Face and scalp

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CHAPTER 29 Face and scalp

SKIN

The scalp and buccolabial tissues are described here. The structure of the eyelids is described in Chapter 39.

SCALP

The scalp extends from the top of the forehead in front to the superior nuchal line behind. Laterally it projects down to the zygomatic arch and external acoustic meatus. It consists of five layers: skin, subcutaneous tissue, occipitofrontalis (epicranius) and its aponeurosis, subaponeurotic loose areolar tissue and periosteum of the skull (pericranium).

The skin of the scalp is hairy and rich in sebaceous glands: it is the commonest site for sebaceous cysts. The dense subcutaneous connective tissue has the richest cutaneous blood supply in the body. The anterior and posterior muscular bellies of occipitofrontalis are connected by a tough, fibrous, epicranial aponeurosis, and this layer is therefore often called the aponeurotic layer (galea aponeurotica). These three upper layers of the scalp can easily slide on the underling layer of loose connective tissue. A scalp flap can be raised within the plane between the galea and the pericranium without compromising either the blood or nerve supply of the scalp, because all of these structures lie in the subcutaneous layer (superficial fascia). Anteriorly based subgaleal scalp flaps (bicoronal) provide excellent access to the craniofacial skeleton for the correction of congenital deformity such as craniosynostoses; treatment of craniofacial fractures involving the frontal bone, nasoethmoid complex, orbit or zygomatic arch; skull base surgery or craniotomies. Pericranial flaps can be used to separate the frontal sinus floor from the nasal cavity in the management of fractures of the posterior wall of the frontal sinus (frontal sinus cranialization). Traumatic scalp avulsion may occur if hair becomes trapped in moving machinery or a shearing force is applied in the subgaleal plane during a road traffic accident or fall injury.

The arterial blood supply to the scalp is particularly rich, and there are free anastomoses between branches of the occipital and superficial temporal vessels. Scalp lacerations continue to bleed profusely because the elastic fibres of the underlying galea aponeurotica prevent initial vessel retraction. Their repair requires a two-layer closure technique to approximate the galea aponeurotica and skin layers. The pericranial layer, if involved, cannot usually be closed because it retracts.

EYEBROWS

The eyebrows are two arched eminences of skin which surmount the orbits. Numerous short, thick hairs are set obliquely in them. Fibres of orbicularis oculi, corrugator and the frontal part of occipitofrontalis, are inserted into the dermis of the eyebrows.

BUCCOLABIAL TISSUE

Cheeks

The cheeks are continuous in front with the lips. The external junction is indicated by the nasolabial groove (sulcus) and further laterally by the nasolabial fold, which descends from the side of the nose to the angle of the mouth. The cheek is covered on the outer surface by skin and on the inner surface by mucosa. Each cheek contains the buccinator muscle, and a variable, but usually considerable, amount of adipose tissue which is often encapsulated to form a biconcave mass, the buccal fat pad (of Bichat), particularly evident in infants. The walls of the cheek also contain fibrous connective tissue, vessels, nerves and numerous small buccal mucous (salivary) glands. The buccal fat pad can be used to reconstruct small intraoral defects of the palate, buccal and retromolar regions following surgery.

Lips

The lips are two fleshy folds surrounding the oral orifice. The centre of each lip contains a thick fibrous strand, consisting of parallel bundles of skeletal muscle fibres (orbicularis oris, together with incisivus superior and inferior, and the direct labial tractors), and their attachments to skin, mucosa or other muscle fibres. The free external surface of each lip is covered by a thin keratinized epidermis, and is continuous with the mucosa at the vermilion (red) zone of the lip. The dermis is well vascularized and contains numerous hair follicles (many of them large in the male), sebaceous glands (Fordyce spots) and sweat glands. Subcutaneous adipose tissue is scanty. The internal mucous surfaces are lined with a thick non-keratinizing stratified squamous epithelium. The submucosa is well vascularized and contains numerous minor salivary glands, which may be harvested for histological confirmation of Sjögren’s connective tissue disease.

Between the skin and mucosa, the vermilion zone is covered with a specialized keratinized stratified squamous epithelium which is thin near the skin, increases in thickness slightly as the mucosa is approached, and then thickens abruptly when true mucosa is reached. The epithelium is covered with transparent, dead squames and its deep surface is highly convoluted, interdigitating with abundant long dermal papillae. The latter carry a rich capillary plexus which imparts a dusky red colour. These surfaces are hairless, their dermis lacks sebaceous, sweat or mucous glands, and they are moistened with saliva by the tip of the tongue. The dense innervation of the lips is reflected in their acute sensitivity to light touch sensation, attributable mainly to the increased density of Meissner’s corpuscles in the dermal papillae.

The size and curvature of the exposed red lip surfaces is subject to considerable individual, gender, and ethnic variation. The line of contact between the lips, the oral fissure, lies just above the incisal edges of the anterior maxillary teeth. On each side a labial commissure forms the angle (corner) of the mouth, usually near the first premolar tooth. The labial epithelia and internal tissues radiate over the boundaries of the commissure to become continuous with those of the cheek. With age, buccolabial (labiomarginal) grooves appear at the corners of the mouth. On each side, the upper lip is separated from the cheek laterally by the nasolabial groove and is continuous above the nasal ala with the circumalar groove (sulcus). The lower lip is separated from the chin by the mentolabial groove (sulcus).

Externally, the central region of the upper lip presents a shallow vertical groove, the philtrum, which is limited above by its attachment to the columella of the nose, and ends below in a slight tubercle limited by lateral ridges. The lower lip shows a small depression in the midline that corresponds to the tubercle. The junction between the external, hair-bearing skin and the red, hairless surface of the upper lip almost invariably takes the form of a double-curved Cupid’s bow. From the centre it rises rapidly on each side to an apex that corresponds to the lower end of each ridge of the philtrum, and then slopes gently downwards towards the angle of the mouth. The line of contact between the red lip surfaces is typically almost horizontal. The Cupid’s bow is interrupted in cleft lip anomalies.

In the upper lip, a narrow band of smooth tissue related to the subnasal maxillae marks the point at which labial mucosa becomes continuous with gingival mucosa. The corresponding reflexion in the lower lip coincides approximately with the mentolabial sulcus, and here the lip is continuous with mental tissues. The upper and lower lips differ in cross-sectional profile in that neither is a simple fold of uniform thickness. The upper lip has a bulbous asymmetrical profile: the skin and red-lip have a slight external convexity, and the adjoining red-lip and mucosa a pronounced internal convexity, creating a mucosal ridge or shelf that can be wrapped around the incisal edges of the parted teeth. The lower lip is on a more posterior plane than the upper lip. In the position of neutral lip contact, the external surface of the lower lip is concave, and there is little or no elevation of the internal mucosal surface. The profile of the lips can be modified by muscular activity.

