2.MAXILLA. PALATINE BONE
3.NASAL, LACRIMAL, ZYGOMATIC BONE, INFERIOR NASAL CONCHA, VOMER, MANDIBLE, HYOID BONE
LESSON # 3
Theme 1. TEMPORAL BONE.
The Temporal Bone (Os Temporale) is situated at the sides and base of the skull. Each consists of the squama, the petrous (pyramid) with mastoid process and tympanic part. The squama has a groove for the middle temporal artery and the temporal line. It is a long the zygomatic process. The mandibular fossa is bounded, in front, by the articular tubercle.
Pars petrosa [pyramis] has a base and apex, and anterior, posterior and inferior surfaces, and three angles, and contains the of the organ of hearing and balance. The base is fused with the internal surfaces of the squama and mastoid portion. The anterior surface has eminentia arcuata, the tegmen tympani; hiatus and sulcus canalis nervi petrosi majoris; laterally hiatus and sulcus canalis nervi petrosi minoris; near the apex of the bone, the termination of the carotid canal (foramen internus); above this canal the trigeminal impression. The posterior surface has the internal acoustic meatus, the aperture externa aqueductus vestibuli, the subarcuate fossa. The inferior surface contain foramen coroticum externus, the jugular fossa, a small opening of the aqueductus cochleae, the styloid process and the stylomastoid foramen. Mastoid Process is perforated by the mastoid foramen and carries the sigmoid sinus sulcus. A section of the mastoid process shows it to be hollowed out into a number of spaces, the mastoid cells and mastoid antrum.
Tympanic Part (pars tympanica) forms the walls and the floor of the bony external acoustic meatus. It borders the tympanomastoid fissure.
Canals of the temporal bone:
1. Carotid canal starts in external foramen on lower surface of the piramis and finishes by internal opening in apex of the piramis.
2. Facial canal starts in internal acoustic meatus on back surface of the piramis and finishes in stylomastoid foramen on lower surface of the piramis.
3. Musculotubarius canal is divided into semicanalis m. tensoris tympani and semicanalis tubae auditivae. It starts on anterior margin of the piramis and leads to the tympanic cavity.
4. Caroticotympanic canaliculi (paired) start on medial wall of the external carotid foramen and lead to the the tympanic cavity.
5. Canaliculus for chorda tympani starts in facial canal, passes through the tympanic cavity and finishes in petrotympanic fissura.
6. Tympanic canaliculus starts in the petrosal fossula and finishes in hiatus canalis nervi petrosi minoris.
7. Mastoid canaliculus on the floor of the jugular fossa and passes to the tympanomastoid fissura.
The temporal bones are situated at the sides and base of the skull. Each consists of five parts, viz., the squama, the petrous, mastoid, and tympanic parts, and thestyloid process.
The Squama (squama temporalis).—The squama forms the anterior and upper part of the bone, and is scale-like, thin, and translucent.
Surfaces.—Its outer surface is smooth and convex; it affords
attachment to the Temporalis muscle, and forms part of the temporal fossa; on
its hinder part is a vertical groove for the middle temporal artery. A curved
line, the temporal line, or supramastoid crest, runs backward and upward across its
posterior part; it serves for the attachment of the temporal fascia, and limits
the origin of the Temporalis muscle. The boundary between the squama and the mastoid portion of the bone, as indicated by
traces of the original suture, lies about
Left temporal bone. Inner surface.
anterior root, continuous with the lower border, is short but broad and strong;
it is directed medialward and ends in a rounded
eminence, the articular tubercle(eminentia articularis).
This tubercle forms the front boundary of the mandibular fossa, and in the
fresh state is covered with cartilage. In front of the articular tubercle is a
small triangular area which assists in forming the infratemporal
fossa; this area is separated from the outer surface of the squama
by a ridge which is continuous behind with the anterior root of the zygomatic process, and in front, in the articulated skull,
with the infratemporal crest on the great wing of the
sphenoid. Between the posterior wall of the external acoustic meatus and the posterior
root of the zygomatic process is the area called the suprameatal
or mastoid fossa, through which an instrument may be
pushed into the tympanic antrum. At the junction of
the anterior root with the zygomatic process is a projection
for the attachment of the temporomandibular ligament;
and behind the anterior root is an oval depression, forming part of the
mandibular fossa, for the reception of the condyle of the mandible. The mandibular fossa (glenoid
fossa) is bounded, in front, by the articular tubercle; behind, by the
tympanic part of the bone, which separates it from the external acoustic
meatus; it is divided into two parts by a narrow slit, the petrotympanic
fissure). The anterior part, formed by the squama,
is smooth, covered in the fresh state with cartilage, and articulates with the
condyle of the mandible. Behind this part of the fossa is a small conical
eminence; this is the representative of a prominent tubercle which, in some
mammals, descends behind the condyle of the mandible, and prevents its backward
displacement. The posterior part of the mandibular fossa, formed by the
tympanic part of the bone, is non-articular, and sometimes lodges a portion of
the parotid gland. The petrotympanic fissure leads
into the middle ear or tympanic cavity; it lodges the anterior process of the
malleus, and transmits the tympanic branch of the internal maxillary artery.
The chorda tympani nerve passes through a canal (
The internal surface of the squama is concave; it presents depressions corresponding to the convolutions of the temporal lobe of the brain, and grooves for the branches of the middle meningeal vessels.
Borders.—The superior border is thin, and bevelled at the expense of the internal table, so as to overlap the squamous border of the parietal bone, forming with it the squamosal suture. Posteriorly, the superior border forms an angle, the parietal notch, with the mastoid portion of the bone. The antero-inferior border is thick, serrated, and bevelled at the expense of the inner table above and of the outer below, for articulation with the great wing of the sphenoid.
Mastoid Portion (pars mastoidea).—The mastoid portion forms the posterior part of the bone.
Surfaces.—Its outer surface is rough, and gives attachment to the Occipitalis and Auricularis posterior. It is perforated by numerous foramina; one of these, of large size, situated near the posterior border, is termed the mastoid foramen; it transmits a vein to the transverse sinus and a small branch of the occipital artery to the dura mater. The position and size of this foramen are very variable; it is not always present; sometimes it is situated in the occipital bone, or in the suture between the temporal and the occipital. The mastoid portion is continued below into a conical projection, the mastoid process, the size and form of which very somewhat; it is larger in the male than in the female.
Coronal section of right temporal bone.
This process serves for the attachment of the Sternocleidomastoideus, Splenius capitis, and Longissimus capitis. On the medial side of the process is a deep groove, the mastoid notch (digastric fossa), for the attachment of the Digastricus; medial to this is a shallow furrow, the occipital groove, which lodges the occipital artery.
