« PrécédentContinuer »
developed, lies close below the infra-orbital groove, and it is not till later that they become separated by the growth of the maxillary sinus, which first makes its appearance as a shallow fossa to the medial side of the orbito-nasal element about the fourth month. In the adult bone the course of the infra-orbital canal and foramen indicates the line of fusion of the orbito-nasal and zygomatic elements, whilst the position of the anterior palatine canal serves to determine the line of union of the incisive with the palatine elements. In addition to the foregoing centres, Rambaud and Renault describe another, the infra-vomerine, which, together with its fellow, is wedged in between the incisive and the palatine elements beneath the vomer, thus explaining the Y-shaped arrangement of the foramina of Stensen, which open into the incisive foramen.
The premaxillæ, which in most vertebrates are independent bones lying in front of the maxillæ, constitute in man and apes the portions of the maxilla which lie in front of the incisive foramen, and support the superior incisor teeth. Each premaxilla is developed from two centres: a facial, which ultimately contains the incisor and canine teeth, and forms the anterior part of the hard palate, as well as the anterior half of the frontal process of the complete maxilla (Fawcett); and a palatine centre (infra vomerine of Rambaud and Renault) which forms the medial wall of the corresponding canal of Stensen. The former develops very early, either before or after the maxilla (Mall), and fuses almost at once with the maxilla along the alveolar margin; the latter appears about the twelfth week, and soon fuses with the facial centre. The line of fusion of the premaxillæ with the maxillæ proper can be readily seen in young skulls, and occasionally also in the adult. It corresponds to a suture which passes on the palate obliquely laterally and forwards, from the incisive foramen to the interval between the lateral incisor and the canine tooth. In cases of alveolar cleft palate the adjacent bones fail to unite along the line of the suture. In some instances, however, the cleft passes outwards between the central and lateral incisor teeth, and this condition suggests the explanation that the premaxillary element is derived from two centres-a lateral and a medial. The researches of Albrecht and Warinski support this view. The latter anatomist further observes that the lateral cleavage may lead to a division of the dental germ of the lateral incisor tooth, and so explain the occurrence of the supernumerary incisor which is occasionally met with. this way the different varieties of cleft palate are readily explained; median cleft palate being due to failure of union between the two premaxillary bones. Lateral cleft palate may be of two types: the cleft in one case passing forwards between the central and lateral incisor, and being due to the non-union of the two elements from which the premaxilla is primarily developed; the other, in which the cleft passes between the lateral incisor and the canine, or between the lateral incisor and a supernumerary incisor, owing to the imperfect fusion of the premaxilla laterally with the maxilla.
FIG. 159.-OSSIFICATION OF THE MAXILLA. A, Lateral side; B, Medial side; C, Under side. Nasal process; b, Orbital plate; c, Anterior nasal spine; d, Infra-orbital groove; e, Infra-orbital foramen; f, Anterior palatine groove;
g, Palatine process; h, Premaxillary suture; i, Alveolar process.
The palate bone, of irregular shape, assists in the formation of the lateral wall of the posterior part of the nasal cavity, the posterior portion of the hard palate, the orbit, the pterygo-palatine, the infra-temporal, and the pterygoid fosseæ. It consists of horizontal and vertical parts, united to each other like the limbs of the letter L. At their point of union there is an irregular outstanding process, called the pyramidal process, whilst capping the summit of the vertical part and separated by a deep cleft are two irregular pieces of bone, called the sphenoidal and orbital processes.
The pars horizontalis (horizontal part) has two surfaces and four borders. As its name implies, it is horizontal in position, and forms the posterior third of the hard palate. Its superior surface, which is smooth, is slightly concave from side to side, and forms the floor of the posterior part of the nasal cavity. Its inferior surface, rougher, is directed towards the mouth, and, near its posterior edge, often displays a transverse ridge for the attachment of a part of the aponeurosis of the tensor veli palatini muscle. The anterior border articulates by means of an irregular suture with the posterior edge of the palatine process of the maxilla. The posterior margin is free and concave from side to side; by its sharp edge it furnishes attachment to the aponeurosis of the soft palate. The medial border is upturned, and when it articulates with its fellow of the opposite side it forms superiorly a central crest continuous in front with the nasal crest of the maxilla; it supports the posterior part of the inferior border of the vomer, and projecting beyond the line of the posterior border forms the posterior nasal spine. The lateral border fuses with the vertical part, forming with it a right angle. The posterior extremity of this edge is grooved by the foramen palatinum majus.
