Skull and Hyoid 



267 



and lambda respectively, and the anterior and posterior lateral at the plerion and asterion 

 (Fig. 212). The bregmatic fontanelle is four-sided, between the two parietals and two 

 halves of the frontal ; the lambdoid is triangular, between occipital and parietals ; 

 and the two lateral intervals are irregular in shape, that at the asterion sometimes 

 extending back for a little distance between the post- and supra- occipital. 



The lateral fontanelles are as a rule practically, although not actually, closed at 

 birth, and the posterior median interval closes shortly after : the bregmatic fontanelle, 

 however, remains open and 

 palpable from the surface till 

 about the end of the second 

 year, and the observation of 

 its condition is of some clinical 

 value. The various bones of 

 the vault come into complete 

 contact by the age of four or 

 five ; in the case of the frontal 

 the two halves are partly joined 

 at that age : occasionally a 

 bone fails to meet its neighbour 

 and the interval is filled by an 

 accessory centre, constituting 

 a Wormian bone. Such true 

 Wormian bones are to be distin- 

 guished from false Wormian 

 bones, which are normal centres 

 that have abnormally failed to 

 join the bone with which they 

 should be fused. 



Fontanelles are not con- 

 fined to the regions mentioned 

 above, for membranous inter- 

 vals sometimes occur elsewhere 

 wherever parts of the bone do 

 not come together adequately 

 at their usual time : thus a 

 metopic fontanelle may be seen 

 at the root of the nose at the 

 lower end of the metopic suture, 

 an occipital one may occur at 

 the hinder margin of the fora- 



men magnum between the 



halves of the post-occipital, 

 or a parietal (sagittal) fontanelle may mark the site of the parietal foramen. 

 There are many things to be noticed about the skull at birth in addition to the fon- 

 tanelles. In Fig. 212 the skull at birth is shown, and it plainly differs from the adult 

 structure. The most striking thing about the foetal skull is the comparative smallness 

 of the face and largeness of the cranium, but another point of distinction can be found 

 by measurement : a line drawn through the condyles practically divides the fcetal 

 base into two equal halves, whereas in the adult the part that lies in front of the line 

 is proportionately much increased and gives a ratio of 17 : i instead of i : i, as in the 



Lamldoid. 

 fmbntllt 



ring. 



FIG. 212. Foetal skull, about the time of birth. The frontal 

 region is artificially depressed to some extent. 



