bones
Note: The temporal bone consists of two main parts based on its embryonic development: a squamous part and a petrous part (see p.22).
D Cranial deformities due to the premature closure of cranial sutures
The premature closure of a cranial suture (craniosynostosis) may lead to characteristic cranial deformities. The following sutures may close prematurely, resulting in various cranial shapes:
a Sagittal suture: scaphocephaly (long, narrow skull)
b Coronal suture: oxycephaly (pointed skull)
c Frontal suture: trigonocephaly (triangular skull)
d Asymmetrical suture closure, usually involving the coronal suture: plagiocephaly (asymmetrical skull)
C The neonatal skull
a Left lateral view, b superior view.
The flat cranial bones must grow as the brain expands, and so the sutures between them must remain open for some time (see F). In the neonate, there are areas between the still-growing cranial bones that are not occupied by bone: the fontanelles. They close at different times (the sphenoid fontanelle in about the 6th month of life, the mastoid fontanelle in the 18th month, the anterior fontanelle in the 36th month). The posterior fontanelle provides a reference point for describing the position of the fetal head during childbirth, and the anterior fontanelle provides a possible access site for drawing a cerebrospinal fluid sample in infants (e.g., in suspected meningitis).
E Hydrocephalus and microcephaly
a Characteristic cranial morphology in hydrocephalus. When the brain becomes dilated due to cerebrospinal fluid accumulation before the cranial sutures ossify (hydrocephalus, “water on the brain”), the neurocranium will expand while the facial skeleton remains unchanged.
b Microcephaly results from premature closure of the cranial sutures. It is characterized by a small neurocranium with relatively large orbits.
F Age at which the principal sutures ossify
|
Suture |
Age at ossification |
|
Frontal suture |
Childhood |
|
Sagittal suture |
20-30years of age |
|
Coronal suture |
30-40 years of age |
|
Lambdoid suture |
40-50 years of age |
1.4 Exterior and Interior of the Calvaria
A Exterior (a) and interior (b) of the calvaria
The external surface of the calvaria (a) is relatively smooth, unlike its internal surface (b). It is defined by the frontal, parietal, and occipital bones, which are interconnected by the coronal, sagittal, and lambdoid sutures. The smooth external surface is interrupted by the parietal foramen, which gives passage to the parietal emissary vein (see F). The internal surface of the calvaria also bears a number of pits and grooves:
• The granular foveolae (small pits in the inner surface of the skull caused by saccular protrusions of the arachnoid membrane covering the brain)
• The groove for the superior sagittal sinus (a dural venous sinus of the brain, see F and p. 65)
• The arterial grooves (which mark the positions of the arterial vessels of the dura mater, such as the middle meningeal artery which supplies most of the dura mater and overlying bone)
• The frontal crest (which gives attachment to the falx cerebri, a sickleshaped fold of dura mater between the cerebral hemispheres, see p. 188).
The frontal sinus in the frontal bone is also visible in the interior view.
В Exterior of the calvaria viewed from above
C The scalp and calvaria
Note the three-layered structure of the calvaria, consisting of the outer table, the diploë, and the inner table.
The diploë has a spongy structure and contains red (blood-forming) bone marrow. With a plasmacytoma (malignant transformation of certain white blood cells), many small nests of tumor cells may destroy the surrounding bony trabeculae, and radiographs will demonstrate multiple lucent areas (“punched-out lesions”) in the skull. Vessels called emissary veins may pass through the calvaria to connect the venous sinuses of the brain with the veins of the scalp (see panels E and F).
D Sensitivity of the inner table to trauma
The inner table of the calvaria is very sensitive to external trauma and may fracture even when the outer table remains intact (look for corresponding evidence on CT Images).
E Diploic veins in the calvaria
The diploic veins are located in the cancellous or spongy tissue of the cranial bones (the diploc) and are visible when the outer table is removed. The diploic veins communicate with the dural venous sinuses and scalp veins by way of the emissary veins, which create a potential route for the spread of infection.
F Emissary veins of the occiput
Emissary veins establish a direct connection between the dural venous sinuses and the extracranial veins. They pass through preformed cranial openings such as the parietal foramen and mastoid foramen. The emissary veins are of clinical interest because they may allow bacteria from the scalp to enter the skull along these veins and infect the dura mater, causing meningitis.
1.5 Base of the Skull, External View
A Bones of the base of the skull
Inferior view. The base of the skull is composed of a mosaic-like assembly of various bones. It is helpful to review the shape and location of these bones before studying further details.
В Relationship of the foramen lacerum to the carotid canal and internal carotid artery
Left lateral view. The foramen lacerum is not a true aperture, being occluded in life by a layer of fibrocartilage; it appears as an opening only in the dried skull. The foramen lacerum is closely related to the carotid canal and to the internal carotid artery that traverses the canal. The greater petrosal nerve and deep petrosal nerve pass through the foramen lacerum (see pp. 81,85, and 90).
C The basal aspect of the skull
Inferior view. The principal external features of the base of the skull are labeled. Note particularly the openings that transmit nerves and vessels. With abnormalities of bone growth, these openings may remain too small or may become narrowed, compressing the neurovascular structures that pass through them. If the optic canal fails to grow normally, it may compress and damage the optic nerve, resulting in visual field defects. The symptoms associated with these lesions depend on the affected opening. All of the structures depicted here will be considered in more detail in subsequent pages.
1.6 Base of the Skull, Internal View
A Bones of the base of the skull, internal view
Different colors are used here to highlight the arrangement of bones in the base of the skull as seen from within the cranium.
В The cranial fossae
a Interior view, b midsagittal section. The interior of the skull base is not flat but is deepened to form three successive fossae: the anterior, middle, and posterior cranial fossae. These depressions become progressively deeper in the frontal-to-occipital direction, forming a terraced arrangement that is displayed most clearly in b.