The inner surface of the mastoid portion presents a deep, curved groove, the sigmoid sulcus, which lodges part of the transverse sinus; in it may be seen the opening of the mastoid foramen. The groove for the transverse sinus is separated from the innermost of the mastoid air cells by a very thin lamina of bone, and even this may be partly deficient.
Borders.—The superior border of the mastoid portion is broad and serrated, for articulation with the mastoid angle of the parietal. The posterior border, also serrated, articulates with the inferior border of the occipital between the lateral angle and jugular process. Anteriorly the mastoid portion is fused with the descending process of the squama above; below it enters into the formation of the external acoustic meatus and the tympanic cavity.
A section of the mastoid process shows it to be hollowed out into a number of spaces, the mastoid cells, which exhibit the greatest possible variety as to their size and number. At the upper and front part of the process they are large and irregular and contain air, but toward the lower part they diminish in size, while those at the apex of the process are frequently quite small and contain marrow; occasionally they are entirely absent, and the mastoid is then solid throughout. In addition to these a large irregular cavity is situated at the upper and front part of the bone. It is called the tympanic antrum, and must be distinguished from the mastoid cells, though it communicates with them. Like the mastoid cells it is filled with air and lined by a prolongation of the mucous membrane of the tympanic cavity, with which it communicates. The tympanic antrum is bounded above by a thin plate of bone, the tegmen tympani, which separates it from the middle fossa of the base of the skull; below by the mastoid process; laterally by the squama just below the temporal line, and medially by the lateral semicircular canal of the internal ear which projects into its cavity. It opens in front into that portion of the tympanic cavity which is known as the attic or epitympanic recess. The tympanic antrum is a cavity of some considerable size at the time of birth; the mastoid air cells may be regarded as diverticula from the antrum, and begin to appear at or before birth; by the fifth year they are well-marked, but their development is not completed until toward puberty.
Petrous Portion (pars petrosa [pyramis]).—The petrous portion or pyramid is pyramidal and is wedged in at the base of the skull between the sphenoid and occipital. Directed medialward, forward, and a little upward, it presents for examination a base, an apex, three surfaces, and three angles, and contains, in its interior, the essential parts of the organ of hearing.
Base.—The base is fused with the internal surfaces of the squama and mastoid portion.
Apex.—The apex, rough and uneven, is received into the angular interval between the posterior border of the great wing of the sphenoid and the basilar part of the occipital; it presents the anterior or internal orifice of the carotid canal, and forms the postero-lateral boundary of the foramen lacerum.
Surfaces.—The anterior surface forms the posterior part of the middle fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the petrosquamous suture, remains of which are distinct even at a late period of life. It is marked by depressions for the convolutions of the brain, and presents six points for examination: (1) near the center, an eminence (eminentia arcuata) which indicates the situation of the superior semicircular canal; (2) in front of and a little lateral to this eminence, a depression indicating the position of the tympanic cavity: here the layer of bone which separates the tympanic from the cranial cavity is extremely thin, and is known as the tegmen tympani; (3) a shallow groove, sometimes double, leading lateralward and backward to an oblique opening, the hiatus of the facial canal, for the passage of the greater superficial petrosal nerve and the petrosal branch of the middle meningeal artery; (4) lateral to the hiatus, a smaller opening, occasionally seen, for the passage of the lesser superficial petrosal nerve; (5) near the apex of the bone, the termination of the carotid canal, the wall of which in this situation is deficient in front; (6) above this canal the shallow trigeminal impression for the reception of the semilunar ganglion.
The posterior surface forms the front part of the posterior
fossa of the base of the skull, and is continuous with the inner surface of the
mastoid portion. Near the center is a large orifice,
the internal acoustic meatus, the size of which varies considerably;
its margins are smooth and rounded, and it leads into a short canal, about
Diagrammatic view of the fundus of the right internal acoustic meatus:
Crista falciformis. 2. Area facialis,
(3) medial to the opening for the carotid canal and close to its posterior border, in front of the jugular fossa, is a triangular depression; at the apex of this is a small opening, the aquaeductus cochleae, which lodges a tubular prolongation of the dura mater establishing a communication between the perilymphatic space and the subarachnoid space, and transmits a vein from the cochlea to join the internal jugular; (4) behind these openings is a deep depression, thejugular fossa, of variable depth and size in different skulls; it lodges the bulb of the internal jugular vein; (5) in the bony ridge dividing the carotid canal from the jugular fossa is the small inferior tympanic canaliculus for the passage of the tympanic branch of the glossopharyngeal nerve; (6) in the lateral part of the jugular fossa is themastoid canaliculus for the entrance of the auricular branch of the vagus nerve;
Left temporal bone. Inferior surface.
behind the jugular fossa is a quadrilateral area, the jugular surface, covered with cartilage in the fresh
state, and articulating with the jugular process of the occipital bone; (8)
extending backward from the carotid canal is the vaginal process, a sheath-like plate of bone, which
divides behind into two laminae; the lateral lamina
is continuous with the tympanic part of the bone, the medial with the lateral
margin of the jugular surface; (9) between these laminae
is the styloid
process, a sharp spine, about
Angles.—The superior angle, the longest, is grooved for the superior petrosal sinus, and gives attachment to the tentorium cerebelli; at its medial extremity is a notch, in which the trigeminal nerve lies. The posterior angle is intermediate in length between the superior and the anterior. Its medial half is marked by a sulcus, which forms, with a corresponding sulcus on the occipital bone, the channel for the inferior petrosal sinus. Its lateral half presents an excavation—the jugular fossa—which, with the jugular notch on the occipital, forms the jugular foramen; an eminence occasionally projects from the center of the fossa, and divides the foramen into two. The anterior angle is divided into two parts—a lateral joined to the squama by a suture (petrosquamous), the remains of which are more or less distinct; a medial, free, which articulates with the spinous process of the sphenoid.
At the angle of junction of the petrous part and the squama are two canals, one above the other, and separated by a thin plate of bone, the septum canalis musculotubarii (processus cochleariformis); both canals lead into the tympanic cavity. The upper one (semicanalis m. tensoris tympani) transmits the Tensor tympani, the lower one (semicanalis tubae auditivae) forms the bony part of the auditory tube.
The tympanic cavity, auditory ossicles, and internal ear, are described with the organ of hearing.
Tympanic Part (pars tympanica).—The tympanic part is a curved plate of bone lying below the squama and in front of the mastoid process.
Surfaces.—Its postero-superior surface is concave, and forms the anterior wall, the floor, and part of the posterior wall of the bony external acoustic meatus. Medially, it presents a narrow furrow, the tympanic sulcus, for the attachment of the tympanic membrane. Its antero-inferior surface is quadrilateral and slightly concave; it constitutes the posterior boundary of the mandibular fossa, and is in contact with the retromandibular part of the parotid gland.