RELAXED SKIN TENSION LINES AND SKIN FLAPS ON THE FACE

The direction in which facial skin tension is greatest varies regionally. Skin tension lines which follow the furrows formed when the skin is relaxed are known as ‘relaxed skin tension lines’ (Borges & Alexander 1962). In the living face, these lines frequently (but not always) coincide with wrinkle lines (Fig. 29.1) and can therefore act as a guide in planning elective incisions.

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Fig. 29.1 A, Distribution of relaxed skin tension lines (Kraissl’s lines) lateral view. B, Anterior view.

When lesions on the face such as scars, pigmented lesions and skin cancers are excised, the dimensions of these lesions often require excision as an ellipse, so that the resulting defect can be closed as a straight line. If the resulting scar is to be aesthetically acceptable it is important to make the long axis of the ellipse parallel to the natural relaxed skin tension lines, so that the scar will look like a natural skin crease. If the excision line runs contrary to the skin tension lines, the scar may be more conspicuous and will tend to stretch transversely as a result of natural expressive facial movements.

When larger lesions are excised it may be necessary to advance or rotate adjacent soft tissue to fill the defect. The ability to raise these skin flaps is entirely dependent on the regional blood supply and both random pattern and axial pattern skin flaps are used surgically. Because of the richness of the subdermal plexus in the face, random pattern flaps can be raised with a greater length : breadth ratio than in any other area of the body.

The following are examples of axial pattern flaps that can be used to reconstruct defects on the face and scalp. Supratrochlear/supraorbital arteries support forehead flaps that are useful for nasal reconstruction: there is usually enough skin laxity to allow the majority of the donor site to be closed directly. The frontal branch of the superficial temporal artery anastomoses in the midline with its opposite number, and consequently the entire forehead skin can be raised on a narrow pedicle based on just one of the superficial temporal arteries. These flaps can be used to repair many facial defects and also intraoral defects, but the donor site defect cannot be closed directly and must be covered by a skin graft. The parietal branch of the superficial temporal artery and the occipital artery can support hair-bearing flaps from the scalp which are useful for reconstructing defects involving the scalp. The nasolabial flap utilizes the lax skin just lateral to the nasolabial groove. It is not supplied by a named axial artery but rather its blood supply is provided by many small branches from the underlying facial artery. These branches run perpendicular to the skin surface. Nasolabial flaps can be either superiorly or inferiorly based.

SOFT TISSUE

FASCIAL LAYERS

Fascia of scalp

The superficial fascia of the scalp is firm, dense and fibroadipose, and adheres closely to both skin and the underlying epicranius and its aponeurosis. Posteriorly it is continuous with the superficial fascia of the back of the neck, and laterally it is prolonged into the temporal region, where it is looser in texture.

Fascial layers and tissue planes in the face

On the basis of gross dissection and complementary histological studies, four distinct tissue planes are recognized on the face superficial to the plane of the facial nerve and its branches. From superficial to deep, these layers are the skin; a subcutaneous layer of fibro-adipose tissue; the superficial musculo-aponeurotic system (SMAS); and the parotid–masseteric fascia.

Subcutaneous fibroadipose tissue

This homogeneous layer is present throughout the face, although the degree of adiposity varies in different parts of the face and with age. Anteriorly, it crosses the nasolabial fold onto the lip, and superiorly it crosses the zygomatic arch. In both locations the layer is more fascial than fatty. The fat content of the subcutaneous tissue in the cheek accounts for the cheek mass: part of the subcutaneous adipose tissue is the malar fat pad, a more or less discrete aggregation of fatty tissue inferolateral to the orbital margin.

Superficial musculo-aponeurotic system (SMAS)

This is described as a single tissue plane in the face. In some areas it is composed of muscle fibres, and elsewhere it is composed of fibrous or fibroaponeurotic tissue: it is not directly attached to bone. When traced below the level of the lower border of the mandible it becomes continuous with platysma in the neck. Microdissection has revealed that the SMAS becomes indistinct on the lateral aspect of the face approximately 1 cm below the level of the zygomatic arch. Anteromedially, the SMAS layer becomes continuous with some of the mimetic muscles including zygomaticus major, frontalis and the peri-orbital fibres of orbicularis oculi.

In most areas of the face, a distinct sub-SMAS plane can be defined deep to SMAS. It is continuous with the plane between platysma and the underlying investing layer of deep cervical fascia in the neck. However, where it overlies the parotid gland, the SMAS is firmly blended with the superficial layer of the parotid fascia, which means that a clear sub-SMAS plane is difficult, if not impossible, to define in the region of the parotid.

Parotid–masseteric fascia

This is a filmy areolar layer that overlies the filamentous branches of the facial nerve and the parotid duct as these structures lie on the surface of masseter. Further anteriorly the parotid–masseteric fascia overlies the buccal fat pad which lies superficial to buccinator. Having crossed the surface of the buccal fat pad, the fascia blends with the epimysium on the surface of buccinator. Below the lower border of the mandible, it is continuous with the investing layer of deep cervical fascia.

Parotid fascia (capsule)

The parotid gland is surrounded by a fibrous capsule called the parotid fascia or capsule. Traditionally this has been described as an upward continuation of the investing layer of deep cervical fascia in the neck which splits to enclose the gland within a superficial and a deep layer. The superficial layer is attached above to the zygomatic process of the temporal bone, the cartilaginous part of the external acoustic meatus, and the mastoid process. The deep layer is attached to the mandible, and to the tympanic plate, styloid and mastoid processes of the temporal bone. The prevailing view is that the deep layer of the parotid gland is derived from the deep cervical fascia. However, the superficial layer of the parotid capsule appears to be continuous with the fascia associated with platysma, and is now regarded as a component of the SMAS (Mitz & Peyronie 1976; Wassef 1987; Gosain et al 1993). It varies in thickness from a thick fibrous layer anteriorly to a thin translucent membrane posteriorly. It may be traced forwards as a separate layer which passes over the masseteric fascia (itself derived from the deep cervical fascia), separated from it by a cellular layer which contains branches of the facial nerve and the parotid duct. Histologically, the parotid fascia is atypical in that it contains muscle fibres which parallel those of platysma, especially in the lower part of the parotid capsule. Although thin fibrous septa may be seen in the subcutaneous layer at the histological level, macroscopically there is little evidence of a distinct layer of superficial fascia.