The cranial fossae are bounded by the following structures:
* Anterior to middle: the lesser wings of the sphenoid bone and the jugum sphenoidale.
* Middle to posterior: the superior border (ridge) of the petrous part of the temporal bone and the dorsum sellae.
C Base of the skull: principal lines of force and common fracture lines
a Principal lines of force, b common fracture lines (interior views). In response to masticatory pressures and other mechanical stresses, the bones of the skull base are thickened to form “pillars” along the principal lines of force (compare with the force distribution in the anterior view on p.5). The intervening areas that are not thickened are sites of predilection for bone fractures, resulting in the typical patterns of basal skull fracture lines shown here. An analogous phenomenon of typical fracture lines is found in the midfacial region (see the anterior views of LeFort fractures on p. 5).
D Interior of the base of the skull
It is interesting to compare the openings in the interior of the base of the skull with the openings visible in the external view (see p. 11). These openings do not always coincide because some neurovascular structures change direction when passing through the bone or pursue a relatively long intraosseous course. An example of this is the internal acoustic meatus, through which the facial nerve, among other structures, passes from the interior of the skull into the petrous part of the temporal bone. Most of its fibers then leave the petrous bone through the stylomastoid foramen, which is visible from the external aspect (see pp. 80,91, and 149 for further details).
In learning the sites where neurovascular structures pass through the base of the skull, it is helpful initially to note whether these sites are located in the anterior, middle, or posterior cranial fossa. The arrangement of the cranial fossae is shown in B. The cribriform plate of the ethmoid bone connects the nasal cavity with the anterior cranial fossa and is perforated by numerous foramina for the passage of the olfactory fibers (see p. 116).
Note: Because the bone is so thin in this area, a frontal head injury may easily fracture the cribriform plate and lacerate the dura mater, allowing cerebrospinal fluid to enter the nose. This poses a risk of meningitis, as bacteria from the nonsterile nasal cavity may enter the sterile cerebrospinal fluid.
1.7 Orbit: Bones and Openings for Neurovascular Structures
A Bones of the right orbit
Anterior view (a), lateral view (b), and medial view (c). The lateral orbital wall has been removed in b, and the medial orbital wall has been removed in c.
The orbit is formed by seven different bones (indicated here by color shading): the frontal bone, zygomatic bone, maxilla, ethmoid bone, sphenoid bone (see a and c), and also the lacrimal bone and palatine bone, which are visible only in the medial view (see b).
The present unit deals with the bony anatomy of the orbits themselves. The relationships of the orbits to each other are described in the next unit.
В Openings in the orbit for neurovascular structures
Noter The supra orbital fora men is an important site in routine clinical examinations because the examiner presses on the supraorbital rim with the thumb to test the sensory function of the supraorbital nerve. The supraorbital nerve is a terminal branch of the first division of the trigeminal nerve (CN V1t see p. 76). When pain is present in the distribution of the trigeminal nerve, tenderness to pressure may be noted at the supraorbital site.
|
Opening or passage |
Neurovascular structures |
|
|
Optic canal |
• Optic nerve (CN II) • Ophthalmic artery |
|
|
Superior orbital fissure |
• Oculomotor nerve (CN III) • Trochlear nerve (CN IV) • Ophthalmic nerve (CNVi) - Lacrimal nerve - Frontal nerve - Nasociliary nerve • Abducent nerve (CN VI) • Superior ophthalmic vein |
|
|
Inferior orbital fissure |
• Zygomatic nerve (CN V2) • Inferior ophthalmic vein • Infraorbital artery, vein, and nerve (CN V2) |
|
|
Nasolacrimal canal |
• Nasolacrimal duct |
|
|
Infraorbital canal |
• Infraorbital artery, vein, and nerve |
|
|
Supraorbital foramen |
• Supraorbital artery • Supraorbital nerve (lateral branch) |
|
|
Frontal incisure |
• Supratrochlear artery • Supraorbital nerve (medial branch) |
|
|
Anterior ethmoidal foramen |
• Anterior ethmoidal artery, vein, and nerve |
|
|
Posterior ethmoidal foramen |
• Posterior ethmoidal artery, vein, and nerve |
|
C Openings in the right orbit for neurovascular structures
Anterior view (a), lateral view (b), and medial view (c). The lateral orbital wall has been removed in b, the medial orbital wall in c. The following openings for the passage of neurovascular structures (see listing in B) can be identified: the superior and inferior orbital fissures (a-c), the optic canal (a, b), the anterior and posterior ethmoidal foramina (b, c), the infraorbital groove (a), the infraorbital canal (b, c), and the infraorbital foramen (a, b).
Diagram b shows the orifice of the nasolacrimal duct, by which lacrimal fluid is conveyed to the inferior meatus of the nose.
The lateral view (b) demonstrates the funnel-like structure of the orbit, which functions like a socket to contain the eyeball and constrain its movements. The inferior orbital fissure opens into the pterygopalatine fossa, which borders on the posterior wall of the maxillary sinus. It contains the pterygopalatine ganglion, an important component of the parasympathetic nervous system (see pp.81, 101). The upper part of the exposed maxillary sinus bears the ostium (in the maxillary hiatus) by which the sinus opens into the nasal cavity superior to the inferior concha (see pp. 20-21).
1.8 Orbit and Neighboring Structures
A Bones of the orbits and adjacent cavities
The color-coding here is the same as for the bones of the orbit on pp.14-15. These bones also form portions of the walls of neighboring cavities. The following adjacent structures are visible in the diagram:
• Anterior cranial fossa
• Frontal sinus
• Middle cranial fossa
• Ethmoid cells*
• Maxillary sinus
Disease processes may originate in the orbit and spread to these cavities, or originate in these cavities and spread to the orbit.
• The Terminologia Anatomica has dropped the term “ethmoid sinus” in favor of “ethmoid cells.”