Borders.—Its lateral border is free and rough, and gives attachment to the cartilaginous part of the external acoustic meatus. Internally, the tympanic part is fused with the petrous portion, and appears in the retreating angle between it and the squama, where it lies below and lateral to the orifice of the auditory tube. Posteriorly, it blends with the squama and mastoid part, and forms the anterior boundary of the tympanomastoid fissure. Its upper border fuses laterally with the back of the postglenoid process, while medially it bounds the petrotympanic fissure. The medial part of the lower border is thin and sharp; its lateral part splits to enclose the root of the styloid process, and is therefore named the vaginal process. The central portion of the tympanic part is thin, and in a considerable percentage of skulls is perforated by a hole, the foramen of Huschke.
The external acoustic meatus is nearly
Structure.—The structure of the squama is like that of the other cranial bones: the mastoid portion is spongy, and the petrous portion dense and hard.
The three principal parts of the tempora bone at birth. 1. Outer surface of petromastoid part. 2. Outer surface of tympanic ring. 3. Inner surface of squama.
Ossification.—The temporal bone is ossified from eight centers, exclusive of those for the internal ear and the tympanic ossicles, viz., one for the squama including the zygomatic process, one for the tympanic part, four for the petrous and mastoid parts, and two for the styloid process. Just before the close of fetal life the temporal bone consists of three principal parts: 1. The squama is ossified in membrane from a single nucleus, which appears near the root of the zygomatic process about the second month. 2. The petromastoid part is developed from four centers, which make their appearance in the cartilaginous ear capsule about the fifth or sixth month. One (prootic) appears in the neighborhood of the eminentia arcuata, spreads in front and above the internal acoustic meatus and extends to the apex of the bone; it forms part of the cochlea, vestibule, superior semicircular canal, and medial wall of the tympanic cavity. A second (opisthotic) appears at the promontory on the medial wall of the tympanic cavity and surrounds the fenestra cochleae; it forms the floor of the tympanic cavity and vestibule, surrounds the carotid canal, invests the lateral and lower part of the cochlea, and spreads medially below the internal acoustic meatus. A third (pterotic) roofs in the tympanic cavity and antrum; while the fourth (epiotic) appears near the posterior semicircular canal and extends to form the mastoid process (Vrolik). 3. The tympanic ring is an incomplete circle, in the concavity of which is a groove, the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. This ring expands to form the tympanic part, and is ossified in membrane from a single center which appears about the third month. The styloid process is developed from the proximal part of the cartilage of the second branchial or hyoid arch by two centers: one for the proximal part, the tympanohyal, appears before birth; the other, comprising the rest of the process, is named thestylohyal, and does not appear until after birth. The tympanic ring unites with the squama shortly before birth; the petromastoid part and squama join during the first year, and the tympanohyal portion of the styloid process about the same time. The stylohyal does not unite with the rest of the bone until after puberty, and in some skulls never at all.
The chief subsequent changes in the temporal bone apart from increase in size are: (1) The tympanic ring extends outward and backward to form the tympanic part. This extension does not, however, take place at an equal rate all around the circumference of the ring, but occurs most rapidly on its anterior and posterior portions, and these outgrowths meet and blend, and thus, for a time, there exists in the floor of the meatus a foramen, the foramen of Huschke; this foramen is usually closed about the fifth year, but may persist throughout life. (2) The mandibular fossa is at first extremely shallow, and looks lateralward as well as downward; it becomes deeper and is ultimately directed downward. Its change in direction is accounted for as follows. The part of the squama which forms the fossa lies at first below the level of the zygomatic process. As, however, the base of the skull increases in width, this lower part of the squama is directed horizontally inward to contribute to the middle fossa of the skull, and its surfaces therefore come to look upward and downward; the attached portion of the zygomatic process also becomes everted, and projects like a shelf at right angles to the squama. (3) The mastoid portion is at first quite flat, and the stylomastoid foramen and rudimentary styloid process lie immediately behind the tympanic ring. With the development of the air cells the outer part of the mastoid portion grows downward and forward to form the mastoid process, and the styloid process and stylomastoid foramen now come to lie on the under surface. The descent of the foramen is necessarily accompanied by a corresponding lengthening of the facial canal. (4) The downward and forward growth of the mastoid process also pushes forward the tympanic part, so that the portion of it which formed the original floor of the meatus and contained the foramen of Huschke is ultimately found in the anterior wall. (5) The fossa subarcuata becomes filled up and almost obliterated.
Articulations.—The temporal articulates with five bones: occipital, parietal, sphenoid, mandible and zygomatic.
Theme 2. MAXILLA. PALATINE BONE
Bones of viscerocranium
Bones of viscerocranium include: maxilla, zygomatic, nasal, palatine, lacrimal bones, vomer, greater portion of the ethmoid, mandible and inferior nasal concha.
Maxilla - paired bone, has a body and four processes: frontal, zygomatic, alveolar and palatine. Frontal process riches the nasal part of the frontal bone, medially it carries ethmoidal crest for middle nasal concha. Zygomatic process joins zygomatic bone. Alveolar process carries dental alveoli for insertion the teeth. Palatine process forms bony palatine.
There are four surfaces in maxillary body: anterior, orbital, infraorbital and nasal. Anterior surface is bordered by infraorbital margin where it is infraorbital foramen nasal surface one can find maxillary hiatus that communicates middle nasal meatus with maxillary (Haymory) sinus.
The maxillae are the largest bones of the face, excepting the mandible, and form, by their union, the whole of the upper jaw. Each assists in forming the boundaries of three cavities, viz., the roof of the mouth, the floor and lateral wall of the nose and the floor of the orbit; it also enters into the formation of two fossae, the infratemporal and pterygopalatine, and two fissures, the inferior orbital and pterygomaxillary.
Each bone consists of a body and four processes—zygomatic, frontal, alveolar, and palatine.
The Body (corpus maxillae).—The body is somewhat pyramidal in shape, and contains a large cavity, the maxillary sinus (antrum of Highmore). It has four surfaces—an anterior, a posterior or infratemporal, a superior or orbital, and a medial or nasal.