The deep fascia covering the muscles forming the parotid bed (digastric and styloid group of muscles) contains the stylomandibular and mandibulostylohyoid ligaments. The stylomandibular ligament passes from the styloid process to the angle of the mandible. The more extensive mandibulostylohyoid ligament (angular tract) passes between the angle of the mandible and the stylohyoid ligament for varying distances, generally reaching the hyoid bone. It is thick posteriorly but thins anteriorly in the region of the angle of the mandible. There is some dispute as to whether the mandibulostylohyoid ligament is part of the deep cervical fascia (Ziarah & Atkinson 1981), or lies deep to it (Shimada & Gasser 1988). The stylomandibular and mandibulostylohyoid ligaments separate the parotid gland region from the superficial part of the submandibular gland, and so are landmarks of surgical interest.

Temporo-parietal and temporal fasciae

Above the level of the zygomatic arch, on the lateral side of the head, the temporo-parietal fascia (superficial temporal fascia) constitutes a fascial layer which lies in the same plane as, but is not continuous with, the SMAS. It is quite separate from, and superficial to, the temporal fascia (deep temporal fascia). More superiorly, it blends with the galea aponeurotica. The plane between the temporo-parietal fascia and the underlying deep temporal fascia contains loose areolar tissue and a small amount of fat. This tissue plane, the temporo-parietal fat pad, is continuous superiorly with the subgaleal plane of loose areolar tissue in the scalp. Running superiorly in the temporo-parietal fascia or just deep to it are the superficial temporal vessels, the auriculotemporal nerve and its branches, and the temporal branches of the facial nerve. When raising a bicoronal flap, identification of the temporoparietal fat pad helps to separate these two fascial layers; subsequent dissection in a plane deep to the temporoparietal fascia protects the temporal branch of the facial nerve. The temporal fascia is a dense aponeurotic layer which lies deep to the temporo-parietal fat pad and covers temporalis: the deep surface of the fascia affords attachment to the superficial fibres of temporalis. Above, it is a single layer attached along the length of the superior temporal line, blending with the periosteum. Below, at approximately the level of the superior orbital rim, it splits into superficial and deep laminae which run downwards to attach to the lateral and medial margins of the upper surface of the zygomatic arch respectively. These fascial attachments have a clinical application in the reduction of fractures of the zygomatic complex via a Gillies approach: an instrument is inserted deep to the deep lamina of temporalis fascia through a scalp incision and used to elevate depressed zygomatic complex fractures. The fat enclosed between these two layers is termed the superficial temporal fat pad; it contains the zygomatico-orbital branch of the superficial temporal artery and a cutaneous nerve, the zygomatico-temporal branch of the maxillary nerve. The temporal fascia is overlapped by auriculares anterior and superior, the epicranial aponeurosis and part of orbicularis oculi, and the superficial temporal vessels and auriculotemporal nerve ascend over it.

Buccopharyngeal fascia

Buccinator is covered by a thin layer of fascia, the buccopharyngeal fascia, which also covers the superior constrictor of the pharynx.

Retaining ligaments of the face

These ligaments are fascial bands at specific sites which serve to anchor the skin to the underlying bone. The general cutaneous laxity that attends the ageing process renders facial skin subject to gravitational pull. However, at sites where retaining ligaments are present, the effect of gravitational pull is resisted. When performing facelift procedures, these ligaments must be surgically divided in order to facilitate redraping of facial skin. Examples of retaining ligaments in the face are the zygomatic ligament (also known as McGregor’s patch) and the mandibular ligament.

Fascial spaces

Two tissue spaces on the face may be involved in spread of odontogenic infection. They are the buccal tissue space, lying between the skin and surface of buccinator, and the infraorbital tissue space, lying between the bony attachments of levator labii superioris and levator anguli oris.

BONES OF THE FACIAL SKELETON AND CRANIAL VAULT

The skull consists of the facial skeleton and cranial vault (calvarium) attached at the skull base. The cranial vault encloses and protects the brain. The facial skeleton is the anterior part of the skull and includes the mandible. The bones of the nasoethmoidal and zygomaticomaxillary complexes are described here. The mandible is described in Chapter 30.

PARIETAL BONE

The two parietal bones form most of the cranial roof and sides of the skull. Each is irregularly quadrilateral and has two surfaces, four borders and four angles (Fig. 29.2).

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Fig. 29.2 Left parietal bone. A, External view; B, Internal view.

The external surface is convex and smooth, with a central parietal tuber (tuberosity). Curved superior and inferior temporal lines cross it and form posterosuperior arches. The temporal fascia is attached to the superior line or arch and temporalis is attached to the inferior line or arch. The epicranial aponeurosis lies above these lines, and part of the temporal fossa lies below. Posteriorly, close to the sagittal (superior) border, an inconstant parietal foramen transmits a vein from the superior sagittal sinus and sometimes a branch of the occipital artery.

The internal surface is concave and marked by impressions of cerebral gyri and by grooves for the middle meningeal vessels. The latter ascend, inclining backwards, from the sphenoidal (anteroinferior) angle and posterior half (or more) of its inferior border. A groove for the superior sagittal sinus lies along the sagittal border, and is completed by the groove on the opposite parietal bone. The falx cerebri is attached to the edges of the groove. Granular foveolae for arachnoid granulations flank the sagittal sulcus, and are most pronounced in old age.

The dentated sagittal border, longest and thickest, articulates with the opposite parietal bone at the sagittal suture. The anterior part of the squamosal (inferior) border is short, thin and truncated, bevelled externally and overlapped by the greater wing of the sphenoid. The middle part of the inferior border is arched, bevelled externally and overlapped by the squamous part of the temporal bone. The posterior part of the inferior border is short, thick and serrated for articulation with the mastoid part.

The frontal border is deeply serrated, bevelled externally above, internally below, and articulates with the frontal bone to form one half of the coronal suture. The occipital border, deeply dentated, articulates with the occipital bone, forming one half of the lambdoid suture.

