206 THE SKELETON. 



the subsequent absorption of the alveoli ; (2) to the size and situation of the dental canal; and (3) 

 to the angle at which the ranms joins with the body. 



At birth (Fig. 168) the bone consists of lateral halves, united by fibrous tissue. The body 

 is a mere shell of bone, containing the sockets of the two incisor, the canine, and the two tem- 

 porary molar teeth, imperfectly 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 (175), and the condyloid portion nearly in the same hori- 

 zontal line with the body ; the neck of the condyle is short, and bent backward. The coronoid 

 process is of comparatively large size, and situated at right angles with the rest of the bone. 



After birth (Fig. 169) the two segments of the bone become joined at the symphysis, from 

 below upward, 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 

 developed 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 

 portion, 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. 

 (About the fourth year it is 140.) 



In the adult (Fig. 170) 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 direction, and joins the body nearly at right angles. 



In old age (Fig. 171) 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 ; conse- 

 quently, the chief part of the bone is beloio 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, the 

 angle obtuse, and the neck of the condyle more or less bent backward. 



The Sutures. 



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

 Sutures. That is, the articulating surfaces or edges of the bones are more or less 

 roughened or uneven, and are closely adapted to each other, a small amount of 

 intervening fibrous tissue fastening them together. The Cranial Sutures may be 

 divided into three sets : 1. Those at the vertex of the skull. 2. Those at the side 

 of the skull. 3. Those at the base. 



The sutures at the vertex of the skull are three : the sagittal, coronal, and 

 lambdoid. 



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

 bones, and extends from the middle of the frontal bone backward to the superior 

 angle of the occipital. In childhood, and occasionally in the adult, when the two 

 halves of the frontal bone are not united, it is continued forward to the root of 

 the nose. This suture is sometimes perforated, near its posterior extremity, by 

 the parietal foramen ; and in front, where it joins the coronal suture, a space is 

 occasionally left which encloses a large Wormian bone. 



The Coronal Suture (fronto-parietafy extends transversely across the vertex of 

 the skull, and connects the frontal with the parietal bones. It commences at the 

 extremity of the greater wing of the sphenoid on one side, and terminates at the 

 same point on the opposite side. The dentations of the suture are more marked 

 at the sides than at the summit, and are so constructed that the frontal rests on 

 the parietal above, whilst laterally the frontal supports the parietal. 



The Lambdoid Suture (occipito-parietal], so called from its resemblance to the 

 Greek letter A, connects the occipital with the parietal bones. It commences on 

 each side at the mastoid portion of the temporal bone, and inclines upward to the 

 end of the sagittal suture. The dentations of this suture are very deep and dis- 

 tinct, and are often interrupted by several small Wormian bones. 



The sutures at the side of the skull extend from the external angular process 

 of the frontal bone to the lower end of the lambdoid suture behind. The anterior 

 portion is formed between the lateral part of the frontal bone above and the malar 

 and great wing of the sphenoid below, forming the fronto-malar and fronto- 