Surfaces.—The anterior surface is directed forward and lateralward. It presents at its lower part a series of eminences corresponding to the positions of the roots of the teeth. Just above those of the incisor teeth is a depression, the incisive fossa, which gives origin to the Depressor alae nasi; to the alveolar border below the fossa is attached a slip of the Orbicularis oris; above and a little lateral to it, the Nasalis arises. Lateral to the incisive fossa is another depression, the canine fossa; it is larger and deeper than the incisive fossa, and is separated from it by a vertical ridge, the canine eminence, corresponding to the socket of the canine tooth; the canine fossa gives origin to the Caninus. Above the fossa is the infraorbital foramen, the end of the infraorbital canal; it transmits the infraorbital vessels and nerve. Above the foramen is the margin of the orbit, which affords attachment to part of the Quadratus labii superioris. Medially, the anterior surface is limited by a deep concavity, thenasal notch, the margin of which gives attachment to the Dilatator naris posterior and ends below in a pointed process, which with its fellow of the opposite side forms the anterior nasal spine.
The infratemporal surface is convex, directed backward and lateralward, and forms part of the infratemporal fossa. It is separated from the anterior surface by the zygomatic process and by a strong ridge, extending upward from the socket of the first molar tooth. It is pierced about its center by the apertures of the alveolar canals, which transmit the posterior superior alveolar vessels and nerves. At the lower part of this surface is a rounded eminence, the maxillary tuberosity, especially prominent after the growth of the wisdom tooth; it is rough on its lateral side for articulation with the pyramidal process of the palatine bone
and in some cases articulates with the lateral pterygoid plate of the sphenoid. It gives origin to a few fibers of the Pterygoideus internus. Immediately above this is a smooth surface, which forms the anterior boundary of the pterygopalatine fossa, and presents a groove, for the maxillary nerve; this groove is directed lateralward and slightly upward, and is continuous with the infraorbital groove on the orbital surface.
The orbital surface is smooth and triangular, and forms the greater part of the floor of the orbit. It is bounded medially by an irregular margin which in front presents a notch, the lacrimal notch; behind this notch the margin articulates with the lacrimal, the lamina papyracea of the ethmoid and the orbital process of the palatine. It is bounded behind by a smooth rounded edge which forms the anterior
Left maxilla. Nasal surface
margin of the inferior orbital fissure, and sometimes articulates at its lateral extremity with the orbital surface of the great wing of the sphenoid.
It is limited in front by part of the circumference of the orbit, which is continuous medially with the frontal process, and laterally with the zyogmatic process. Near the middle of the posterior part of the orbital surface is the infraorbital groove, for the passage of the infraorbital vessels and nerve. The groove begins at the middle of the posterior border, where it is continuous with that near the upper edge of the infratemporal surface, and, passing forward, ends in a canal, which subdivides into two branches. One of the canals, the infraorbital canal, opens just below the margin of the orbit; the other, which is smaller, runs downward in the substance of the anterior wall of the maxillary sinus, and transmits the anterior superior alveolar vessels and nerve to the front teeth of the maxilla. From the back part of the infraorbital canal, a second small canal is sometimes given off; it runs downward in the lateral wall of the sinus, and conveys the middle alveolar nerve to the premolar teeth. At the medial and forepart of the orbital surface just lateral to the lacrimal groove, is a depression, which gives origin to the Obliquus oculi inferior.
The nasal surface presents a large, irregular opening leading into the maxillary sinus. At the upper border of this aperture are some broken air cells, which, in the articulated skull, are closed in by the ethmoid and lacrimal bones. Below the aperture is a smooth concavity which forms part of the inferior meatus of the nasal cavity, and behind it is a rough surface for articulation with the perpendicular part of the palatine bone; this surface is traversed by a groove, commencing near the middle of the posterior border and running obliquely downward and forward; the groove is converted into a canal, the pterygopalatine canal, by the palatine bone. In front of the opening of the sinus is a deep groove, the lacrimal groove, which is converted into the nasolacrimal canal, by the lacrimal bone and inferior nasal concha; this canal opens into the inferior meatus of the nose and transmits the nasolacrimal duct. More anteriorly is an oblique ridge, the conchal crest, for articulation with the inferior nasal concha. The shallow concavity above this ridge forms part of the atrium of the middle meatus of the nose, and that below it, part of the inferior meatus.
Left maxillary sinus opened from the exterior
The Maxillary Sinus or Antrum of Highmore (sinus maxillaris).—The maxillary sinus is a large pyramidal cavity, within the body of the maxilla: its apex, directed lateralward, is formed by the zygomatic process; its base, directed medialward, by the lateral wall of the nose. Its walls are everywhere exceedingly thin, and correspond to the nasal orbital, anterior, and infratemporal surfaces of the body of the bone. Its nasal wall, or base, presents, in the disarticulated bone, a large, irregular aperture, communicating with the nasal cavity. In the articulated skull this aperture is much reduced in size by the following bones: the uncinate process of the ethmoid above, the ethmoidal process of the inferior nasal concha below, the vertical part of the palatine behind, and a small part of the lacrimal above and in front; the sinus communicates with the middle meatus of the nose, generally by two small apertures left between the above-mentioned bones. In the fresh state, usually only one small opening exists, near the upper part of the cavity; the other is closed by mucous membrane. On the posterior wall are the alveolar canals, transmitting the posterior superior alveolar vessels and nerves to the molar teeth. The floor is formed by the alveolar process of the maxilla, and, if the sinus be of an average size, is on a level with the floor of the nose; if the sinus be large it reaches below this level.
Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second molar teeth; in some cases the floor is perforated by the fangs of the teeth. The infraorbital canal usually projects into the cavity as a well-marked ridge extending from the roof to the anterior wall; additional ridges are sometimes seen in the posterior wall of the cavity, and are caused by the alveolar canals. The size of the cavity varies in different skulls, and even on the two sides of the same skull.
The Zygomatic Process (processus zygomaticus; malar process).—The zygomatic process is a rough triangular eminence, situated at the angle of separation of the anterior, zygomatic, and orbital surfaces. In front it forms part of the anterior surface; behind, it is concave, and forms part of the infratemporal fossa; above, it is rough and serrated for articulation with the zygomatic bone; while below, it presents the prominent arched border which marks the division between the anterior and infratemporal surfaces.
The Frontal Process (processus frontalis; nasal process).—The frontal process is a strong plate, which projects upward, medialward, and backward, by the side of the nose, forming part of its lateral boundary. Its lateral surface is smooth, continuous with the anterior surface of the body, and gives attachment to the Quadratus labii superioris, the Orbicularis oculi, and the medial palpebral ligament. Its medial surface forms part of the lateral wall of the nasal cavity; at its upper part is a rough, uneven area, which articulates with the ethmoid, closing in the anterior ethmoidal cells; below this is an oblique ridge, the ethmoidal crest, the posterior end of which articulates with the middle nasal concha, while the anterior part is termed the agger nasi; the crest forms the upper limit of the atrium of the middle meatus. The upper border articulates with the frontal bone and the anterior with the nasal; the posterior border is thick, and hollowed into a groove, which is continuous below with the lacrimal groove on the nasal surface of the body: by the articulation of the medial margin of the groove with the anterior border of the lacrimal a corresponding groove on the lacrimal is brought into continuity, and together they form the lacrimal fossa for the lodgement of the lacrimal sac. The lateral margin of the groove is named theanterior lacrimal crest, and is continuous below with the orbital margin; at its junction with the orbital surface is a small tubercle, the lacrimal tubercle, which serves as a guide to the position of the lacrimal sac.