The frontal (anterosuperior) angle, which is approximately 90°, is at the bregma, where sagittal and coronal sutures meet, and marks the site of the anterior fontanelle in the neonatal skull. The sphenoidal (anteroinferior) angle lies between the frontal bone and greater wing of the sphenoid. Its internal surface is marked by a deep groove or canal that carries the frontal branches of the middle meningeal vessels. The frontal, parietal, sphenoid and temporal bones usually meet at the pterion, which marks the site of the sphenoidal fontanelle in the embryonic skull. The frontal bone sometimes meets the squamous part of the temporal bone, in which case the parietal bone fails to reach the greater wing of the sphenoid bone. The rounded occipital (posterosuperior) angle is at the lambda, the meeting of the sagittal and lambdoid sutures, which marks the site of the posterior fontanelle in the neonatal skull. The blunt mastoid (posteroinferior) angle articulates with the occipital bone and the mastoid portion of the temporal bones at the asterion. Internally it bears a broad, shallow groove for the junction of the transverse and sigmoid sinuses.

Ossification

Each parietal bone is ossified from two centres which appear in dense mesenchyme near the tuberosity, one above the other, at about the seventh week in utero. These two centres unite early and ossification subsequently radiates from them towards the margins. The angles are therefore the last parts to be ossified, and fontanelles occur at these sites. At birth the temporal lines are low down; they only reach their final position after the eruption of the molar teeth. Occasionally the parietal bone is divided by an anteroposterior suture.

FRONTAL BONE

The frontal bone is like half a shallow, irregular cap forming the forehead or frons (Fig. 29.3). It has three parts, and contains two cavities, the frontal sinuses.

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Fig. 29.3 Frontal bone. A, anterior view; B, inferior view, including the articulations between the frontal, ethmoid and nasal bones.

(From Sobotta 2006.)

Squamous part

The squamous part forms the major portion of the frontal bone. Its external surface has a rounded frontal tuber (tuberosity) approximately 3 cm above the midpoint of each supraorbital margin. These tubera vary, but are especially prominent in young skulls and more so in adult females than males. Below them and separated by a shallow groove, are two curved superciliary arches, medially prominent and joined by a smooth median elevated glabella. The arches are more prominent in males; prominence depends partly on the size of the frontal sinuses, but is occasionally associated with small sinuses. The curved supraorbital margins of the orbital openings lie inferior to the superciliary arches. The lateral two-thirds of each margin are sharp, the medial third rounded; a supraorbital notch or foramen, which transmits the supraorbital vessels and nerve, lies at the junction between them. A small frontal notch or foramen lies medial to the supraorbital notch in 50% of skulls. Both features show sexual dimorphism. The supraorbital margin ends laterally in a strong, prominent zygomatic process that articulates with the zygomatic bone. A line curves posterosuperiorly from the process and divides into superior and inferior temporal lines, which are continued on the squamous part of the temporal bone. The area of the frontal bone below and behind the temporal lines is known as the temporal surface and forms the anterior part of the temporal fossa. The parietal (posterior) margin is thick, deeply serrated, and bevelled internally above and externally below. Inferiorly it becomes a rough, triangular surface that articulates with the greater wing of the sphenoid.

The internal surface of the frontal bone is concave. Its upper, median, part displays a vertical sulcus whose edges unite below as the frontal crest. The sulcus contains the anterior part of the superior sagittal sinus. The crest ends in a small notch which is completed by the ethmoid bone to form a foramen caecum. The anterior portion of the falx cerebri is attached to the margins of the sulcus and to the frontal crest. The internal surface shows impressions of cerebral gyri, small furrows for meningeal vessels, and granular foveolae for arachnoid granulations near the sagittal sulcus.

Nasal part

The nasal part of the frontal bone lies between the supraorbital margins. A serrated nasal notch articulates with the nasal bones inferiorly and with the frontal processes of the maxillae and the lacrimal bones laterally. From the centre of the notch posteriorly the bone projects anteroinferiorly behind the nasal bones and the frontal processes of the maxillae, and supports the nasal bridge. The region ends in a sharp nasal spine, on each side of which a small grooved surface partly roofs the ipsilateral nasal cavity. The nasal spine makes a very small contribution to the nasal septum: it articulates anteriorly with the crest of the nasal bones and posteriorly with the perpendicular plate of the ethmoid bone.

Orbital parts

Most of the frontal bone is thick, and consists of trabecular tissue lying between two compact laminae. In contrast, the orbital plates consist entirely of compact bone and are thin and often translucent posteriorly, indeed they may be partly absorbed in old age.

The orbital plates form the largest part of the orbital roofs and are two thin, curved, triangular laminae separated by a wide ethmoidal notch. The orbital surface of each plate is smooth and concave, and bears a shallow anterolateral fossa for the lacrimal gland. The trochlear fovea (or spine) for attachment of a fibrocartilaginous trochlea through which the tendon of superior oblique plays, lies below and behind the medial end of the supraorbital margin, midway between the supraorbital notch and frontolacrimal suture. The convex cerebral surface is marked by frontal gyri and faint grooves for meningeal vessels.

The quadrilateral ethmoidal notch is occupied by the cribriform plate of the ethmoid bone. Inferior to its lateral margins, the bone articulates with the labyrinths of the ethmoid bone and impressions of the ethmoidal air cells can be seen on this surface. Two transverse grooves across each margin are converted into anterior and posterior ethmoidal canals by articulation with the ethmoid bone: these canals open on the medial orbital wall and transmit the anterior and posterior ethmoidal nerves and vessels. The posterior borders of the orbital plates are thin and serrated and articulate with the lesser wings of the sphenoid; their lateral parts usually appear in the middle cranial fossa between the greater and lesser wings of the sphenoid.

The frontal sinuses are two irregular cavities that ascend posterolaterally for a variable distance between the frontal laminae. They are separated by a thin septum and usually deflected from the median plane, which means that they are rarely symmetrical. The sinuses are variable in size and usually larger in males. Their openings lie anterior to the ethmoidal notch and lateral to the nasal spine, and each communicates with the middle meatus in the ipsilateral nasal cavity by a frontonasal canal.

The frontal sinuses are rudimentary at birth and can barely be distinguished. They show a primary expansion with eruption of the first deciduous molars at about 18 months, and again when the permanent molars begin to appear in the sixth year. Growth is slow in the early years but it can be detected radiographically by 6 years. They reach full size after puberty, although with advancing age osseous absorption may lead to further enlargement. Their degree of development appears to be linked to the prominence of the superciliary arches, which is thought to be a response to masticatory stresses. The frontal sinuses are described in Chapter 32.

