Skull and Hyoid 



269 



choanae, like the anterior openings, are proportionately less high than in the adult, 

 and their plane is more nearly horizontal. 



The mandible is in two halves joined by fibrous tissue. Each half shows a short 

 ramus making a very large angle (175 degrees) with the body. The body is poorly 

 developed both in its alveolus and its splenial portion, and presents prominences that 

 mark the position of the dental sacs in the bone. The coronoid process is higher 

 with reference to the condyle than in the adult. 



The growth of the alveolar regions in both jaws depends on dentition, and thus 

 its full development in each is not reached before the twentieth year or thereabouts, 

 although the temporary dentition brings about a rapid growth in the first five or six 

 years of life. The space required by the teeth and the increasing height of the maxillae 

 push the mandible down, and in this way lead (Fig. 209) to the lessening of the angle 

 between ramus and body in the lower jaw, in order to keep the teeth in apposition ; 

 the angle at the first dentition is about 140 degrees and in adult jaws about 120 degrees. 

 Thus the appearance of an angle in the 

 mandible may to some extent be also looked 

 on as a result of the later growth of the upper 

 jaws. The body of the mandible increases 

 in length proportionately in keeping with 

 the antero-posterior growth of the face, and 

 in this way provides room for the three 

 permanent molars. 



The smallness of the maxilla is reflected 

 in the orbit, where the position of the two 

 main fissures is decidedly lower, and the 

 spheno-maxillary fissure is also much wider, 

 than in the adult : as the maxilla grows the 

 fissure is narrowed, and at the same time the 

 malar region is pushed and rotated into a 

 more external position, thus raising the 

 levels of the fissures. 



The frontal bone, in two halves, is seen 

 above the face : each half shows a marked frontal eminence from which ridges radiate 

 on the surface of the bone. 



Among numerous other minor details of the front aspect it may be pointed out 

 that the lower edges of the nasal bones are straight and not notched or serrated as 

 in the adult, and that the lachrymal may be somewhat rotated so that its surface looks 

 forwards as well as outwards. 



The lateral view of the new-born skull shows well-marked parietal eminences, 

 where the skull is broadest. At a lower level the junction of bones at the asterion 

 and pterion may be more or less incomplete (see previous reference to fontanelles). 

 The temporal squama is not fused with the underlying petrous, nor is there any appear- 

 ance of a mastoid process : the tympanic plate is represented by the tympanic ring, 

 so that the membrane is practically on the surface of the skull.* The ring is fused with 

 the squama in part, and its circumference is not much less than that of the adult mem- 

 brane, so that this membrane is proportionately larger in the newly-born. The want 

 of development of the mastoid and tympanic plate leaves the stylo-mastoid foramen 



FIG. 214. Section through foetal skull. 



* This does not mean that the membrane is visible in the living infant from the surface, for the outer 

 fibrous and cartilaginous meatus is of proper proportionate length and thus conceals the membrane. 



