Skull and Hyoid 



189 



most purely descriptive classifications, it therefore cannot be pushed to include every 

 detail, or even every skull bone, in its scope. 



A cursory examination of the skull reveals the fact that it is composed of several 

 bones which, with the exception of the lower jaw, are immovably articulated with 

 each other. This is most apparent when the complete skull is viewed from above 

 (norma verticalis) or from the side (norma lateralis), when the several bones are seen 

 at once articulating with each other along irregular lines, known as sutures* 



Before proceeding to a detailed examination of these bones and of the complete 

 skull it is advisable to become acquainted with the relative positions of the bones in 

 the skull. 



CRANIAL BONES. 



Examine the skull from above (Fig. 159). We perceive that the front part is formed 

 by a single bone, the frontal, behind which are the parietal bones, right agd left, placed 

 therefore laterally and above, and also markedly on the posterior aspect of the curved 

 skull. The pronto-parietal suture is also termed coronal, while the inter-parietal suture, 



wftw 



. sagittal suhre. 

 FIG. 159. Norma verticalis. Norma occipitalis. Norma frontalis. 



being in the middle line and running from before backwards, is usually termed the 

 sagittal suture : observe that these sutures are very serrated. 



Now turn to the posterior aspect of the skull (norma occipitalis) (Fig. 159), and 

 see that another bone, the occipital, comes up in a point between the two parietals, 

 so that the sagittal suture becomes continuous with two parieto-occipital sutures : 

 these two together constitute the lambdoid suture, so termed on account of the figure 

 formed by them. Small additional ossicles, known as Wormian bones, are frequently 

 found in this suture, as also in many others in the skull, and may in exceptional cases 

 number some hundreds : they are due to accessory centres of ossification occurring 

 where the normal centres have failed to meet and form a normal suture. 



Now look at the front of the skull (norma frontalis), and observe that the frontal 

 bone alone forms this wall of the cranial cavity. It comes down to make the upper 

 margins of the openings of the orbits, and turns back here to form the greater part of 

 each roof of these cavities, so that the bone may be said to possess two orbital plates 

 in the roof of the orbit, in addition to the large frontal plate. There is often a suggestion 

 of a suture, more or less complete, running up the frontal plate in the middle line, 



* In skulls from subjects of advanced years the sutures are frequently obliterated in part or altogether. 