Ossification

The frontal bone is ossified in fibrous mesenchyme from two primary centres that appear in the eighth week in utero, one near each frontal tuber. Ossification extends superiorly to form half of the main part of the bone; posteriorly to form the orbital part, and inferiorly to form nasal parts. Two secondary centres for the nasal spine appear about the tenth year. At birth the bone consists of two halves. The median suture usually disappears by about 8 years, but may persist as the metopic suture. Metopism has been assessed at 0–7.4% of individuals in various ethnic groups.

ETHMOID BONE

The ethmoid bone is cuboidal and fragile (Fig. 29.3B, Fig. 29.4, Fig. 29.5, Fig. 29.6). It lies anteriorly in the cranial base and contributes to the medial walls of the orbit, the nasal septum and the roof and lateral walls of the nasal cavity. It has a horizontal perforated cribriform plate, a median perpendicular plate, and two lateral labyrinths that contain the ethmoidal air cells.

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Fig. 29.4 Sagittal view of the facial skeleton, viewed from the right side of the nasal septum, looking towards the left.

(From Sobotta 2006.)

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Fig. 29.5 A, B Sagittal view of the facial skeleton, showing the floor of the anterior cranial fossa, the lateral wall of the nasal cavity, the hard palate and the sphenoidal air sinus. In B, the middle concha in the lateral wall of the nasal cavity has been removed to reveal the uncinate process.

(From Sobotta 2006.)

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Fig. 29.6 A, Sagittal view of the facial skeleton, showing the bones forming the lateral wall of the left orbit and the maxillary air sinus. B, The bones forming the medial wall and the floor of the left orbit.

(From Sobotta 2006.)

Cribriform plate and crista galli

The cribriform plate fills the ethmoidal notch of the frontal bone and forms a large part of the nasal roof. It derives its name from the fact that it is penetrated by numerous foramina that transmit branches of the olfactory nerves and their associated meninges. A thick, smooth, triangular, median process, the crista galli, projects upwards from the centre of the cribriform plate. The falx cerebri is attached to its thin and curved posterior border, while its shorter, thick, anterior border articulates with the frontal bone by two small alae, so completing the foramen caecum. Its sides are generally smooth, but may show slight bulges that are related to underlying ethmoidal air cells. On both sides of the crista galli, the cribriform plate is narrow and depressed and is related to the gyrus rectus and the olfactory bulb which lie above it. On each side of the crista anteriorly there is a small slit occupied by dura mater. Just anterolateral to the slit, a foramen transmits the anterior ethmoidal nerve and vessels to the nasal cavity. A groove runs forwards to the foramen caecum from the anterior ethmoidal canal.

Perpendicular plate

The perpendicular plate is thin, flat, quadrilateral and median. It descends from the cribriform plate to form the upper part of the nasal septum, usually deviating slightly from the midline. Its anterior border articulates with the nasal spine of the frontal bone and the crests of the nasal bones, and its posterior border articulates with the crest of the body of the sphenoid bone above and the vomer below. The thick inferior border is attached to the nasal septal cartilage. Its surfaces are smooth except above, where numerous grooves and canals that transmit filaments of the olfactory nerves lead to medial foramina in the cribriform plate.

Ethmoidal labyrinths

The ethmoidal labyrinths consist of thin-walled ethmoidal air cells between two vertical plates. The lateral surface (orbital plate) of the labyrinth is part of the medial orbital wall. The air cells are arranged in anterior, middle and posterior groups. On average there are 11 anterior ethmoidal air cells, three middle, and six posterior. In the disarticulated bone, many air cells are open, but in life, and in the articulated skull, they are closed by proximity to adjoining bones, except where they open into the nasal cavity. The superior surface is crossed by two grooves that are converted into the anterior and posterior ethmoidal canals by the frontal bone; it shows open-air cells that are covered by the edges of the ethmoidal notch of the frontal bone. On the posterior surface open air cells are covered by the sphenoidal conchae and the orbital process of the palatine bone. The middle and posterior ethmoidal air cells are covered by a thin, smooth, oblong orbital plate that articulates superiorly with the orbital plate of the frontal bone, inferiorly with the maxilla and orbital process of the palatine bone, anteriorly with the lacrimal bone and posteriorly with the sphenoid bone. The walls of the air cells lying anterior to the orbital plate are completed by the lacrimal bone and frontal process of the maxilla.

A thin, curved uncinate process, variable in size, projects posteroinferiorly from the labyrinth. The upper edge of this process is a medial boundary of the hiatus semilunaris in the middle meatus. The uncinate process appears in the medial wall of the maxillary sinus as it crosses the ostium of the maxillary sinus to join the ethmoidal process of the inferior nasal concha.

The medial surface of the labyrinth forms part of the lateral nasal wall. It appears as a thin lamella that descends from the inferior surface of the cribriform plate and ends as the convoluted middle nasal concha. Superiorly the surface contains numerous vertical grooves that transmit bundles of olfactory nerves. Posteriorly it is divided by the narrow, oblique superior meatus, bounded above by the thin, curved superior nasal concha. Posterior ethmoidal air cells open into the superior meatus. The convex surface of the middle nasal concha extends along the entire medial surface of the labyrinth, anteroinferior to the superior meatus. Its lower edge is thick and its lateral surface is concave and forms part of the middle meatus. Middle ethmoidal air cells produce a swelling, the bulla ethmoidalis, on the lateral wall of the middle meatus, and open into the meatus, either on the bulla or above it. A curved infundibulum extends up and forwards from the middle meatus and communicates with the anterior ethmoidal sinuses. In more than 50% of crania it continues up as the frontonasal duct to include the drainage point for the frontal sinus. (The ethmoidal air cells are described further in Chapter 32.)

Ossification

The ethmoid bone ossifies in the cartilaginous nasal capsule from three centres, one in the perpendicular plate, and one in each labyrinth. The latter two appear in the orbital plates between the fourth and fifth months in utero, and extend into the ethmoid conchae. At birth, the labyrinths, although ill-developed, are partially ossified, and the remainder are cartilaginous. The perpendicular plate begins to ossify from the median centre during the first year, and fuses with the labyrinths early in the second year. The cribriform plate is ossified partly from the perpendicular plate, and partly from the labyrinths. The crista galli ossifies during the second year. The parts of the ethmoid bone unite to form a single bone at around 3 years of age. Ethmoidal air cells begin to develop at about 3 months in utero, and are therefore present at birth, however, they are difficult to visualize radiographically until the end of the first year. They grow slowly and have almost reached adult size by the age of 12 years.

