THE INFERIOR MAXILLARY BONE. 141 
that muscle is powerfully developed the bone is usually marked by a series of 
oblique curved ridges, best seen towards the angle. About the middle of the deep 
or inner surface is the large opening (foramen mandibulare) of the inferior dental 
canal, which runs downwards and forwards to reach the body, and transmits the 
inferior dental vessels and nerves. This aperture is overhung in front by a pointed 
scale of bone, the lingula, to the edges of which the internal lateral hgament of the 
temporo-maxillary articulation is attached. Behind the lingula and leading down-- 
wards and forwards for an inch or so from the opening of the inferior dental canal 
is the mylo-hyoid groove (sulcus mylo-hyoideus), along which the mylo-hyoid artery 
and nerve pass. Behind and below this groove the inner 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 condyle behind, the two being 
separated by the wide sigmoid notch (incisura mandibule), 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 sigmoid notch. In front its 
anterior edge is convex from above downwards and forwards, and becomes con- 
fluent below with the anterior border of the ramus and the external oblique line. 
To the inner side of this ridge there is a grooved elongated triangular surface, 
the inner margin of which, commencing above near the summit of the coronoid 
process, leads downwards along the inner side of the root of the last molar tooth 
towards the internal oblique line. Behind this ridge the thickness of the ramus 
is much reduced. The temporal muscle is inserted into the margins and inner 
surface of the coronoid process. The posterior border of the ramus is continued 
upwards to support the condyle (capitulum mandibulze), below which it is some- 
what constricted to form the neck (collum mandibule), which is compressed from 
before backwards, and bounds the sigmoid hollow posteriorly. To the inner 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 inclined nearly horizontally from within outwards and a little forwards. 
The convexity of the condyle is more marked in its antero-posterior than in its 
transverse diameter. 
Architecture.—The mandible is remarkable for the density and thickness of its inner and 
outer walls. Where these coalesce below at the base of the body, the bone is particularly stout. 
Superiorly, where they form the walls of the alveoli, they gradually thin, being thicker, however, 
on the inner than the outer side, except in the region of the last molar tooth where the inner 
wall is the thinner. The cancellous substance is open-meshed below, finer and more condensed 
where it surrounds the alveoli. The inferior dental canal is large and has no very definite wall ; 
it is prolonged beyond the mental foramen to reach the incisor teeth From it numerous 
channels pass upwards to the sockets of the teeth, and it communicates freely with the 
surrounding cancellous tissue. Above the canal the substance of the bone is broken up by the 
alveoli for the reception of the roots of the teeth. In the substance of the condyle the cancellous 
tissue is more compact, with a general striation vertical to the articular surface. 
Variations —Considerable differences are met with in the height of the coronoid process: 
usually its summit reaches the same level as the condyle, or slightly above it ; occasionally, how- 
ever, 1t rises to a much higher level; in other cases it is much reduced. These differences 
naturally react on the form of the sigmoid notch. The projection of the mental protuberance 
is also liable to vary. Occasionally the mental foramen is double, and sometimes the mylo-hyoid 
groove is for a short distance converted into a canal. 
Ossification.—The development of the lower jaw 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. Their distal ends are united in the region of the symphysis. It is in the 
connective tissue overlying the outer surface of this cartilaginous arch that the bulk of 
the lower jaw is developed. The cartilage itself is not converted into bone, but undergoes 
resorption, except its anterior extremity, which is stated to undergo ossification to 
form the part of the jaw lying between the mental foramen and the symphysis. In a 
third or fourth month foetus the cartilage can be traced from the under surface of the 
fore part of the tympanic ring downwards and forwards to reach the jaw, to which it is 
attached at the opening of the inferior dental canal ; from this it may be traced forwards 
as a narrow strip applied to the inner surface of the mandible, which it sensibly grooves. 
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