В Clinically Important relationships between the orbits and surrounding structures
|
Relationship to the orbit |
Neighboring structure |
|
Inferior |
• Maxillary sinus |
|
Superior |
• Frontal sinus • Anterior cranial fossa |
|
(contains the frontal lobes of the brain) |
|
|
Medial |
• Ethmoid cells |
Deeper structures that have a clinically important relationship to the orbit:
• Sphenoid sinus
• Middle cranial fossa
• Optic chiasm
• Pituitary
• Cavernous sinus
• Pterygopalatine fossa
C Orbits and neighboring structures
Coronal section through both orbits, viewed from the front. The walls separating the orbit from the ethmoid cells (0.3 mm, lamina papyracea) and from the maxillary sinus (0.5 mm, orbital floor) are very thin. Thus, both of these walls are susceptible to fractures and provide routes for the spread of tumors and inflammatory processes into or out of the orbit. The superior orbital fissure communicates with the middle cranial fossa, and so several structures that are not pictured here—the sphenoid sinus, pituitary gland, and optic chiasm—are also closely related to the orbit.
D Close-up view of the left pterygopalatine fossa
Lateral view. The pterygopalatine fossa is a crossroads between the middle cranial fossa, orbit, and nose, being traversed by many nerves and vessels that supply these structures. The pterygopalatine fossa is continuous laterally with the infratemporal fossa. This diagram shows the lateral approach to the pterygopalatine fossa through the infratemporal fossa, which is utilized in surgical operations on tumors in this region (e.g., nasopharyngeal fibroma).
F Connections of the left pterygopalatine fossa with adjacent structures
Detail from D. The contents of the pterygopalatine fossa include the pterygopalatine ganglion (see pp. 81, 101), which is an important ganglion in the parasympathetic nervous system.
C Structures bordering the pterygopalatine fossa
|
Direction |
Bordering structure |
|
Anterior |
Maxillary tuberosity |
|
Posterior |
Pterygoid process |
|
Medial |
Perpendicular plate of the palatine bone |
|
Lateral |
Infratemporal fossa (via the pterygo- maxil la ry fissure) |
|
Superior |
Greater wing of the sphenoid bone, junction with the inferior orbital fissure |
|
Inferior |
Retropharyngeal space |
E Structures adjacent to the right pterygopalatine
Inferior view. The arrow indicates the approach to the pterygopalatine fossa from the skull base. The fossa itself (not visible in this view) is lateral to the lateral plate of the pterygoid process of the sphenoid bone.
H Pathways to the pterygopalatine fossa
|
Pathways |
From: |
Transmitted structures |
|
Foramen rotund um |
Middle cranial fossa |
• Maxillary nerve (CN V2) |
|
Pterygoid canal (Vidian canal) |
Skull base (inferior surface) |
• Greater petrosal nerve (parasympathetic branch of facial nerve) • Deep petrosal nerve (sympathetic fibers from carotid plexus) • Artery of pterygoid canal with accompanying veins • Nerve of pterygoid canal |
|
Greater palatine canal (foramen) |
Palate |
• Greater palatine nerve • Descending palatine artery • Greater palatine artery |
|
Lesser palatine canals |
Palate |
• Lesser palatine nerves • Lesser palatine arteries (terminal branches of descending palatine artery) |
|
Sphenopalatine foramen |
Nasal cavity |
• Sphenopalatine artery (plus accompanying veins) • Lateral and medial superior posterior nasal branches of the nasopalatine nerve (CNV2) |
|
Inferior orbital fissure |
Orbit |
• Infraorbital nerve • Zygomatic nerve • Orbital branches (of CN V2) • Infraorbital artery (plus accompanying veins) • Inferior ophthalmic vein |
1.9 Nose: Nasal Skeleton
A Skeleton of the external nose
Left lateral view. The skeleton of the nose is composed of bone, cartilage, and connective tissue. Its upper portion is bony and frequently involved in midfacial fractures, while its lower, distal portion is cartilaginous and therefore more elastic and less susceptible to injury. The proximal lower portion of the nostrils (alae) is composed of connective tissue with small embedded pieces of cartilage. The lateral nasal cartilage is a winglike lateral expansion of the cartilaginous nasal septum rather than a separate piece of cartilage.
C Bones of the lateral wall of the right nasal cavity
Left lateral view. The lateral wall of the right nasal cavity is formed by six bones: the maxilla, nasal bone, ethmoid bone, inferior nasal concha, palatine bone, and sphenoid bone. Of the nasal concha, only the inferior is a separate bone; the middle and superior conchae are parts of the ethmoid bone.
В Nasal cartilage
Inferior view. Viewed from below, each of the major alar cartilages is seen to consist of a medial and lateral crus. This view also displays the two nares, which open into the nasal cavities. The right and left nasal cavities are separated by the nasal septum, whose inferior cartilaginous portion is just visible in the diagram. The wall structure of a single nasal cavity will be described in this unit, and the relationship of the nasal cavity to the paranasal sinuses will be explored in the next unit.
D Bones of the nasal septum
Parasagittal section. The nasal septum is formed by the following bones: the nasal bone (roof of the septum), ethmoid bone, vomer, sphenoid bone, palatine bone, and maxilla. The latter three contribute only small bony projections to the nasal septum.
E Lateral wall of the right nasal cavity
Medial view. Air enters the bony nasal cavity through the anterior nasal aperture and travels through the three nasal passages: the superior me atus, middle meatus, and inferior meatus. Air leaves the nose through the choanae, entering the nasopharynx. The three nasal passages are separated into meatuses by the inferior, middle, and superior conchae.
F Nasal septum
Parasagittal section viewed from the left side. The left lateral wall of the nasal cavity has been removed with the adjacent bones. The nasal septum consists of an anterior cartilaginous part, the septal cartilage, and a posterior bony part (see D). The posterior process of the cartilaginous septum extends deep into the bony septum. Deviations of the nasal septum are common and may involve the cartilaginous part of the septum, the bony part, or both. Cases in which the septal deviation is sufficient to cause obstruction of nasal breathing can be surgically corrected.
