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temporal bones. The anterior or horizontal part, which contains the teeth, is called the corpus mandibula (body); the posterior or vertical portions constitute the rami mandibulæ.
The body displays in the median plane, in front, a faint vertical ridge, the symphysis, which indicates the line of fusion of the two symmetrical halves from which the bone is primarily developed. Inferiorly this ridge divides so as to enclose, in well-marked specimens, a triangular area-the protuberantia mentalis (mental protuberance), the centre of which is somewhat depressed, thus emphasising the inferior angles, which are known as the tubera mentalia (mental tubercles). The lateral surface is crossed by a faint, elevated ridge, the linea obliqua (oblique line), which runs upwards and backwards from the mental tubercle to the lower part of the anterior border of the ramus, with which it is confluent. From this ridge arise the m. quadratus labiiinferioris and the triangular muscle. A littleabove this, midway between the upper and lower borders of the mandible, and in line with the root of the second premolar tooth, the bone is pierced by the mental foramen; this is the anterior opening of the the inferior
FIG. 164. THE MANDIBLE AS SEEN FROM THE LEFT SIDE.
4. Coronoid processes.
1. Mental tubercle.
8. Oblique line.
9. Mental foramen.
Through this aperture
alveolar canal, which traverses the body of the bone. the mental vessels and nerves reach the surface. The upper border supports the sixteen teeth of the mandible. It is thick behind and thinner in front, in correspondence with the size of the roots of the teeth. Anteriorly the sockets of the incisor and canine teeth produce a series of vertical elevations (juga alveolaria), of which that corresponding to the canine tooth is the most prominent. When this is outstanding it gives rise to a hollowing of the surface between it and the symphysis, often referred to as the incisor fossa; frequently, however, this is only faintly marked. Below the oblique line the bone is full and rounded, and ends below in the basis mandibulæ (inferior border). This slopes laterally at the sides, and forwards in front, where it is thick and hollowed out on either side of the symphysis to form the digastric fosse, to which the anterior bellies of the digastric muscles are attached; narrowing somewhat behind this, the base again expands opposite the molar teeth, and finally becoming reduced in width, terminates posteriorly at the angle formed between it and the posterior border of the ramus. The medial surface of the body is crossed by the mylohyoid line. This slants from above downwards and forwards towards the lower part of the symphysis. It serves for the origin of the mylohyoid muscle, and also, just behind the last molar tooth, furnishes an attachment to the superior constrictor of the pharynx. Below the posterior part of this ridge the surface is hollowed to form a fossa for the lodgment of the submaxillary gland. Above the anterior part of the mylo-hyoid line the bone is smooth and usually convex. Here the sublingual gland lies in relation to it. In the angle formed by the convergence of the two mylo-hyoid lines, and in
correspondence with the back of the lower part of the symphysis, there is a raised tubercle surmounted by two laterally placed spines, the mental spines. Occasionally these are again subdivided into an upper and lower pair, or it may be that the lower pair may fuse to form a rough median ridge. To the upper pair of spines the genio-glossi muscles are attached, whilst the lower pair serve for the origin of the genio-hyoid muscles. Immediately above the tubercle there is a median foramen for the transmission of a nutrient vessel, and close to the alveolar border opposite the intervals between the central and lateral incisors, there are two little vascular canals.
The ramus mandibulæ passes upwards from
the posterior part
of the body, forming by the junction of its posterior border with the base of the body the angulus mandibulæ (angle), which is usually rounded and more or less everted. The lateral surface of the ramus affords attachment to the masseter muscle, and when that muscle is powerfully developed the bone
FIG. 165. THE MEDIAL SIDE OF THE RIGHT HALF OF THE MANDIBLE.