The Alveolar Process (processus alveolaris).—The alveolar process is the thickest and most spongy part of the bone. It is broader behind than in front, and excavated into deep cavities for the reception of the teeth. These cavities are eight in number, and vary in size and depth according to the teeth they contain. That for the canine tooth is the deepest; those for the molars are the widest, and are subdivided into minor cavities by septa; those for the incisors are single, but deep and narrow. The Buccinator arises from the outer surface of this process, as far forward as the first molar tooth. When the maxillae are articulated with each other, their alveolar processes together form the alveolar arch; the center of the anterior margin of this arch is named the alveolar point.
The Palatine Process (processus palatinus; palatal process).—The palatine process, thick and strong, is horizontal and projects medialward from the nasal surface of the bone. It forms a considerable part of the floor of the nose and the roof of the mouth and is much thicker in front than behind. Its inferior surface is concave, rough and uneven, and forms, with the palatine process of the opposite bone, the anterior three-fourths of the hard plate. It is perforated by numerous foramina for the passage of the nutrient vessels; is channelled at the back part of its lateral border by a groove, sometimes a canal, for the transmission of the descending palatine vessels and the anterior palatine nerve from the spheno-palatine ganglion; and presents little depressions for the lodgement of the palatine glands. When the two maxillae are articulated, a funnel-shaped opening, the incisive foramen, is seen in the middle line, immediately behind the incisor teeth. In this opening the orifices of two lateral canals are visible; they are named the incisive canals or foramina of Stenson; through each of them passes the terminal branch of the descending palatine artery and the nasopalatine nerve. Occasionally two additional canals are present in the middle line; they are termed the foramina of Scarpa, and when present transmit the nasopalatine nerves, the left passing through the anterior, and the right through the posterior canal. On the under surface of the palatine process, a delicate linear suture, well seen in young skulls, may sometimes be noticed extending lateralward and forward on either side from the incisive foramen to the interval between the lateral incisor and the canine tooth. The small part in front of this suture constitutes the premaxilla (os incisivum), which in most vertebrates forms an independent bone; it includes the whole thickness of the alveolus, the corresponding part of the floor of the nose and the anterior nasal spine, and contains the sockets of the incisor teeth. The upper surface of the palatine process is concave from side to side, smooth, and forms the greater part of the floor of the nasal cavity. It presents, close to its medial margin, the upper orifice of the incisive canal. The lateral border of the process is incorporated with the rest of the bone. The medial border is thicker in front than behind, and is raised above into a ridge, the nasal crest, which, with the corresponding ridge of the opposite bone, forms a groove for the reception of the vomer. The front part of this ridge rises to a considerable height, and is named the incisor crest; it is prolonged forward into a sharp process, which forms, together with a similar process of the opposite bone, the anterior nasal spine. The posterior border is serrated for articulation with the horizontal part of the palatine bone.
The bony palate and alveolar arch
Ossification.—The maxilla is ossified in membrane. Mall 40 and Fawcett 41 maintain that it is ossified from two centers only, one for the maxilla proper and one for the premaxilla. These centers appear during the sixth week of fetal life and unite in the beginning of the third month, but the suture between the two portions persists on the palate until nearly middle life. Mall states that the frontal process is developed from both centers. The maxillary sinus appears as a shallow groove on the nasal surface of the bone about the fourth month of fetal life, but does not reach its full size until after the second dentition. The maxilla was formerly described as ossifying from six centers, viz., one, the orbitonasal, forms that portion of the body of the bone which lies medial to the infraorbital canal, including the medial part of the floor of the orbit and the lateral wall of the nasal cavity; a second, the zygomatic, gives origin to the portion which lies lateral to the infraorbital canal, including the zygomatic process; from a third, the palatine, is developed the palatine process posterior to the incisive canal together with the adjoining part of the nasal wall; a fourth, thepremaxillary, forms the incisive bone which carries the incisor teeth and corresponds to the premaxilla of the lower vertebrates; 42 a fifth, the nasal, gives rise to the frontal process and the portion above the canine tooth; and a sixth, the infravomerine, lies between the palatine and premaxillary centers and beneath the vomer; this center, together with the corresponding center of the opposite bone, separates the incisive canals from each other.
Anterior surface of maxilla at birth
Inferior surface of maxilla at birth
Articulations.—The maxilla articulates with nine bones: two of the cranium, the frontal and ethmoid, and seven of the face, viz., the nasal, zygomatic, lacrimal, inferior nasal concha, palatine, vomer, and its fellow of the opposite side. Sometimes it articulates with the orbital surface, and sometimes with the lateral pterygoid plate of the sphenoid.
Changes Produced in the Maxilla by Age At birth the transverse and antero-posterior diameters of the bone are each greater than the vertical. The frontal process is well-marked and the body of the bone consists of little more than the alveolar process, the teeth sockets reaching almost to the floor of the orbit. The maxillary sinus presents the appearance of a furrow on the lateral wall of the nose. In the adult the vertical diameter is the greatest, owing to the development of the alveolar process and the increase in size of the sinus. In old age the bone reverts in some measure to the infantile condition; its height is diminished, and after the loss of the teeth the alveolar process is absorbed, and the lower part of the bone contracted and reduced in thickness.
Left palatine bone. Nasal aspect. Enlarged.
The palatine bone is situated at the back part of the nasal cavity between the maxilla and the pterygoid process of the sphenoid. It contributes to the walls of three cavities: the floor and lateral wall of the nasal cavity, the roof of the mouth, and the floor of the orbit; it enters into the formation of two fossae, the pterygopalatineand pterygoid fossae; and one fissure, the inferior orbital fissure. The palatine bone somewhat resembles the letter L, and consists of a horizontal and a vertical part and three outstanding processes—viz., the pyramidal process, which is directed backward and lateralward from the junction of the two parts, and the orbital and sphenoidal processes, which surmount the vertical part, and are separated by a deep notch, the sphenopalatine notch.
The Horizontal Part (pars horizontalis; horizontal plate).—The horizontal part is quadrilateral, and has two surfaces and four borders.