1.10 Nose: Paranasal Sinuses
A Projection of the paranasal sinuses onto the skull
a Anterior view, b lateral view.
The paranasal sinuses are air-filled cavities that reduce the weight of the skull. Because they are subject to inflammation that may cause pain over the affected sinus (e.g., frontal headache due to frontal sinusitis), knowing the location of the sinuses is helpful in making the correct diagnosis.
В Pneumatization of the maxillary and frontal sinuses
Anterior view. The frontal and maxillary sinuses develop gradually during the course of cranial growth (pneumatization)—unlike the ethmoid sinuses, which are already pneumatized at birth. As a result, sinusitis in children is most likely to involve the ethmoid cells (with risk of orbital penetration: red, swollen eye; see D).
D Bony structure of the paranasal sinuses
Anterior view. The central structure of the paranasal sinuses is the ethmoid bone (red). Its cribriform plate forms a portion of the anterior skull base. The frontal and maxillary sinuses are grouped around the ethmoid bone. The inferior, middle and superior meatuses can be identified within the nasal cavity and are bounded by the coordinately-named conchae. The bony ostium of the maxillary sinus opens into the middle meatus, lateral to the middle concha. Below the middle concha and above the maxillary sinus ostium is the ethmoid bulla, which contains the middle ethmoid cells. At its anterior margin is a bony hook, the uncinate process, which bounds the maxillary sinus ostium anteriorly. The middle concha is a useful landmark in surgical procedures on the maxillary sinus and anterior ethmoid. The lateral wall separating the ethmoid bone from the orbit is the paper-thin orbital plate (= lamina papyracea). Inflammatory processes and tumors may penetrate this thin plate in either direction.
E Nasal cavity and paranasal sinuses
Transverse section viewed from above. The mucosal surface anatomy has been left intact to show how narrow the nasal passages are. Even relatively mild swelling of the mucosa may obstruct the nasal cavity, impeding aeration of the paranasal sinuses.
This diagram also shows that the pituitary gland, located behind the sphenoid sinus in the hypophyseal fossa (see C), is accessible to transnasal surgical procedures.
C Ostiomeatal unit on the left side of the nose
Coronal section. When the mucosa (ciliated respiratory epithelium) in the ethmoid cells (green) becomes swollen due to inflammation (sinusitis), it blocks the flow of secretions (see arrows) from the frontal sinus (yellow) and maxillary sinus (orange) in the ostiomeatal unit (red). Because of this blockage, microorganisms also become trapped in the other sinuses, where they may incite an inflammation. Thus, while the anatomical focus of the disease lies in the ethmoid cells, inflammatory symptoms are also manifested in the frontal and maxillary sinuses. In patients with chronic sinusitis, the narrow sites can be surgically widened to establish an effective drainage route, thereby curing the disease.
F Sites where the nasolacrimal duct and paranasal sinuses open into the nose
|
Nasal passage |
Structures that open into the meatus |
|
Inferior |
* Nasolacrimal duct |
|
meatus |
|
|
Middle |
• Frontal sinus |
|
meatus |
• Maxillary sinus |
|
• Anterior ethmoid cells |
|
|
• Middle ethmoid cells |
|
|
Superior |
* Posterior ethmoid cells |
|
meatus |
|
|
Spheno- |
• Sphenoid sinus |
|
ethmoid |
|
|
recess |
1.11 Temporal Bone
A Position of the temporal bone In the skull
Left lateral view. The temporal bone is a major component of the base of the skull. It forms the capsule for the auditory and vestibular apparatus and bears the articular fossa of the temporomandibularjoint.
В Ossification centers of the left temporal bone
a Left lateral view, b inferior view.
The temporal bone develops from three centers that fuse to form a single bone:
• The squamous part, or temporal squama (light green), bears the articular fossa of the temporomandibularjoint (mandibular fossa).
• The petrous part, or petrous bone (pale green), contains the auditory and vestibular apparatus.
• The tympanic part (darker green) forms large portions of the external auditory canal.
Note: The styloid process appears to belong to the tympanic part of the temporal bone because of its location. Developmentally, however, it is part of the petrous bone.
C Projection of clinically important structures onto the left temporal bone
The tympanic membrane is shown translucent in this lateral view. Because the petrous bone contains the middle and inner ear and the tympanic membrane, a knowledge of its anatomy is of key importance in otological surgery. The internal surface of the petrous bone has openings (see D) for the passage of the facial nerve, internal carotid artery, and internal jugular vein. A fine nerve, the chorda tympani, passes through the tympanic cavity, and lies medial to the tympanic membrane. The chorda tympani arises from the facial nerve, which is susceptible to injury during surgical procedures (see C, p. 79). The mastoid process of the petrous bone forms air-filled chambers, the mastoid cells, that vary greatly in size. Because these chambers communicate with the middle ear, which in turn communicates with the nasopharynx via the pharyngotympanic (auditory) tube (also called Eustachian tube) bacteria in the nasopharynx may pass up the pharyngotympanic tube and gain access to the middle ear. From there they may pass to the mastoid air cells and finally enter the cranial cavity, causing meningitis.
D Left temporal bone
a Lateral view. The principal structures of the temporal bone are labeled in the diagram. An emissary vein (see p. 9) passes through the mastoid foramen (external orifice shown in a, internal orifice in c), and the chorda tympani passes through the medial part of the petrotympanic fissure (see p. 147). The mastoid process develops gradually in life due to traction from the sternocleidomastoid muscle and is pneumatized from the inside (see C). b Inferior view. The shallow articular fossa of the temporomandibular joint (the mandibular fossa) is clearly seen from the inferior view. The facial nerve emerges from the base of the skull through the stylomastoid foramen. The initial part of the internal jugular vein is adherent to the jugular fossa, and the internal carotid artery passes through the carotid canal to enter the skull, c Medial view. This view displays the internal orifice of the mastoid foramen and the internal acoustic meatus. The facial nerve and vestibulocochlear nerve are among the structures that pass through the internal meatus to enter the petrous bone. The part of the petrous bone shown here is also called the petrous pyramid, whose apex (often called the “petrous apex”) lies on the interior of the base of the skull.