Surface in relation to
5. Coronoid process.
10. Fossa for submaxillary gland.
is usually marked by a series of oblique curved ridges, best seen towards the angle. About the middle of the deep or medial surface is the large opening (foramen mandibulare) of the inferior alveolar canal, which runs downwards and forwards to reach the body, and transmits the inferior alveolar vessels and nerve. This aperture is overhung in front by a pointed scale of bone, the lingula mandibulæ, to the edges of which the spheno-mandibular ligament is attached. Behind the lingula and leading downwards and forwards for an inch or so from the opening of the inferior alveolar canal is the sulcus mylohyoideus (mylo-hyoid groove), along which the mylo-hyoid artery and nerve pass. Behind and below this groove the medial surface of the angle is rough for the attachment of the internal pterygoid muscle. Superiorly the ramus supports the coronoid process in front, and the condyloid process behind, the two being separated by the wide incisura mandibulæ (mandibular notch), over which there pass in the recent condition the vessels and nerve to the masseter muscle. The coronoid process, of variable length and beak-shaped, is limited behind by a thin curved margin, which forms the anterior boundary of the mandibular notch. In front its anterior edge is convex from above downwards and forwards, and becomes confluent below with the anterior border of the ramus and the oblique line. To the medial side of this edge there is a grooved elongated triangular surface, the medial margin of which, commencing above near the summit of the coronoid process, leads downwards along the medial side of the root of the last molar tooth towards the mylo-hyoid line. Behind this ridge the thickness of the ramus is much reduced. The temporal muscle is inserted into the margins and medial surface of the coronoid process. The posterior border of the ramus is continued upwards to support the capitulum mandibulæ (condyle), below which it is somewhat constricted to form the collum mandibula (neck), which is compressed from
7. Mandibular foramen.
11. Mylo-hyoid line.
before backwards, and bounds the mandibular notch posteriorly. To the medial side of the neck, immediately below the condyle, there is a little depression (fovea pterygoidea) for the insertion of the external pterygoid muscle. The convex surface of
the condyle is transversely elongated, and so disposed that its long axis is inelined nearly horizontally medio - laterally and little forwards. The convexity of the condyle is more marked in its anteroposterior than in its transverse diameter, and tends slightly to overhang the mandibular notch. The medial and lateral ends of the condyle terminate in tubercles which serve for the attachment of part of the articular capsule of the joint.
FIG. 166.-DEVELOPMENT OF THE MANDIBLE.
(In A and B Meckel's cartilage is coloured blue.)
Ossification.-Its development is intimately associated with Meckel's cartilage, the cartilaginous bar of the first visceral or mandibular arch. Meckel's cartilages, of which there are two, are connected proximally with the periotic capsule and cranial base. These distal ends meet, but do not fuse, in the region of the symphysis. Ossification takes place chiefly from membrane, in part from primordial cartilage (Meckel's cartilage), and also in part from accessory (metaplastic) cartilages, which have no connexion with Meckel's cartilage, but arise in the membrane from which the greater part of the bone is formed. Before ossification commences three structures are seen lying side by side in the mandibular arch of the embryo. These are, from medial to lateral side, Meckel's cartilage, the inferior alveolar nerve, which anteriorly divides, into its two terminal branches, viz., the incisor and mental nerves, and a dense connective tissue which stretches from before backwards from close to the mid-line anteriorly to near the acoustic region posteriorly. Ossification in membrane commences about the fortieth to forty-fifth day in the angle between the incisor and mental nerves; it extends rapidly backwards under the mental nerve, which grooves its upper surface, and is ultimately enclosed within the mental foramen. At the same time the outer alveolar wall is formed by the extension of this ossifying membrane bone, from which later, about the third month, is developed by backward growth the angle and ramus, the latter surmounted by a well-defined coronoid process. About the forty-fifth day the inner alveolar wall, the so-called splenial element, is formed by an ingrowth from the anterior part of the floor of the mental groove. This passes below the incisor nerve and passes up between it and Meckel's cartilage, which it subsequently overlaps, extending rapidly forwards and backwards to end