Left palatine bone. Posterior aspect. Enlarged
Surfaces.—The superior surface, concave from side to side, forms the back part of the floor of the nasal cavity. The inferior surface, slightly concave and rough, forms, with the corresponding surface of the opposite bone, the posterior fourth of the hard palate. Near its posterior margin may be seen a more or less marked transverse ridge for the attachment of part of the aponeurosis of the Tensor veli palatini.
Borders.—The anterior border is serrated, and articulates with the palatine process of the maxilla. The posterior border is concave, free, and serves for the attachment of the soft palate. Its medial end is sharp and pointed, and, when united with that of the opposite bone, forms a projecting process, the posterior nasal spine for the attachment of the Musculus uvulae. The lateral border is united with the lower margin of the perpendicular part, and is grooved by the lower end of the pterygopalatine canal. The medial border, the thickest, is serrated for articulation with its fellow of the opposite side; its superior edge is raised into a ridge, which, united with the ridge of the opposite bone, forms the nasal crest for articulation with the posterior part of the lower edge of the vomer.
The Vertical Part (pars perpendicularis; perpendicular plate)—The vertical part is thin, of an oblong form, and presents two surfaces and four borders.
Surfaces.—The nasal surface exhibits at its lower part a broad, shallow depression, which forms part of the inferior meatus of the nose. Immediately above this is a well-marked horizontal ridge, the conchal crest, for articulation with the inferior nasal concha; still higher is a second broad, shallow depression, which forms part of the middle meatus, and is limited above by a horizontal crest less prominent than the inferior, the ethmoidal crest, for articulation with the middle nasal concha. Above the ethmoidal crest is a narrow, horizontal groove, which forms part of the superior meatus.
The maxillary surface is rough and irregular throughout the greater part of its extent, for articulation with the nasal surface of the maxilla; its upper and back part is smooth where it enters into the formation of the pterygopalatine fossa; it is also smooth in front, where it forms the posterior part of the medial wall of the maxillary sinus. On the posterior part of this surface is a deep vertical groove, converted into the pterygopalatine canal, by articulation with the maxilla; this canal transmits the descending palatine vessels, and the anterior palatine nerve.
Borders.—The anterior border is thin and irregular; opposite the conchal crest is a pointed, projecting lamina, the maxillary process, which is directed forward, and closes in the lower and back part of the opening of the maxillary sinus. The posterior border presents a deep groove, the edges of which are serrated for articulation with the medial pterygoid plate of the sphenoid. This border is continuous above with the sphenoidal process; below it expands into the pyramidal process. Thesuperior border supports the orbital process in front and the sphenoidal process behind. These processes are separated by the sphenopalatine notch, which is converted into the sphenopalatine foramen by the under surface of the body of the sphenoid. In the articulated skull this foramen leads from the pterygopalatine fossa into the posterior part of the superior meatus of the nose, and transmits the sphenopalatine vessels and the superior nasal and nasopalatine nerves. The inferior borderis fused with the lateral edge of the horizontal part, and immediately in front of the pyramidal process is grooved by the lower end of the pterygopalatine canal.
The Pyramidal Process or Tuberosity (processus pyramidalis).—The pyramidal process projects backward and lateralward from the junction of the horizontal and vertical parts, and is received into the angular interval between the lower extremities of the pterygoid plates. On its posterior surface is a smooth, grooved, triangular area, limited on either side by a rough articular furrow. The furrows articulate with the pterygoid plates, while the grooved intermediate area completes the lower part of the pterygoid fossa and gives origin to a few fibers of the Pterygoideus internus. The anterior part of the lateral surface is rough, for articulation with the tuberosity of the maxilla; its posterior part consists of a smooth triangular area which appears, in the articulated skull, between the tuberosity of the maxilla and the lower part of the lateral pterygoid plate, and completes the lower part of the infratemporal fossa. On the base of the pyramidal process, close to its union with the horizontal part, are the lesser palatine foramina for the transmission of the posterior and middle palatine nerves.
The Orbital Process (processus orbitalis).—The orbital process is placed on a higher level than the sphenoidal, and is directed upward and lateralward from the front of the vertical part, to which it is connected by a constricted neck. It presents five surfaces, which enclose an air cell. Of these surfaces, three are articular and two non-articular. The articular surfaces are: (1) the anterior or maxillary, directed forward, lateralward, and downward, of an oblong form, and rough for articulation with the maxilla; (2) the posterior or sphenoidal, directed backward, upward, and medialward; it presents the opening of the air cell, which usually communicates with the sphenoidal sinus; the margins of the opening are serrated for articulation with the sphenoidal concha; (3) the medial or ethmoidal, directed forward, articulates with the labyrinth of the ethmoid. In some cases the air cell opens on this surface of the bone and then communicates with the posterior ethmoidal cells. More rarely it opens on both surfaces, and then communicates with the posterior ethmoidal cells and the sphenoidal sinus. The non-articular surfaces are: (1) the superior or orbital, directed upward and lateralward; it is triangular in shape, and forms the back part of the floor of the orbit; and (2) the lateral, of an oblong form, directed toward the pterygopalatine fossa; it is separated from the orbital surface by a rounded border, which enters into the formation of the inferior orbital fissure.
The Sphenoidal Process (processus sphenoidalis).—The sphenoidal process is a thin, compressed plate, much smaller than the orbital, and directed upward and medialward. It presents three surfaces and two borders. The superior surface articulates with the root of the pterygoid process and the under surface of the sphenoidal concha, its medial border reaching as far as the ala of the vomer; it presents a groove which contributes to the formation of the pharyngeal canal. Themedial surface is concave, and forms part of the lateral wall of the nasal cavity. The lateral surface is divided into an articular and a non-articular portion: the former is rough, for articulation with the medial pterygoid plate; the latter is smooth, and forms part of the pterygopalatine fossa. The anterior border forms the posterior boundary of the sphenopalatine notch. The posterior border, serrated at the expense of the outer table, articulates with the medial pterygoid plate.
The orbital and sphenoidal processes are separated from one another by the sphenopalatine notch. Sometimes the two processes are united above, and form between them a complete foramen, or the notch may be crossed by one or more spicules of bone, giving rise to two or more foramina.
Ossification.—The palatine bone is ossified in membrane from a single center, which makes its appearance about the sixth or eighth week of fetal life at the angle of junction of the two parts of the bone. From this point ossification spreads medialward to the horizontal part, downward into the pyramidal process, and upward into the vertical part. Some authorities describe the bone as ossifying from four centers: one for the pyramidal process and portion of the vertical part behind the pterygopalatine groove; a second for the rest of the vertical and the horizontal parts; a third for the orbital, and a fourth for the sphenoidal process. At the time of birth the height of the vertical part is about equal to the transverse width of the horizontal part, whereas in the adult the former measures about twice as much as the latter.