INFERIOR NASAL CONCHA

The inferior nasal conchae are curved horizontal laminae in the lateral nasal walls (Fig. 29.5) (see also Ch. 32). Each has two surfaces (medial and lateral), two borders (superior and inferior) and two ends (anterior and posterior). The medial surface is convex, much perforated, and longitudinally grooved by vessels. The lateral surface is concave and part of the inferior meatus. The superior border, thin and irregular, may be divided into three regions: an anterior region articulating with the conchal crest of the maxilla; a posterior region articulating with the conchal crest of the palatine bone; and a middle region with three processes, which are variable in size and form. The lacrimal process is small and pointed and lies towards the front. It articulates apically with a descending process from the lacrimal bone, and at its margins with the edges of the nasolacrimal groove on the medial surface of the maxilla, thereby helping to complete the nasolacrimal canal. Most posteriorly, a thin ethmoidal process ascends to meet the uncinate process of the ethmoid bone. An intermediate thin maxillary process curves inferolaterally to articulate with the medial surface of the maxilla at the opening of the maxillary sinus. The inferior border is thick and spongiose, especially in its midpart. Both the anterior and posterior ends of the inferior nasal concha are more or less tapered, the posterior more than the anterior.

Ossification

Ossification is from one centre which appears at about the fifth month in utero in the incurved lower border of the cartilaginous lateral wall of the nasal capsule. The inferior concha loses continuity with the nasal capsule during ossification.

LACRIMAL BONE

The lacrimal bones are the smallest and most fragile of the cranial bones and lie anteriorly in the medial walls of the orbits (Fig. 29.5B). Each has two surfaces (medial and lateral) and four borders (anterior, posterior, superior and inferior). The lateral (orbital) surface is divided by a vertical posterior lacrimal crest. Anterior to the crest is a vertical groove whose anterior edge meets the posterior border of the frontal process of the maxilla to complete the fossa that houses the lacrimal sac. The medial wall of the groove is prolonged by a descending process that contributes to the formation of the nasolacrimal canal by joining the lips of the nasolacrimal groove of the maxilla and the lacrimal process of the inferior nasal concha. A smooth part of the medial orbital wall lies behind the posterior lacrimal crest: the lacrimal part of orbicularis oculi is attached to this surface and crest. The surface ends below in the lacrimal hamulus which, together with the maxilla, completes the upper opening of the nasolacrimal canal. The hamulus may appear as a separate lesser lacrimal bone. The anteroinferior region of the medial (nasal) surface is part of the middle meatus. Its posterosuperior part meets the ethmoid to complete some of the anterior ethmoidal air cells. The anterior border of the lacrimal bone articulates with the frontal process of the maxilla, the posterior border with the orbital plate of the ethmoid bone, the superior border with the frontal bone, and the inferior border with the orbital surface of the maxilla.

Ossification

Ossification is from a centre that appears at about the 12th week in utero in mesenchyme around the nasal capsule. In later life the lacrimal bone is subject to patchy erosion.

NASAL BONE

The nasal bones are small, oblong, variable in size and form, and placed side by side between the frontal processes of the maxillae (Fig. 29.4, Fig. 29.5, Fig. 29.6B). They jointly form the nasal bridge. Each nasal bone has two surfaces (external and internal) and four borders (superior, inferior, lateral and mesial). The external surface has a descending concavo-convex profile and is transversely convex. It is covered by procerus and nasalis and perforated centrally by a small foramen that transmits a vein. The internal surface, transversely concave, bears a longitudinal groove that houses the anterior ethmoidal nerve. The superior border, thick and serrated, articulates with the nasal part of the frontal bone. The inferior border, thin and notched, is continuous with the lateral nasal cartilage. The lateral border articulates with the frontal process of the maxilla. The medial border, thicker above, articulates with its fellow and projects behind as a vertical crest, thereby forming a small part of the nasal septum. It articulates from above with the nasal spine of the frontal bone, the perpendicular plate of the ethmoid bone, and the nasal septal cartilage.

Ossification

Ossification is from a centre that appears early in the third month in utero in the mesenchyme overlying the cartilaginous anterior part of the nasal capsule.

VOMER

The vomer is thin, flat, and almost trapezoid (Fig. 29.4). It forms the posteroinferior part of the nasal septum and presents two surfaces and four borders. Both surfaces are marked by grooves for nerves and vessels. A prominent groove for the nasopalatine nerve and vessels lies obliquely in an anteroinferior plane. The superior border is thickest, and possesses a deep furrow between projecting alae which fits the rostrum of the body of the sphenoid bone. The alae articulate with the sphenoidal conchae, the vaginal processes of the medial pterygoid plates of the sphenoid bone, and the sphenoidal processes of the palatine bones. Where each ala lies between the body of the sphenoid and the vaginal process, its inferior surface helps to form the vomerovaginal canal. The inferior border articulates with the median nasal crests of the maxilla and palatine bones. The anterior border is the longest, and articulates in its upper half with the perpendicular plate of the ethmoid bone. Its lower half is cleft to receive the inferior margin of the nasal septal cartilage (see Ch. 32). The concave posterior border is thick and bifid above and thin below: it separates the posterior nasal apertures. The anterior extremity of the vomer articulates with the posterior margin of the maxillary incisor crest and descends between the incisive canals.

Ossification

The nasal septum is at first a plate of cartilage, part of which is ossified above to form the perpendicular plate of the ethmoid. Its anteroinferior region persists as septal cartilage. The vomer is ossified in a layer of connective tissue which covers the cartilage posteroinferiorly on each aspect. About the eighth week in utero, two centres appear flanking the midline, and in the twelfth week these unite below the cartilage, to form a deep groove for the nasal septal cartilage. Union of the bony lamellae progresses anterosuperiorly while the intervening cartilage is absorbed. By puberty the lamellae are almost united, but evidence of their bilaminar origin remains in the everted alae and anterior marginal groove.

ZYGOMATIC BONE

Each zygomatic bone forms the prominence of a cheek, contributes to the floor and lateral wall of the orbit and the walls of the temporal and infratemporal fossae, and completes the zygomatic arch. Each is roughly quadrangular and is described as having three surfaces, five borders and two processes (Fig. 29.7).

image

Fig. 29.7 Zygomatic bone. A, Anterolateral aspect. B, Posterolateral aspect. Muscle attachments shown in A.