posteriorly in the lingula anterior to the point of origin of the mylo-hyoid nerve. The mandible, in point of time, is the second bone to ossify, being preceded only by the clavicle. Ossification in Meckel's cartilage. This commences a little later than the first formation of the coronoid process, opposite the first and second incisor tooth germs, not by independent ossification, but by invasion of osteoblasts from the neighbouring membrane bone. The cartilage becomes surrounded by shelves of bone projected medially both above and below it from the main membrane bone. A bony tube is thus formed which extends from near the mid-line anteriorly to the second milk tooth posteriorly. Within these limits Meckel's cartilage becomes incorporated within the mandible. The extreme anterior end of the cartilage does not, however, undergo ossification, and the posterior end, save that part concerned in the formation of the malleus and incus, degenerates and ultimately disappears. Ossification in accessory cartilages.-These appear at the following sites: one,
a carrot-like mass, at the condyle; the large end forms the condyle; the tapering end is wedged into the ossifying ramus under the root of the coronoid process. This cartilage appears about the eleventh week. About the thirteenth week a strip of cartilage appears along the anterior border of the coronoid process. Along the anterior end of the alveolar walls close to the middle line, and turning down the symphysial surface of the mandible to end below in the region of the future digastric impression, another mass of cartilage appears about the fourteenth week. All the above cartilages are ossified by invasion from the surrounding membrane bone and are not therefore independent centres. is possible that the symphysial cartilages may be occasionally independently' ossified and thus give rise to the ossa mentalia when they exist. From what has been stated it thus appears that under normal conditions each half of the mandible ossifies from one centre only. The above account is based on the researches of Low and Fawcett.2
In a third or fourth month fœtus the cartilage can be traced from the under surface of the anterior part of the tympanic ring downwards and forwards to reach the jaw, to which it is attached at the opening of the mandibular canal ;' from this it may be traced forwards as a narrow strip applied to the medial surface of the mandible, which it sensibly grooves. The proximal end of this furrow remains permanently as the mylo-hyoid groove. The part of the cartilage between the tympanic ring and the mandible disappears, and its sheath becomes converted into fibrous tissue, and persists in the adult as the spheno-mandibular ligament, its proximal end being continuous, through the petro-tympanic fissure, with the slender process of the malleus, with the development of which bone it is intimately associated. I. Chaine (Comptes Rendus, Biologie, 1903) takes exception to this view and regards the spheno-mandibular ligament as the remnant of a muscular slip.
At birth the mandible consists of two halves united at the symphysis by fibrous tissue; towards the end of the first, or during the second year, osseous union between the two halves is complete. In infancy the mandible is shallow and the rami proportionately small; further, owing to the obliquity of the ramus, the angle is large, averaging about 150°. The mental foramen lies near the lower border of the bone. Coincident with the eruption of the teeth and the use of the mandible in mastication, the rami rapidly increases in size, and the angle becomes more acute. After the completion of the permanent dentition it approaches more nearly a right angle varying from 110° to 120°. The body of the bone is stout and deep, and the mental foramen usually lies midway between the upper and lower borders. As age advances, owing to the loss of the teeth and the consequent shrinkage and absorption of the alveolar border of the bone, the body becomes narrow and attenuated, and the mental foramen now lies close to the upper border. At the same time the angle opens out again (130° to 140°), in this respect resembling the infantile condition. In old age the coronoid process and the condyle form a more open angle with each other than in the young adult.
The hyoid bone, though placed in the neck, is developmentally connected with the skull. It lies between the mandible above and the larynx below, and is connected with the root of the tongue. Of U-shaped form, as its name implies (Greek v and eldos, like), it consists in the adult of a central part, or body, with which are united two long processes extending backwards-the greater cornua -one on each side. At the point where these are ossified with the body, the lesser cornua, which project upwards and backwards, are placed.