1.12 Sphenoid Bone
A Position of the sphenoid bone in the skull
The sphenoid bone is the most structurally complex bone in the human
body. It must be viewed from various aspects in order to appreciate all its features (see also B):
a Base of the skull, external aspect. The sphenoid bone combines with the occipital bone to form the load-bearing midline structure of the skull base.
b Base of the skull, internal aspect. The sphenoid bone forms the boundary between the anterior and middle cranial fossae. The openings for the passage of nerves and vessels are clearly displayed (see details in B).
c Lateral view. Portions of the greater wing of the sphenoid bone can be seen above the zygomatic arch, and portions of the pterygoid process can be seen below the zygomatic arch.
Note the bones that border on the sphenoid bone in each view.
В Isolated sphenoid bone
a Inferior view (its position in situ is shown in A). This view demonstrates the medial and lateral plates of the pterygoid process. Between them is the pterygoid fossa, which is occupied by the medial pterygoid muscle. The foramen spinosum and foramen rotundum provide pathways through the base of the skull (see also in c). b Anterior view. This view illustrates why the sphenoid bone was originally called the sphecoid bone (“wasp bone") before a transcription error turned it into the sphenoid (“wedge-shaped”) bone. The apertures of the sphenoid sinus on each side resemble the eyes of the wasp, and the pterygoid processes of the sphenoid bone form its dangling legs, between which are the pterygoid fossae. This view also displays the superior orbital fissure, which connects the middle cranial fossa with the orbit on each side. The two sphenoid sinuses are separated by an internal septum (see p.21).
c Superior view. The superior view displays the sella turcica, whose central depression, the hypophyseal fossa, contains the pituitary gland. The foramen spinosum, foramen ovale, and foramen rotun- dum can be identified posteriorly.
d Posterior view. The superior orbital fissure is seen particularly clearly in this view, while the optic canal is almost completely obscured by the anterior clinoid process. The foramen rotundum is open from the middle cranial fossa to the external base of the skull (the foramen spinosum is not visible in this view; compare with a). Because the sphenoid and occipital bones fuse together during puberty (“tribasilar bone”), a suture is no longer present between the two bones. The cancellous trabeculae are exposed and have a porous appearance.
A Integration of the occipital bone into the external base of the skull Inferior view. Note the relationship of the occipital bone to the adjacent bones.
The occipital bone fuses with the sphenoid bone during puberty to form the “tribasilar bone.”
В Isolated occipital bone
a Inferior view. This view shows the basilar part of the occipital bone, whose anterior portion is fused to the sphenoid bone. The condylar canal terminates posterior to the occipital condyles, while the hypoglossal canal passes superior to the occipital condyles. The condylar canal is a venous channel that begins in the sigmoid sinus and ends in the occipital vein (emissary vein, see p. 9). The hypoglossal canal contains a venous plexus in addition to the hypoglossal nerve (CN XII). The pharyngeal tubercle gives attachment to the pharyngeal muscles, while the external occipital protuberance provides a palpable bony landmark on the occiput.
b Left lateral view. The extent of the occipital squama, which lies above the foramen magnum, is clearly appreciated in this view. The internal openings of the condylar canal and hypoglossal canal are visible along with the jugular process, which forms part of the wall of the jugular foramen (see p. 11). This process is analogous to the transverse process of a vertebra.
c Internal surface. The grooves for the dural venous sinuses of the brain can be identified in this view. The cruciform eminence overlies the confluence of the superior sagittal sinus and transverse sinuses. The configuration of the eminence shows that in some cases the sagittal sinus drains predominantly into the left transverse sinus.
1.13 Occipital Bone and Ethmoid Bones
E Isolated ethmoid bone
a Superior view. This view demonstrates the crista galli, which gives attachment to the falx cerebri (see p. 188) and the horizontally directed cribriform plate. It is perforated by foramina through which the olfactory fibers pass from the nasal cavity into the anterior cranial fossa. With its numerous foramina, the cribriform plate is a mechanically weak structure that fractures easily in response to trauma. This type of fracture is manifested clinically by cerebrospinal fluid leakage from the nose (“runny nose” in a patient with head injury).
b Anterior view. The anterior view displays the midline structure that separates the two nasal cavities: the perpendicular plate (which resembles the pendulum of a grandfather clock). Note also the middle concha, which is part of the ethmoid bone (of the conchae, only the inferior concha is a separate bone), and the ethmoid cells, which are clustered on both sides of the middle conchae.
c Left lateral view. Viewing the bone from the leftside, we observe the perpendicular plate and the opened anterior ethmoid cells. The orbit is separated from the ethmoid cells by a thin sheet of bone called the orbital plate.
d Posterior view. This is the only view that displays the uncinate process, which is almost completely covered by the middle concha when in situ. It partially occludes the entrance to the maxillary sinus, the semilunar hiatus, and it is an important landmark during endoscopic surgery of the maxillary sinus. The narrow depression between the middle concha and uncinate process is called the ethmoid infundibulum. The frontal sinus, maxillary sinus, and anterior ethmoid cells open into this “funnel.” The superior concha is located at the posterior end of the ethmoid bone.
A Integration of the hard palate into the base of the skull.
Inferior view.
В Bones of the hard palate
a Superior view. The hard palate is a horizontal bony plate formed by parts of the maxilla and palatine bone. It serves as a partition between the oral and nasal cavities. In this view we are looking down at the floor of the nasal cavity, whose inferior surface forms the roof of the oral cavity. The upper portion of the maxilla has been removed. The palatine bone is bordered posteriorly by the sphenoid bone.
b Inferior view. The choanae, the posterior openings of the nasal cavity, begin at the posterior border of the hard palate.
c Oblique posterior view. This view demonstrates the close relationship between the oral and nasal cavities.