The pars perpendicularis (perpendicular part) is very much broader below than above. Composed of thin bone, particularly at its superior part, it is liable to be broken in the process of disarticulation, so that it is somewhat uncommon to meet with a perfect specimen. It may be described as possessing two surfaces and four borders. Its medial surface, which is directed towards the cavity of the nose, is crossed horizontally, about its middle, by the crista conchalis (conchal crest) with which the posterior end of the superior border of the inferior conchal bone articulates; above and below this, it enters into the formation of the lateral wall of the middle and inferior meatuses of the nose, respectively. Near the superior extremity of the perpendicular part, and below the processes which spring from it, there is another ridge more or less parallel to that already described. This is the crista ethmoidalis (ethmoidal crest), and with this the posterior extremity of the middle concha articulates. The lateral surface, which forms the medial wall of the pterygopalatine fossa, is channelled by a vertical groove (sulcus pterygopalatinus), converted into the pterygo-palatine canal by articulation with the maxilla. This canal, called at its lower end the greater palatine foramen, transmits the posterior palatine nerve and greater palatine vessels. Anteriorly the lateral surface projects forwards to a variable extent, and helps to close in the maxillary sinus by its maxillary process. The anterior border is a thin edge, of irregular outline, which articulates above with the ethmoid, with the posterior edge of the maxillary process of the inferior conchal bone about its middle, and below with the maxilla. The posterior border, thin above,
where it articulates with the anterior part of the medial pterygoid lamina, expands
For medial pterygoid lamina
For lateral pterygoid lamina
The processus pyramidalis (pyramidal process) is directed backwards and laterally from the angle formed by the perpendicular and horizontal parts, and presents, on its posterior surface, a central
smooth vertical groove, bounded on each side by rough articular furrows which unite above in a V-shaped manner with the upper thin posterior edge. These latter articulate with the anterior parts of the lower portions of the medial and lateral pterygoid laminæ, while the central groove fits into the wedge-like interval between the two pterygoid laminæ, thus entering into the formation of the pterygoid fossa. The lateral surface of the pyramidal process is rough above, where it is confluent with the lateral surface of the perpendicular part which articulates with the tuberosity of the maxilla; below, there is a small, smooth, triangular area which appears between the tuberosity of the maxilla and the lateral surface of the lateral pterygoid lamina, and so enters into the medial wall of the infra-temporal fossa. Passing through the pyramidal process in a vertical direction are the foramina palatina minora (lesser palatine foramina) for the transmission of the lesser palatine nerves and vessels.
FIG. 161. THE RIGHT PALATE BONE.
The processus orbitalis (orbital process), shaped like a hollow cube, surmounts the anterior part of the vertical plate. The open mouth of the cube is usually directed backwards and medially towards the anterior part of the body of the sphenoid, with the cavity of which it commonly communicates; the anterior part of the cube articulates with the medial end of the angle formed by the orbital plate and infra-temporal surface of the maxilla. Of the remaining four surfaces, one directed forwards medially articulates with the ethmoid. The others are non-articular: the superior enters into the formation of the floor of the orbit; the lateral is directed towards the pterygo-palatine fossa; whilst the inferior, which is confluent with the medial surface of the vertical part, is of variable extent, and overhangs the superior meatus of the nose.
The processus sphenoidalis (sphenoidal process), much smaller than the orbital, curves upwards, medially, and backwards from the posterior part of the summit of the perpendicular part. Its superior surface, which is grooved, articulates with the anterior part of the inferior surface of the body of the sphenoid and the root of the medial pterygoid lamina, thereby converting the groove into the pharyngeal canal, which transmits an artery of the same name together with a pharyngeal branch from the spheno-palatine ganglion. Its lateral side enters into the formation of part of the medial wall of the pterygo-palatine fossa. Its medial curved aspect is directed towards the nasal cavity, whilst its medial edge is in contact with the ala of the vomer.
Connexions. The palate bone articulates with its fellow of the opposite side, with the ethmoid, vomer, sphenoid, maxilla, and inferior concha.
Ossification. The palate bone ossifies in membrane at the side of the nasal segment of the bucco-nasal cavity, medial to the descending palatine nerves, at a time when each half of the developing palatine shelf is hanging down by the side of the tongue. When the palatine shelf becomes horizontal, as it does in the fifth week, bone extends into it to form the horizontal plate. From this common centre all parts of the palate bone develop but the orbital process may be ossified from an independent centre, which either fuses with the palate bone, or with the sphenoid, or with the ethmoid.
The zygomatic bone (O.T. malar) underlies the most prominent part of the cheek, and is hence often called the cheek-bone. Placed to the lateral side of the orbital cavity, it forms the sharp lateral border of that hollow, and serves to separate that space from the temporal and infra-temporal fosse which lie behind; below, it rests upon and is united to the maxilla; behind, it enters into the formation of the zygomatic arch, which bridges across the temporal fossa.
