50 INFERIOR MAXILLARY BONE. 



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. Immediately 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 foramen, for the exit of the inferior dental 

 nerve and artery, and below this foramen, 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 muscles. Near to the posterior 

 part of this surface is a rough impression made by the masseter 

 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 

 tubercles less marked and pointed, beneath them a ridge, and beneath 

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

 tachment from above downwards to the genio-hyo-glossi, genio-hyoidei, 

 part of the mylo-hyoidei and to the digastric muscles. Running out- 

 wards 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 constrictor muscle. Immediately above the ridge, and by 

 the side of the symphysis, is a smooth concave surface, which cor- 

 responds with the sublingual 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 in the angle of the 

 bone. 



The ramus is a strong square-shaped process, differing in direc- 

 tion at various periods of life ; thus, in the foetus and infant, it 

 is almost parallel with the body ; in youth it is oblique, and gradually 

 increases in the vertical direction until manhood ; in old age, after 

 the loss of the teeth, it again declines and assumes the oblique 

 direction. Upon its external surface it is rough, for the attachment of 

 of the masseter muscle ; and at the junction of its posterior border 

 with the body of the bone, is a rough tuberosity, the angle of the 