Articulations.—The palatine articulates with six bones: the sphenoid, ethmoid, maxilla, inferior nasal concha, vomer, and opposite palatine.
Theme 3. NASAL, LACRIMAL, ZYGOMATIC BONE, INFERIOR NASAL CONCHA, VOMER, MANDIBLE, HYOID BONE
Nasal bone (paired) forms bony part of nasal dorsum.
Articulation of nasal and lacrimal bones with maxilla.
Lacrimal bones (paired) forms anterior part of the medial orbital wall.
Zygomatic bone (paired) has frontal and temporal processes; lateral, temporal and orbital surfaces.
Left zygomatic bone in situ
The zygomatic bone is small and quadrangular, and is situated at the upper and lateral part of the face: it forms the prominence of the cheek, part of the lateral wall and floor of the orbit, and parts of the temporal and infratemporal fossae. It presents a malar and a temporal surface; four processes, the frontosphenoidal, orbital, maxillary, and temporal; and four borders.
Surfaces.—The malar surface is convex and perforated near its center by a small aperture, the zygomaticofacial foramen, for the passage of the zygomaticofacial nerve and vessels; below this foramen is a slight elevation, which gives origin to the Zygomaticus.
The temporal surface directed backward and medialward, is concave, presenting medially a rough, triangular area, for articulation with the maxilla, and laterally a smooth, concave surface, the upper part of which forms the anterior boundary of the temporal fossa, the lower a part of the infratemporal fossa. Near the center of this surface is the zygomaticotemporal foramen for the transmission of the zygomaticotemporal nerve.
process is thick and
serrated, and articulates with the zygomatic process
of the frontal bone. On its orbital surface, just within the orbital margin and
Left zygomatic bone. Malar surface.
Its anterior margin, smooth and rounded, is part of the circumference of the orbit. Its superior margin, rough, and directed horizontally, articulates with the frontal bone behind the zygomatic process. Its posterior margin is serrated for articulation, with the great wing of the sphenoid and the orbital surface of the maxilla. At the angle of junction of the sphenoidal and maxillary portions, a short, concave, non-articular part is generally seen; this forms the anterior boundary of the inferior orbital fissure: occasionally, this non-articular part is absent, the fissure then being completed by the junction of the maxilla and sphenoid, or by the interposition of a small sutural bone in the angular interval between them. The maxillary process presents a rough, triangular surface which articulates with the maxilla. The temporal process, long, narrow, and serrated, articulates with the zygomatic process of the temporal.
Left zygomatic bone. Temporal surface.
Borders.—The antero-superior or orbital border is smooth, concave, and forms a considerable part of the circumference of the orbit. The antero-inferior ormaxillary border is rough, and bevelled at the expense of its inner table, to articulate with the maxilla; near the orbital margin it gives origin to the Quadratus labii superioris. The postero-superior or temporal border, curved like an italic letter f, is continuous above with the commencement of the temporal line, and below with the upper border of the zygomatic arch; the temporal fascia is attached to it. The postero-inferior or zygomatic border affords attachment by its rough edge to the Masseter.
—The zygomatic bone is generally described as ossifying from three centers—one for the malar and two for the orbital portion; these appear about the eighth week and fuse about the fifth month of fetal life. Mall describes it as being ossified from one center which appears just beneath and to the lateral side of the orbit.
After birth, the bone is sometimes divided by a horizontal suture into an upper larger, and a lower smaller division. In some quadrumana the zygomatic bone consists of two parts, an orbital and a malar.
Articulations.—The zygomatic articulates with four bones: the frontal, sphenoidal, temporal, and maxilla.
Vomer (unpaired) is located in nasal cavity and forms bony septum nasi with perpendicular lamina (ethmoid bone).
Mandible (unpaired) has a body and two rami. There is alveolar arch with teeth alveoli. Mandibular angle is located between, body and rami and it carries masseteric and pterygoid tuberosities for attachment of mastication muscles. Ramus contains coronal and condilar processes. Mandibular canal starts on the inner surface of the ramus and terminates by mental foramen in body. It contains vessels and nerves that supply teeth in the lower jaw. This is only bone that form synovial joint.
The Mandible, the largest and strongest bone of the face, serves for the reception of the lower teeth. It consists of a curved, horizontal portion, the body, and two perpendicular portions, the rami, which unite with the ends of the body nearly at right angles.
The Body (corpus mandibulae).—The body is curved somewhat like a horseshoe and has two surfaces and two borders.
Surfaces.—The external surface is marked in the median line by a faint ridge, indicating the symphysis or line of junction of the two pieces of which the bone is composed at an early period of life. This ridge divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle. On either side of the symphysis, just below the incisor teeth, is a depression, the incisive fossa, which gives origin to the Mentalis and a small portion of the Orbicularis oris. Below the second premolar tooth, on either side, midway between the upper and lower borders of the body, is the mental foramen, for the passage of the mental vessels and nerve. Running backward and upward from each mental tubercle is a faint ridge, theoblique line, which is continuous with the anterior border of the ramus; it affords attachment to the Quadratus labii inferioris and Triangularis; the Platysma is attached below it.
Mandible. Outer surface. Side view
The internal surface is concave from side to side. Near the lower part of the symphysis is a pair of laterally placed spines, termed the mental spines, which give origin to the Genioglossi. Immediately below these is a second pair of spines, or more frequently a median ridge or impression, for the origin of the Geniohyoidei. In some cases the mental spines are fused to form a single eminence, in others they are absent and their position is indicated merely by an irregularity of the surface. Above the mental spines a median foramen and furrow are sometimes seen; they mark the line of union of the halves of the bone. Below the mental spines, on either side of the middle line, is an oval depression for the attachment of the anterior belly of the Digastricus. Extending upward and backward on either side from the lower part of the symphysis is the mylohyoid line, which gives origin to the Mylohyoideus; the posterior part of this line, near the alveolar margin, gives attachment to a small part of the Constrictor pharyngis superior, and to the pterygomandibular raphé. Above the anterior part of this line is a smooth triangular area against which the sublingual gland rests, and below the hinder part, an oval fossa for the submaxillary gland.
Borders.—The superior or alveolar border, wider behind than in front, is hollowed into cavities, for the reception of the teeth; these cavities are sixteen in number, and vary in depth and size according to the teeth which they contain. To the outer lip of the superior border, on either side, the Buccinator is attached as far forward as the first molar tooth. The inferior border is rounded, longer than the superior, and thicker in front than behind; at the point where it joins the lower border of the ramus a shallow groove; for the external maxillary artery, may be present.
Mandible. Inner surface. Side view
The Ramus (ramus mandibulae; perpendicular portion).—The ramus is quadrilateral in shape, and has two surfaces, four borders, and two processes.