The body is arched from side to side and compressed from before backwards, so that its surfaces slope downwards and forwards. Its anterior surface displays a slight median ridge, on either side of which the bone is marked by the attachment of the mylo-hyoid muscles. Its posterior surface, deeply hollowed, is concave from side to side and from above downwards. Herein lie a quantity of
1 Journal of Anatomy and Physiology, vol. xliv. p. 82.
2 Graduation Thesis, Edinburgh, 1906.
fat and a bursa, which separates this aspect from the thyreo-hyoid membrane. The upper border, usually described with the anterior surface, is broad; it is separated from the anterior aspect by a transverse ridge, behind which are the impressions for the attachment of the genio-hyoid muscles. Its posterior edge is thin and sharp; to this, above, are attached the genio-glossi, whilst behind and below the thyreo-hyoid membrane is connected with it. The inferior border is well defined and narrow; it serves for the attachment of the omo-hyoid, sterno-hyoid, thyreohyoid, and stylo-hyoid muscles.
The greater cornua are connected on either side with the lateral parts of the body. At first, union is effected by synchondroses, which, however, ultimately Ossify. These cornua curve backwards, as well as upwards, and terminate in more or less rounded and expanded extremities. Compressed laterally, they serve for the attachments laterally of the thyreo-hyoid and hyo-glossi muscles, and the middle constrictor of the pharynx from below upwards, whilst medially they are connected with the lateral expansions of the thyreo-hyoid membrane, the free edges of which are somewhat thickened, and connect the extremities of the greater cornua with the ends of the superior cornua of the thyreoid cartilage below.
The lesser cornua, frequently cartilaginous in part, are about the size of grains of wheat. They rest upon the upper surface of the bone at the junctions of the greater cornua with the body. In youth they are separated from, but in advanced life become ossified with, the rest of the bone, from which they are directed upwards, backwards, and a little laterally. Their summits are connected with the stylohyoid ligaments; they also serve for the attachment of muscles.
Connexions.-The hyoid is slung from the styloid processes of the temporal bones by the stylo-hyoid ligaments. Inferiorly it is connected with the thyreoid cartilage of the larynx by the thyreo-hyoid ligaments and membrane. Posteriorly it is intimately associated with the epiglottis.
Ossification. In considering the development of the hyoid bone it is necessary to refer to the arrangement and disposition of the cartilaginous bars of the second and third visceral arches. That of the second visceral arch, the hyoid bar-or Reichert's cartilage, as it is sometimes called-is united above to the petrous part of the temporal, whilst ventrally it is joined to its fellow of the opposite side by an independent median cartilage. Chondrification of the third visceral arch only occurs towards its ventral extremity, forming what is known as the thyreo-hyoid bar. This also unites with the median cartilage above mentioned. In these cartilaginous processes ossific centres appear in certain definite situations. Towards the end of fœtal life a single centre (by some authorities regarded as primarily double) appears in the median cartilage, and forms the body of the bone (basihyal). About the same time ossification begins in the lower ends of the thyreo-hyoid bars, and from these the greater cornua are developed (thyreo-hyals). During the first year the lower ends of the hyoid bars begin to ossify and form the lesser cornua (cerato-hyals). The cephalic ends of the same cartilages meanwhile ossify to form the styloid process (stylohyal) on either side and one of the auditory ossicles called the stapes, whilst the intervening portions of cartilage undergo resorption and become converted into the fibrous tissue of the stylo-hyoid ligaments, which in the adult connect the lesser cornua with the styloid processes of the temporal bone. The greater cornua fuse with the body in middle life; the lesser cornua only at a more advanced period. Variations in the course of development lead to interesting anomalies of the hyoid apparatus. The lesser cornua may be unduly long or the stylo-hyoid ligament may be bony; in this case the cartilage has not undergone resorption, but has passed on to the further stage of ossification, thus forming an epihyal element comparable to that in the dog. The ossified stylo-hyoid ligament, as felt through the pharyngeal wall, may be mistaken for a foreign body. (Farmer, G. W. S., Brit. Med. Journ. 1900, vol. i. p. 1405.)
THE SKULL AS A WHOLE.
The skull as a whole may be studied as seen from the front (norma frontalis) from the side (norma lateralis), from the back (norma occipitalis), from above (norma verticalis), and from below (norma basalis).