As viewed from the lateral side, the bone is convex from side to side, and has four processes, of which three are prominent. These are the fronto-sphenoidal
FIG. 162.-THE RIGHT ZYGOMATIC BONE. A, Lateral Side; B, Medial Side.
(processus frontosphenoidalis), the marginal or pointed extremity of the maxillary border, and the temporal (processus temporalis). The most elevated part of the convex malar surface (facies malaris) forms the malar tuberosity.
The temporal process ends posteriorly in an oblique edge, which articulates with the extremity of the zygomatic process of the temporal bone. The frontosphenoidal process, the most prominent of the three, is united superiorly to the zygomatic process of the frontal bone. The edge between the frontal and temporal processes is thin and sharp; it affords attachment to the temporal fascia, and near its upper end there is usually a pronounced angle (processus marginalis), formed by a sudden change in the direction of the border of the bone. It is just below this point that the zygomatico-temporal branch of the zygomatic nerve becomes cutaneous. The inferior margin of the temporal process is somewhat thicker and rounded; it extends downwards and forwards towards the inferior angle, where the bone articulates with the maxilla, and is there confluent with the ridge which separates the facial from the infra-temporal aspect of the maxilla. This edge of the bone is sometimes called the masseteric border, since it affords attachment to the fibres of origin of the masseter muscle. Sweeping downwards, in front of the fronto-sphenoidal process, is a curved edge which terminates inferiorly in a pointed process. This border forms the lateral and, in part, the inferior margin of the orbital cavity. Between the anterior extremity of the masseteric edge and the pointed anterior angle there is an irregular suture by which the bone is joined to the maxilla. The opening of the foramen zygomaticofaciale (zygomatico
facial foramen) is seen on the lateral surface of the bone; its size and position are very variable.
The medial aspect of the bone is distinguished by a curved elevated crest, called the orbital process, which extends medially and backwards, and is confluent laterally with the orbital margin. This process has two surfaces-one anterior, which forms a part of the lateral and lower wall of the orbit, and one posterior, which is directed towards the temporal fossa above and the infra-temporal fossa below. The free edge of the orbital process is thin and serrated; a little below its middle it is usually interrupted by a non-articular notch, which corresponds to the anterior extremity of the inferior orbital fissure. The part above this articulates with the great wing of the sphenoid, the portion below with the orbital surface of the maxilla. Behind the orbital process the medial surface of the bone is concave from side to side, and extends backwards along the medial aspect of the temporal process and upwards over the posterior half of the medial side of the frontal process, thus entering into the formation of the infra-temporal and temporal fossæ respectively. The orbital surface of the orbital process usually displays the openings of two canals (foramina zygomatico-orbitalia)-one which traverses the bone below the orbital margin and appears on the front of the bone as already described, the other which passes obliquely upwards and laterally through the orbital process and appears in the temporal fossa, to the medial side of the frontal process (foramen zygomaticotemporale). The former transmits the zygomatico-facial branch, the latter the zygomatico-temporal branch of the zygomatic nerve. Just under the orbital margin and a short distance below the zygomatico-frontal sutures there is usually a small tubercle serving for the attachment of the lateral palpebral raphe. (Whitnall, Journ. Anat. and Physiol., vol. xlv.)
Below the orbital process there is a rough triangular area, bounded laterally by the maxillary border. This articulates with the zygomatic process of the maxilla, and occasionally forms the lateral wall of the maxillary sinus.
Connexions. The zygomatic bone articulates with the frontal, sphenoid, maxilla, and temporal bones.
Ossification. The zygomatic ossifies in membrane. Its basis appears about the tenth week as a thin ossifying lamina which corresponds to the orbital margin, attached to which there is a backward expansion corresponding to the body of the bone; from this posteriorly there extends the element of the temporal process. On the medial side, and lying within the angle formed by the orbital and temporal elements, there appears a secondary thickening, which develops into a cup-shaped layer which fits into the recess and ultimately forms the surface of the bone directed to the temporal fossa. Below the orbital margin on the medial side, and extending backwards towards the temporal process, is another secondary thickening, which forms a club-shaped nodule, the thick end of which is directed forwards, whilst posteriorly it forms, in part, the lower margin of the body and temporal process. The overlap of these several parts leads to the formation of grooves which may persist in the adult as sutures. (Karl Toldt, junr., Sitzsbr. des Akad. des Wiss., Wien, July 1902.) Regarding the ossification of this bone there are great differences of opinion; not a few anatomists describe it as developed from a single centre. Support, however, is given to its origin from multiple centres owing to the frequency with which in the adult it is met with in a divided condition.
The mandible or lower jaw, of horse-shoe shape, with the extremities upturned, is the only movable bone of the face. Stout and strong, it supports the teeth of the lower dental arch, and articulates with the base of the cranium, by the joints, on either side, between its condyles and the mandibular fossæ of the