Note how the pyramidal process of the palatine bone is integrated into the lateral plate of the pterygoid process of the sphenoid bone.
1.14 Hard Palate
C Hard palate
a Superior view of the floor of the nasal cavity (= upper portion of hard palate) with the upper part of the maxilla removed. The hard palate separates the oral cavity from the nasal cavities. The small canal that links the oral and nasal cavities, the incisive canal (present here on both sides), merges within the bone to form one canal, which opens on the inferior surface by a single orifice, the incisive foramen (see b).
b Inferior view. The two horizontal processes of the maxilla, the palatine processes, grow together during development and become fused at the median palatine suture. Failure of this fusion results in a cleft palate. The boundary line between anterior clefts (cleft lip, alone or combined with a cleft alveolus) and posterior clefts (cleft palate) is the incisive foramen. These anomalies may also take the form of cleft lip and palate (with a defect involving the lip, alveolus, and palate). Note: the nasal cavity (whose floor is formed by the hard palate) communicates with the nasopharynx byway of the choanae.
c Oblique posterior view of the posterior part of the sphenoid bone at the level of the sphenoid body, displaying both sphenoid sinuses separated by a septum. The close topographical relationship between the nasal cavity and hard palate can be appreciated in this view. If the hard palate is unfused in a nursing infant due to a cleft anomaly (see b), some of the ingested milk will be diverted from the oral cavity and will enter the nose. This defect should be closed with a plate immediately after birth to permit satisfactory oral nutrition.
A Mandible
a Anterior view. The mandible is connected to the viscerocranium at the temporomandibular joint, whose convex surface is the head of the mandibular condyle. This “head of the mandible" is situated atop the vertical (ascending) ramus of the mandible, which joins with the body of the mandible at the mandibular angle. The teeth are set in the alveolar processes (alveolar part) along the upper border of the mandibular body. This part of the mandible is subject to typical age- related changes as a result of dental development (see B). The mental branch of the trigeminal nerve exits through the mental foramen to enter its bony canal. The location of this foramen is important in clinical examinations, as the tenderness of the nerve to pressure can be tested at that location (e.g., in trigeminal neuralgia, p.77).
b Posterior view. The mandibular foramen is particularly well displayed in this view. It transmits the inferior alveola г nerve, which supplies sensory innervation to the mandibular teeth. Its terminal branch emerges from the mental foramen. The two mandibular foramina are interconnected by the mandibular canal, c Oblique left lateral view. This view displays the coronoid process, the condylar process, and the mandibular notch between them. The coronoid process is a site for muscular attachments, while the condylar process bears the head of the mandible, which articulates with the mandibular fossa of the temporal bone. A depression on the medial side of the condylar process, the pterygoid fovea, gives attachment to portions of the lateral pterygoid muscle.
1.15 Mandible and Hyoid Bone
В Age-related changes in the mandible
The structure of the mandible is greatly influenced by the alveolar processes of the teeth. Because the angle of the mandible adapts tc changes in the alveolar process, the angle between the body and ramus also varies with age-related changes in the dentition. The angle measures approximately 150° at birth, and approximately 120—130° in adults, decreasing to 140° in the edentulous mandible of old age.
a At birth the mandible is without teeth and the alveolar part has not yet formed.
b In children the mandible bears the deciduous teeth. The alveolar part is still relatively poorly developed because the deciduous teeth are considerably smaller than the permanent teeth.
c In adults the mandible bears the permanent teeth, and the alveolar part of the bone is fully developed.
d Old age is characterized by an edentulous mandible with resorption of the alveolar process.
Note: the resorption of the alveolar process with advanced age leads to a change in the position of the mental foramen (which is normally located below the second premolar tooth, as in c). This change must be taken into account in surgery or dissections involving the mental nerve.
C Hyoid bone
a Anterior view, b posterior view, c oblique left lateral view. The hyoid bone is suspended by muscles between the oral floor and larynx in the neck, although it is listed among the cranial bones in the Terminologia Anatomica. The greater horn and body of the hyoid bone are palpable in the neck. The physiological movement of the hyoid bone during swallowing is also palpable.
1.16 Temporomandibular Joint
A Mandibular fossa of the temporomandibular j'oint
Inferior view. The head of the mandible articulates with the mandibular fossa in the temporomandibular joint The mandibular fossa is a depression in the squamous part of the temporal bone. The articular tubercle is located on the anterior side of the mandibular fossa. The head of the mandible (see B) is markedly smaller than the mandibular fossa, allowing it to have an adequate range of movement (see p. 35). Unlike other articular surfaces, the mandibular fossa is covered by fibrocartilage rather than hyaline cartilage. As a result, it is not as clearly delineated on the skull as other articular surfaces. The externa I auditory canal lies just behind the mandibular fossa. This proximity explains why trauma to the mandible may damage the auditory canal.
В Head of the mandible in the right temporomandibular joint
a Anterior view, b posterior view. The head of the mandible is not only markedly smallerthan the articular fossa but also has a cylindrical shape. This shape further increases the mobility of the mandibular head, as it allows rotational movements abouta vertical axis.
C Ligaments of the left temporomandibular joint
Lateral view. The temporomandibular joint is surrounded by a relatively lax capsule, which permits physiological dislocation during jaw opening. The joint is stabilized by three ligaments (see C and D). This lateral view demonstrates the strongest of these ligaments, the lateral ligament, which stretches over the capsule and is blended with it. The weaker stylomandibular ligament is also shown.
D Right temporomandibular joint and ligaments
Medial view. The sphenomandibular ligament can also be identified in this view.
E Opened left temporomandibular joint
Lateral view. The capsule extends posteriorly to the petrotympanic fissure (not shown here). Interposed between the mandibular head and fossa is the articular disk, which is attached to the joint capsule on all sides.