The lateral (facial) surface is convex and is pierced near its orbital border by the zygomaticofacial foramen, which is often double and occasionally absent, and transmits the zygomaticofacial nerve and vessels. This surface gives attachment to zygomaticus major posteriorly and zygomaticus minor anteriorly. The posteromedial (temporal) surface has a rough anterior area for articulation with the zygomatic process of the maxilla, and a smooth, concave posterior area that extends up posteriorly on its frontal process as the anterior aspect of the temporal fossa. It also extends back on the medial aspect of the temporal process as an incomplete lateral wall for the infratemporal fossa. The zygomaticotemporal foramen pierces this surface near the base of the frontal process. The smooth and concave orbital surface forms the anterolateral part of the floor and adjoining lateral wall of the orbit, and extends up on the medial aspect of its frontal process. It usually bears zygomatico-orbital foramina which represent the openings of canals leading to the zygomaticofacial and zygomaticotemporal foramina.

The smoothly concave anterosuperior (orbital) border forms the inferolateral circumference of the orbital opening, and separates the orbital and lateral surfaces of the bone. The anteroinferior (maxillary) border articulates with the maxilla. Its medial end tapers to a point above the infraorbital foramen. A part of levator labii superioris is attached at this surface. The posterosuperior (temporal) border is sinuous, convex above and concave below, and is continuous with the posterior border of the frontal process and upper border of the zygomatic arch. The temporal fascia is attached to this border. There is often a small, easily palpable, marginal tubercle below the frontozygomatic suture. The posteroinferior border is roughened for the attachment of masseter. The serrated posteromedial border articulates with the greater wing of the sphenoid bone above, and the orbital surface of the maxilla below. Between these serrated regions a short, concave, non-articular part usually forms the lateral edge of the inferior orbital fissure. Occasionally absent, the fissure is then completed by the articulation of the maxilla and sphenoid (or with a small sutural bone between them).

The frontal process, thick and serrated, articulates above with the zygomatic process of the frontal bone and behind with the greater wing of the sphenoid bone. A tubercle of varying size and form, Whitnall’s tubercle, is usually present on its orbital aspect, within the orbital opening and about 1 cm below the frontozygomatic suture. This tubercle provides attachment for the lateral palpebral ligament, the suspensory ligament of the eye, and part of the aponeurosis of levator palpebrae superioris. The temporal process, directed backwards, has an oblique, serrated end that articulates with the zygomatic process of the temporal bone to complete the zygomatic arch.

Ossification

Ossification is from one centre, which appears in fibrous tissue about the eighth week in utero. The bone is sometimes divided by a horizontal suture into a larger upper and smaller lower part.

MAXILLA

The maxillae are the largest of the facial bones, other than the mandible, and jointly form the whole of the upper jaw. Each bone forms the greater part of the floor and lateral wall of the nasal cavity, and of the floor of the orbit, contributes to the infratemporal and pterygopalatine fossae, and bounds the inferior orbital and pterygomaxillary fissures. Each maxilla has a body and four processes, namely the zygomatic, frontal, alveolar and palatine processes (Fig. 29.6, Fig. 29.8).

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Fig. 29.8 Maxilla and palatine bone. A, Maxilla. B, Maxilla and palatine bone.

(From Sobotta 2006.)

Body

The body of the maxilla is roughly pyramidal, and has anterior, infratemporal (posterior), orbital and nasal surfaces that enclose the maxillary sinus.

Anterior surface

This surface faces anterolaterally and displays inferior elevations overlying the roots of teeth. There is a shallow incisive fossa above the incisors to which depressor septi is attached. A slip of orbicularis oris is attached to the alveolar border below this fossa, and nasalis is attached superolateral to it. Lateral to the incisive fossa is a larger, deeper canine fossa: levator anguli oris is attached to the bone of this fossa. The incisive and canine fossae are separated by the canine eminence, which overlies the socket of the canine tooth. The infraorbital foramen lies above the fossa and transmits the infraorbital vessels and nerve. Above the foramen a sharp border separates the anterior and orbital surfaces of the bone and contributes to the infraorbital margin. Levator labii superioris is attached here above the infraorbital foramen and levator anguli oris below it. Medially the anterior surface ends at a deeply concave nasal notch, terminating in a pointed process which, with its contralateral fellow, forms the anterior nasal spine. Nasalis and depressor septi are attached to the anterior surface near the notch.

Infratemporal surface

This surface is concave and faces posterolaterally, forming the anterior wall of the infratemporal fossa. It is separated from the anterior surface by the zygomatic process and a ridge (jugal crest) that ascends to it from the first molar socket. Near its centre are the openings of two or three alveolar canals which transmit posterior superior alveolar vessels and nerves. Posteroinferior is the maxillary tuberosity, roughened superomedially where it articulates with the pyramidal process of the palatine bone. A few fibres of medial pterygoid are attached here. Above the tuberosity the smooth anterior boundary of the pterygopalatine fossa is grooved by the maxillary nerve as it passes laterally and slightly upwards into the infraorbital groove on the orbital surface.

Orbital surface

This surface is smooth and triangular, and forms most of the floor of the orbit. Anteriorly its medial border bears a lacrimal notch, behind which it articulates with the lacrimal bone, the orbital plate of the ethmoid and, posteriorly, with the orbital process of the palatine bone. Its posterior border is smoothly rounded, and forms most of the anterior edge of the inferior orbital fissure. The infraorbital groove lies centrally. The anterior border is part of the orbital margin, and is continuous medially with the lacrimal crest of the frontal process of the maxilla. The infraorbital groove transmits the infraorbital vessels and nerve, and begins midway on the posterior border, where it is continuous with a groove on the posterior surface. It passes forwards into the infraorbital canal which opens on the anterior surface below the infraorbital margin. Near its midpoint, the infraorbital canal gives off a small lateral branch, the canalis sinuosus, that transmits the anterior superior alveolar nerve and vessels. The canalis sinuosus descends in the orbital floor lateral to the infraorbital canal, curves medially in the anterior wall of the maxillary sinus, and then passes below the infraorbital foramen to the margin of the anterior nasal aperture in front of the anterior end of the inferior concha. It follows the lower margin of the aperture and opens near the nasal septum in front of the incisive canal. The site of the attachment of inferior oblique may be indicated by a small depression in the bone at the anteromedial corner of the orbital surface, lateral to the lacrimal groove.

