INFERIOR MAXILLARY BONE. 81 



border is more or less deeply grooved for the reception of the central la- 

 mella of the ethmoid and the cartilage of the septum. This groove is an 

 indication of the early constitution of the bone of two lamellae, united at 

 the inferior border. The vomer not unfrequently presents a convexity to 

 one or the other side, generally, it is said, to the left. 



Development. By a single centre, which makes its appearance at the 

 same time with those of the vertebrae. Ossification begins from below and 

 proceeds upwards. At birth, the vomer presents the form of a trough, in 

 the concavity of which the cartilage of the septum nasi is placed ; it is this 

 disposition which subsequently enables the bone to embrace the rostrum of 

 the sphenoid. 



'A i*e vomer has no muscles attached to it. 



^reticulations. With six bones: the sphenoid, ethmoid, two superior 

 maxillary, and two palate bones, and with the cartilage of the septum. 



INFERIOR MAXILLARY BONE. The lower jaw is the arch of bone which 

 contains the inferior teeth ; it is divisible into a horizontal portion or body, 

 and a perpendicular portion, the ramus, at each side. 



Upon the external surface of the body of the bone, at the middle line, 

 and extending from between the two first incisor teeth to the chin, is a 

 slight ridge, crista mentalis, which indicates the point of conjunction of 

 the lateral halves of the bone in the young subject, the symphysis. Im- 

 mediately external to this ridge is a depression which gives origin to the 

 depressor labii inferioris muscle ; and, corresponding with the root of the 

 lateral incisor tooth, another depression, the incisive fossa, for the levator 

 labii inferioris. Further outwards is an oblique opening, the mental fora- 

 men, for the exit of the inferior dental nerve and artery ; and below this 

 foramen is the commencement of an oblique ridge which runs upwards 

 and outwards to the base of the coronoid process and gives attachment to 

 the depressor anguli oris, platysma myoides, and buccinator muscle. Near 

 the posterior part of this surface is a rough impression made by the mas- 

 seter muscle ; and immediately in front of this impression, a groove may 

 occasionally be seen for the facial artery. The projecting tuberosity at the 

 posterior extremity of the lower jaw, at the point where the body and ramus 

 meet, is the angle. 



Upon the internal surface of the body of the bone, at the symphysis, are 

 two small pointed tubercles ; immediately beneath these, two other tuber- 

 cles, less marked and pointed ; beneath them a ridge, and beneath the 

 ridge two rough depressions of some size. These four points give attach- 

 ment, from above dow r nwards, to the genio-hyo-glossi, genio-hyoidei, part 

 of the mylo-hyoidei, and to the digastric muscles. Running outwards 

 into the body of the bone from the above ridge is a prominent line, the 

 mylo-hyoidean ridge, which gives attachment to the mylo-hyoideus muscle, 

 and by its extremity to the pterygo-maxillary ligament and superior con- 

 strictor muscle. Immediately above the ridge, and by the side of the 

 symphysis, is a smooth concave surface, which corresponds with the sub- 

 lingual gland ; and below the ridge, and more externally, a deeper fossa 

 for the submaxillary gland. 



The superior border of the body of the bone is the alveolar process, 

 furnished in the adult with alveoli for sixteen teeth. The inferior border 

 or base is rounded and smooth ; thick and everted in front to form the 

 chin, and thin behind where it merges into the angle of the bone. 



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