F Dislocation of the temporomandibular joint
The head of the mandible may slide past the articular tubercle when the mouth is opened, dislocating the temporomandibular joint. This may result from heavy yawning ora blow to the opened mandible. When the joint dislocates, the mandible becomes locked in a protruded position and can no longer be closed. This condition is easily diagnosed clinically and is reduced by pressing on the mandibular row of teeth.
C Sensory innervation of the temporomandibular joint capsule (after Schmidt)
Superior view. The temporomandibular joint capsule is supplied by articular branches arising from three branches of the mandibular division of the trigeminal nerve (CN V3):
• Auriculotemporal nerve
• Deep temporal nerve
• Masseteric nerve
1.17 Temporomandibular Joint, Biomechanics
A Movements of the mandible in the temporomandibular j'oint
Superior view. Most of the movements in the temporomandibular joint are complex motions that have three main components:
• Rotation (opening and closing the mouth)
• Translation (protrusion and retrusion of the mandible)
• Grinding movements during mastication
a Rotation. The axis for joint rotation runs transversely through both heads of the mandible. The two axes intersect at an angle of approximately 1505 (range of 110—180° between individuals). During this movement the temporomandibular joint acts as a hinge joint (ab- duction/depression and adduction/elevation of the mandible). In humans, pure rotation in the temporomandibular joint usually occurs only during sleep with the mouth slightly open (aperture angle up to approximately 15°, see Bb). When the mouth is opened past 15°, rotation is combined with translation (gliding) of the mandibular head.
b Translation. In this movement the mandible is advanced (protruded) and retracted (retruded). The axes for this movement are parallel to the median axes through the center of the mandibular heads, c Grinding movements in the left temporomandibular joint. In describing these lateral movements, a distinction is made between the “resting condyle” and the “swinging condyle.” The resting condyle on the left working side rotates about an almost vertical axis through the head of the mandible (also a rotational axis), while the swinging condyle on the right balance side swings forward and inward in a translational movement. The lateral excursion of the mandible is measured in degrees and is called the Bennett angle. During this movement the mandible moves in laterotrusion on the working side and in mediotrusion on the balance side, d Grinding movements in the right temporomandibular joint. Here, the right temporomandibular joint is the working side. The right resting condyle rotates about an almost vertical axis, while the left condyle on the balance side swings forward and inward.
В Movements of the temporomandibular joint
Left lateral view. Each drawing shows the left temporomandibular joint including the articular disk and capsule and the lateral pterygoid muscle, and each schematic diagram at right shows the corresponding axis of joint movement. The muscle, capsule, and disk form a functionally coordinated musculo-disco-capsular system and work closely together when the mouth is opened and closed.
a Mouth closed. When the mouth is in a closed position, the head of the mandible rests against the mandibular fossa of the temporal bone.
b Mouth opened to 15°. Up to 15° of abduction, the head of the mandible remains in the mandibular fossa, c Mouth opened past 15°. At this point the head of the mandible glides forward onto the articular tubercle. The joint axis that runs transversely through the mandibular head is shifted forward. The articular diskis pulled forward by the superior part of the lateral pterygoid muscle, and the head of the mandible is drawn forward by the inferior part of that muscle.
1.18 The Teeth in situ
В Permanent teeth of an adult
a Maxilla. Inferior view displaying the occlusal surfaces of the teeth.
b Mandible. Superior view.
Each tooth is given an identification code (see p. 38) to describe the specific location of dental lesions such as caries.
Each half of the maxilla and mandible contains the following set of anterior and posterior (postcanine) teeth:
• Anterior teeth: two incisors and one canine tooth.
• Posterior teeth: two premolars and three molars.
C Occlusal plane and dental arches
a, b Types of teeth and the occlusal plane. The maxilla and mandible present a symmetrical arrangement. With the mouth dosed (occlusal position), the maxillary teeth are apposed to their mandibular counterparts. They are offset relative to one another so that the cusps of one tooth fit into the fissures of the two opposing teeth (cusp-and- fissure dentition). Because of this arrangement, every tooth comes into contact with two opposing teeth. This offset results from the slightly greater width of the maxillary incisors (see p.39). The occlusal plane often forms a superiorly open arch (von Spee curve), c Dental arches. The teeth of the maxilla (green) and mandible (blue) are arranged in superior and inferior arches. The superior dental arch forms a semi-ellipse while the inferior arch is shaped like a parabola.
D Histology of a tooth
Illustrated here for a mandibular incisor. This diagram shows the hard tissues of the tooth (enamel, dentine, cementum) as well as the soft tissues (dental pulp).
E Supporting structures of the tooth: the periodontium
The tooth is anchored in the alveolus by a special type of syndesmosis called a gomphosis. The tissues that invest and support the tooth, the periodontium, consist of:
* The periodontal ligament
* The cementum
* The alveolar wall
* The gingiva.
The Sharpey fibers are collagenous fibers that pass obliquely downward from the alveolar bone and insert into the cementum of the tooth. This downward obliquity of the fibers transforms masticatory pressures on the dental arch into tensile stresses acting on the fibers and anchored bone (pressure would lead to bony atrophy).
F Connective tissue fibers in the gingiva
Many of the tough collagenous fiber bundles in the connective-tissue core of the gingiva above the alveolar bone are arranged in a screw-like pattern around the tooth, further strengthening its attachment.
1.19 Permanent Teeth and the Dental Panoramic Tomogram
A Coding the permanent teeth
In the United States, the permanent teeth are numbered sequentially rather than being assigned to quadrants. Progressing in a clockwise fashion (from the perspecive of the viewer), the teeth of the upper arc
are numbered 1 to 16, while those of the lower are considered 17 to 32. Note: The third upper molar (wisdom tooth) on the patient’s right is considered 1.