Nasal surface

This surface displays posterosuperiorly a large, irregular maxillary hiatus that leads into the maxillary sinus. Parts of air sinuses that are completed by articulation with the ethmoid and lacrimal bones lie at the upper border of the hiatus. The smooth concave surface below the hiatus is part of the inferior meatus. Posteriorly, the surface is roughened where it articulates with the perpendicular plate of the palatine bone. This surface is traversed by a groove which descends forwards from the midposterior border, and is converted into a greater palatine canal by the perpendicular plate. Anterior to the hiatus, a deep nasolacrimal groove, continuous above with the lacrimal groove, makes up about two-thirds of the circumference of the nasolacrimal canal. The rest is contributed by the descending part of the lacrimal bone and the lacrimal process of the inferior nasal concha. This canal conveys the nasolacrimal duct to the inferior meatus. More anteriorly, an oblique conchal crest articulates with the inferior nasal concha. The concavity below it is part of the inferior meatus, while the surface above it is part of the atrium of the middle meatus.

Zygomatic process

Anterior, infratemporal and orbital surfaces of the maxilla converge at a pyramidal projection, the zygomatic process. Anteriorly, the process merges into the facial surface of the body of the maxilla. Posteriorly, it is concave and continuous with the infratemporal surface. Superiorly, it is roughly serrated for articulation with the zygomatic bone. Inferiorly, a bony arched ridge, the zygomaticoalveolar ridge or jugal crest, separates the facial (anterior) and infratemporal surfaces.

Frontal process

The frontal process projects posterosuperiorly between the nasal and lacrimal bones. Its lateral surface is divided by a vertical anterior lacrimal crest which gives attachment to the medial palpebral ligament and is continuous below with the infraorbital margin. A small palpable tubercle at the junction of the crest and orbital surface is a guide to the lacrimal sac. The smooth area anterior to the lacrimal crest merges below with the anterior surface of the body of the maxilla. Parts of orbicularis oculi and levator labii superioris alaeque nasi are attached here. Behind the crest, a vertical groove combines with a groove on the lacrimal bone to complete the lacrimal fossa. The medial surface is part of the lateral nasal wall. A rough subapical area articulates with the ethmoid, and closes anterior ethmoidal air cells. Below this an oblique ethmoidal crest articulates posteriorly with the middle nasal concha, and anteriorly underlies the agger nasi, a ridge anterior to the concha on the lateral nasal wall. The ethmoidal crest forms the upper limit of the atrium of the middle meatus. The frontal process articulates above with the nasal part of the frontal bone. Its anterior border articulates with the nasal bone and its posterior border articulates with the lacrimal bone.

Alveolar process

The alveolar process is thick and arched, wide behind, and socketed for the roots of the upper teeth. The eight sockets on each side vary according to the tooth type. The socket for the canine is deepest, the sockets for the molars are widest and subdivided into three by septa, those for the incisors and second premolar are single, and that for the first premolar usually double. Buccinator is attached to the external alveolar aspect as far forwards as the first molar. In articulated maxillae the processes form the alveolar arch. Occasionally a variably prominent maxillary torus is present in the midline of the palate.

Palatine process

The palatine process, thick and horizontal, projects medially from the lowest part of the medial aspect of the maxilla. It forms a large part of the nasal floor and hard palate and is much thicker in front. Its inferior surface is concave and uneven, and with its contralateral fellow it forms the anterior three-fourths of the osseous (hard) palate. The palatine process displays numerous vascular foramina and depressions for palatine glands and, posterolaterally, two grooves that transmit the greater palatine vessels and nerves. The infundibular incisive fossa is placed between the two maxillae, behind the incisor teeth. The median intermaxillary palatal suture runs posterior to the fossa, and although a little uneven, is usually relatively flat on its oral aspect. Its bony margins are sometimes raised into a prominent longitudinal palatine torus. Two lateral incisive canals, each ascending into its half of the nasal cavity, open in the incisive fossa: they transmit the terminations of the greater palatine artery and nasopalatine nerve. Two additional median openings, anterior and posterior incisive foramina, are occasionally present: they transmit the nasopalatine nerves, the left usually passing through the anterior, and the right through the posterior foramen. On the inferior palatine surface a fine groove, sometimes termed the incisive suture, and prominent in young skulls, may be observed in adults. It extends anterolaterally from the incisive fossa to the interval between the lateral incisor and canine teeth. The superior surface of the palatine process is smooth, concave transversely, and forms most of the nasal floor. The incisive canal lies anteriorly, near its median margin. The lateral border is continuous with the body of the maxilla. The medial border, thicker in front, is raised into a nasal crest that, with its contralateral fellow, forms a groove for the vomer. The front of this ridge rises higher as an incisor crest, prolonged forwards into a sharp process which, with its fellow, forms an anterior nasal spine. The posterior border is serrated for articulation with the horizontal plate of the palatine bone.

Maxillary sinus

The maxillary sinus is the largest of the paranasal sinuses and is situated in the body of the maxilla. It is described in detail in Chapter 32.

Ossification

The maxilla ossifies from a single centre in a sheet of mesenchyme that appears above the canine fossa at about the sixth week in utero and spreads into the rest of the maxilla and its processes. The pattern of spread of ossification may initially leave an unmineralized zone roughly corresponding to a site where a premaxillary suture may occur. However, this deficiency is soon ossified; there is no evidence of a separate centre of ossification for the incisor-bearing portion of the maxilla (i.e. premaxilla).

The maxillary sinus appears as a shallow groove on the nasal aspect at about the fourth month in utero. Though small at birth, the sinus is identifiable radiologically. After birth it enlarges with the growing maxilla, though it is only fully developed following the eruption of the permanent dentition. The infraorbital vessels and nerve are for a time in an open groove in the orbital floor; the anterior part of the groove is subsequently converted into a canal by a lamina that grows in from the lateral side.

At birth the transverse and sagittal maxillary dimensions are greater than the vertical. The frontal process is prominent, but the body is little more than an alveolar process, because the alveoli reach almost to the orbital floor. In adults the vertical dimension is the greatest, reflecting the development of the alveolar process and enlargement of the sinus. When teeth are lost, the bone reverts towards its infantile shape: its height diminishes, the alveolar process is absorbed, and the lower parts of the bone contract and become reduced in thickness at the expense of the labial wall.

PALATINE BONE

The palatine bones are posteriorly placed in the nasal cavity, between the maxillae and the pterygoid processes of the sphenoid bones. They contribute to the floor and lateral walls of the nose, to the floor of the orbit and the hard palate, to the pterygopalatine and pterygoid fossae, and to the inferior orbital fissures. Each has two plates (horizontal and perpendicular) arranged as an L-shape, and three processes (pyramidal, orbital and sphenoidal) (Fig. 29.9).

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Fig. 29.9 Palatine bone, posterior view.

Horizontal plate

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