174 THE SKELETON. 



The Sigmoid Notch, separating the two processes, is a deep semilunar depres- 

 sion, crossed by the raasseteric artery and nerve. 



Development. This bone is formed at such an early period of life, before in- 

 deed, any other bone except the clavicle, that it has been found impossible at 

 present to determine its earliest condition. It appears probable, however, that 

 it is developed by two centres, one for each lateral half, the two segments 

 meeting at the symphysis, where they become united. Additional centres have 

 also been described for the coronoid process, the condyle, the angle, and the 

 thin plate of bone, which forms the inner side of the alveolus. 



CHANGES PRODUCED IN THE LOWER JAW BY AGE. 



The changes which the Lower Jaw undergoes after birth, relate 1. To the alterations effected 

 in the body of the bone by the first and second dentitions, the loss of the teeth in the aged 

 the subsequent absorption of the alveoli. 2. To the size and situation of the dental canal; and, 

 3. To the angle at which the ramus joins with the body. 



At birth (Fig. 125), the bone consists of two lateral halves, united by fibro-cartilaginon 

 sue, in which one or two osseous nuclei are generally found. The body is a mere shell of 

 containing the sockets of the two incisor, the canine, and the two temporary molar teeth, imper- 

 fectly partitioned from one another. The dental canal is of large size, and runs near the lower 

 border of the bone, the mental foramen opening beneath the socket of the first molar. The 

 angle is obtuse, from the jaws not being as yet separated by the eruption of the teeth. 



After birth (Fig. 126), the two segments of the bone become joined at the symphysis, from 

 below upwards, in the first year ; but a trace of separation may be visible in the beginning of the 

 second year, near the alveolar margin. The body becomes elongated in its whole length but 

 more especially behind the mental foramen, to provide space for the three additional teeth deve- 

 loped in this part. The depth of the body becomes greater, owing to increased growth of the 

 alveolar part, to afford room for the fangs of the teeth, and by thickening of the subdental por- 

 tion which enables the jaw to withstand the powerful action of the masticatory muscles ; but the 

 alveolar portion is the deeper of the two, and, consequently, the chief part of the body lies 

 above the oblique line. The dental canal, after the second dentition, is situated just above the 

 level of the mylo-hyoid ridge ; and the mental foramen occupies the position usual to it in the 

 adult. The angle becomes less obtuse, owing to the separation of the jaws by the teeth. 



In the adult (Fig. 127), the alveolar and basilar portions of the body are usually of equal depth. 

 The mental foramen opens midway between the upper and lower border of the bone, and the 

 dental canal runs nearly parallel with the mylo-hyoid line. The ramus is almost vertical in direc- 

 tion, and joins the body at nearly right angles. 



In old age (Fig. 128), the bone becomes greatly reduced in size; for, with the loss -of the teeth, 

 the alveolar process is absorbed, and the basilar part of the bone alone remains ; consequently 

 the chief part of the bone is below the oblique line. The dental canal, with the mental foramen 

 opening from it, is close to the alveolar border. The rami are oblique in direction, and the angL 

 obtuse. 



Articulations. With the glenoid fossae of the two temporary bones. 



Attachment of Muscles. To its external surface, commencing at the symphysis 

 and proceeding backwards: Levator Menti, Depressor Labii Inferioris, De 

 pressor Anguli Oris, Platysma Myoides, Buccinator, Masseter. To its intern; 

 surface, commencing at the same point: Genio-hyo-glossus, Genio-hyoideus 

 Mylo-hyoideus, Digastric, Superior Constrictor, Temporal, Internal Pterygoic 1 

 External Pterygoid. 



THE SUTURES. 



The bones of the cranium and face are connected to each other by means 

 Sutures. The sutures are rows of dentated processes of bone, projecting fror 

 the edge of either bone, and locking into each other; the dentations, however 

 are confined to the external table, the edges of the internal table lying mereb 

 in apposition. The Cranial Sutures may be divided into three sets: 1. Tliost 

 at the vertex of the skull. 2. Those at the side of the skull. 8. Those at tin 

 base. 



The sutures at the vertex of the skull are three ; the sagittal, coronal, anc 

 lambdoid. 



The Sagittal Suture (interparietal) is formed by the junction of the two pariet 

 bones, and extends from the middle of the frontal bone, backwards to the si 