В Designation of tooth surfaces
Superior view of the mandibular dental arch. These designations are used in describing the precise location of small carious lesions. The term labial is used for incisors and canine teeth, and buccal is used for premolar and molar teeth. The term lingual is used for the mandibular teeth and palatal for the maxillary teeth.
C Dental panoramic tomogram
The dental panoramic tomogram (DPT) is a survey radiograph that allows a preliminary assessment of the temporomandibular joints, maxillary sinuses, maxillomandibular bone, and dental status (carious lesions, location of the wisdom teeth). It is based on the principle of conventional tomography in which the X-ray tube and film are moved about the plane of interest to blur out the shadows of structures outside the sectional plane. The plane of interest in the DPT is shaped like a parabola, conforming to the shape of the jaws. In the case shown here, all four wisdom teeth (third molars) should be extracted: teeth 1,16, and 17 are not fully erupted and tooth 32 is horizontally impacted (cannot erupt). If the DPT raises suspicion of caries or root disease, it should be followed with spot radiographs so that specific regions of interest can be evaluated at higher resolution.
(Tomogram courtesy of Prof. Dr. U. J. Rother, director of the Department of Diagnostic Radiology, Center for Dentistry and Oromaxillofacial Surgery, Eppendorf University Medical Center, Hamburg, Germany.)
Note: The upper incisors are broader than the lower incisors, leading to a “cusp-and-fissure” type of occlusion (see p.37).
1.20 Individual Teeth
A Incisors
a Central incisor (9); b lateral incisor (10); c lower incisors (23-26; 24 and 25 central; see p. 38 for coding). The incisor teeth have a sharp- edged crown that is consistent with their function of biting off bits of food. The palatal surface often bears a blind pit, the foramen cecum (not shown here), which is a site of predilection for dental caries.
В Canines (Cuspids)
a Upper canine (11); b lower canine (22); * = the tip of the crown, which represents the occlusal surface. The crown is thicker mesially than distally, and has greater curvature (arrow). In dogs, these teeth (also known as cuspids or eye teeth) are developed into fangs for gripping the prey between the jaws—hence the term “canine.”
C Premolars (Bicuspids)
a First premolar (1st bicuspid, 12); b second premolar (2nd bicuspid, 13); c first premolar (21); d second premolar (20). The premolars represent a transitional form between the incisors and molars. Like the molars, they have cusps and fissures indicating that their primary function is the grinding of food, rather than biting and tearing. The upper left first premolar (12, a) is the only premolar that has two roots. Its mesial surface which borders the neighboring proximal tooth often bears a small pit that is difficult to clean and vulnerable to caries. The other premolars have one root that is divided by a longitudinal groove and contains two root canals.
D Molars
a First molar (6-yr molar, 14); b second molar (12-yr molar, 15); c third molar (wisdom tooth, 16); d first molar (19); e second molar (18); f third molar (17). Most of the molars have three roots to withstand the greater masticatory pressures in the molar region. The roots of the third molars (the wisdom teeth, which erupt after 16 years of age, if at all) are commonly fused together, particularly in the upper third molars. Because the molars crush and grind food, they have a crown with a plateau. The fissures between the cusps are a frequent site of caries formation in adolescents.
Note: The term lingual is used for the mandibular teeth, the term palatal for the maxillary teeth.
1.21 Deciduous Teeth
A Deciduous teeth of the left side
The deciduous dentition (baby teeth) consists of only 20 teeth. Each of the four quadrants contains the following teeth:
a Central incisor (first incisor)
b Lateral incisor (second incisor)
c Canine (cuspid)
d First molar (6-yr molar)
e Second molar (12-yr molar)
To distinguish the deciduous teeth from the permanent teeth, they are coded with letters. The upper arch is labeled A to J, the lower is labeled К to T (see D).
В Eruption of the teeth
The eruptions of the deciduous and permanent teeth are called the first and second dentitions, respectively. The individual teeth are listed from left to right (viewer’s perspective) and the types of teeth are ordered according to the time of eruption.
C Eruption pattern of the deciduous and permanent teeth (after Meyer)
The eruption pattern is illustrated for the upper left teeth (deciduous teeth in black, permanent teeth in red). Knowing the times of eruption of the teeth is clinically important, as these data can provide a basis for diagnosing growth delays in children.
|
First dentition |
Type of tooth |
Individual tooth (see D) |
Time of eruption |
|
Central incisor |
E, F; P, 0 |
6-8 months |
|
|
Lateral incisor |
D, G; Q, N |
8-12 months |
|
|
First molar |
В, 1; S, L |
12-16 months |
|
|
Canines |
C, H; R, M |
15-20 months |
|
|
Second molar |
A,J;T,K |
20-40 months |
|
|
Second dentition |
Type of tooth |
Individual tooth (see p. 38) |
Time of eruption |
|
First molar |
3,14;30,19 |
6-8 years (“6-yr molar”) |
|
|
Central incisor |
8,9;25,24 |
6-9 years |
|
|
Lateral incisor |
7,10:26, 23 |
7-10 years |
|
|
First premolar |
5,12:28,21 |
9-13 years |
|
|
Canine |
6,11:27,22 |
9-14 years |
|
|
Second premolar |
4,13:29,20 |
11-14 years |
|
|
Second molar |
2,15:31,18 |
10-14 years (“12-yr molar”) |
|
|
Third molar |
1,16;32,17 |
16-30 years (“wisdom tooth”) |
D Coding the deciduous teeth
The upper right molar is considered A. The lettering then proceeds clockwise along the upper arc and back across the lower.
E Dentition of a 6-year-old child
a, b Anterior view; c, d left lateral view. The anterior bony plate over the roots of the deciduous teeth has been removed to display the underlying permanent tooth buds (pale blue). This age was selected because all of the deciduous teeth have erupted by this time and are all still present. The first permanent tooth, the “6-year molar,” also begins to erupt at this age (see C).
Anterior view of maxilla (a) and mandible (b); left lateral view of maxilla (c) and mandible (d).