Surfaces.—The lateral surface is flat and marked by oblique ridges at its lower part; it gives attachment throughout nearly the whole of its extent to the Masseter. Themedial surface presents about its center the oblique mandibular foramen, for the entrance of the inferior alveolar vessels and nerve. The margin of this opening is irregular; it presents in front a prominent ridge, surmounted by a sharp spine, the lingula mandibulae, which gives attachment to the sphenomandibular ligament; at its lower and back part is a notch from which the mylohyoid groove runs obliquely downward and forward, and lodges the mylohyoid vessels and nerve. Behind this groove is a rough surface, for the insertion of the Pterygoideus internus. The mandibular canal runs obliquely downward and forward in the ramus, and then horizontally forward in the body, where it is placed under the alveoli and communicates with them by small openings. On arriving at the incisor teeth, it turns back to communicate with the mental foramen, giving off two small canals which run to the cavities containing the incisor teeth. In the posterior two-thirds of the bone the canal is situated nearer the internal surface of the mandible; and in the anterior third, nearer its external surface. It contains the inferior alveolar vessels and nerve, from which branches are distributed to the teeth. The lower border of the ramus is thick, straight, and continuous with the inferior border of the body of the bone. At its junction with the posterior border is the angle of the mandible, which may be either inverted or everted and is marked by rough, oblique ridges on each side, for the attachment of the Masseter laterally, and the Pterygoideus internus medially; the stylomandibular ligament is attached to the angle between these muscles. The anterior border is thin above, thicker below, and continuous with the oblique line. The posterior border is thick, smooth, rounded, and covered by the parotid gland. Theupper border is thin, and is surmounted by two processes, the coronoid in front and the condyloid behind, separated by a deep concavity, the mandibular notch.
The Coronoid Process (processus coronoideus) is a thin, triangular eminence, which is flattened from side to side and varies in shape and size. Its anterior border is convex and is continuous below with the anterior border of the ramus; its posterior border is concave and forms the anterior boundary of the mandibular notch. Itslateral surface is smooth, and affords insertion to the Temporalis and Masseter. Its medial surface gives insertion to the Temporalis, and presents a ridge which begins near the apex of the process and runs downward and forward to the inner side of the last molar tooth. Between this ridge and the anterior border is a grooved triangular area, the upper part of which gives attachment to the Temporalis, the lower part to some fibers of the Buccinator.
The Condyloid Process (processus condyloideus) is thicker than the coronoid, and consists of two portions: the condyle, and the constricted portion which supports it, the neck. The condyle presents an articular surface for articulation with the articular disk of the temporomandibular joint; it is convex from before backward and from side to side, and extends farther on the posterior than on the anterior surface. Its long axis is directed medialward and slightly backward, and if prolonged to the middle line will meet that of the opposite condyle near the anterior margin of the foramen magnum. At the lateral extremity of the condyle is a small tubercle for the attachment of the temporomandibular ligament. The neck is flattened from before backward, and strengthened by ridges which descend from the forepart and sides of the condyle. Its posterior surface is convex; its anterior presents a depression for the attachment of the Pterygoideus externus.
The mandibular notch, separating the two processes, is a deep semilunar depression, and is crossed by the masseteric vessels and nerve.
Ossification.—The mandible is ossified in the fibrous membrane covering the outer surfaces of Meckel’s cartilages. These cartilages form the cartilaginous bar of the mandibular arch , and are two in number, a right and a left. Their proximal or cranial ends are connected with the ear capsules, and their distal extremities are joined to one another at the symphysis by mesodermal tissue. They run forward immediately below the condyles and then, bending downward, lie in a groove near the lower border of the bone; in front of the canine tooth they incline upward to the symphysis. From the proximal end of each cartilage the malleus and incus, two of the bones of the middle ear, are developed; the next succeeding portion, as far as the lingula, is replaced by fibrous tissue, which persists to form the sphenomandibular ligament. Between the lingula and the canine tooth the cartilage disappears, while the portion of it below and behind the incisor teeth becomes ossified and incorporated with this part of the mandible.
Ossification takes place in the membrane covering the outer surface of the ventral end of Meckel’s cartilage and each half of the bone is formed from a single center which appears, near the mental foramen, about the sixth week of fetal life. By the tenth week the portion of Meckel’s cartilage which lies below and behind the incisor teeth is surrounded and invaded by the membrane bone. Somewhat later, accessory nuclei of cartilage make their appearance, viz., a wedge-shaped nucleus in the condyloid process and extending downward through the ramus; a small strip along the anterior border of the coronoid process; and smaller nuclei in the front part of both alveolar walls and along the front of the lower border of the bone. These accessory nuclei possess no separate ossific centers, but are invaded by the surrounding membrane bone and undergo absorption. The inner alveolar border, usually described as arising from a separate ossific center (splenial center), is formed in the human mandible by an ingrowth from the main mass of the bone. At birth the bone consists of two parts, united by a fibrous symphysis, in which ossification takes place during the first year.
Articulations.—The mandible articulates with the two temporal bones.
In the adult.
Side view of the mandible at different periods of life.
After birth the two segments of the bone become joined at the symphysis, from below upward, in the first year; but a trace of separation may be visible in the beginning of the second year, near the alveolar margin. The body becomes elongated in its whole length, but more especially behind the mental foramen, to provide space for the three additional teeth developed in this part. The depth of the body increases owing to increased growth of the alveolar part, to afford room for the roots of the teeth, and by thickening of the subdental portion which enables the jaw to withstand the powerful action of the masticatory muscles; but the alveolar portion is the deeper of the two, and, consequently, the chief part of the body lies above the oblique line. The mandibular canal, after the second dentition, is situated just above the level of the mylohyoid line; and the mental foramen occupies the position usual to it in the adult. The angle becomes less obtuse, owing to the separation of the jaws by the teeth; about the fourth year it is 140°.
In the adult the alveolar and subdental portions of the body are usually of equal depth. The mental foramen opens midway between the upper and lower borders of the bone, and the mandibular canal runs nearly parallel with the mylohyoid line. The ramus is almost vertical in direction, the angle measuring from 110° to 120°.
In old age the bone becomes greatly reduced in size, for with the loss of the teeth the alveolar process is absorbed, and, consequently, the chief part of the bone is below the oblique line. The mandibular canal, with the mental foramen opening from it, is close to the alveolar border. The ramus is oblique in direction, the angle measures about 140°, and the neck of the condyle is more or less bent backward.
Inferior nasal concha (paired) - thin plate that separate middle and inferior nasal meatus.
Hyoid bone is positioned in neck between mandible and larynx. It has body, large and mall horns.
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