Skull and Hyoid 



241 



The superficial, external, or orbital surface, on the whole slightly concave, has a 

 posterior part k'\vl with the os planum, separated by a lachrymal crest from the deep 

 fossa that holds the lachrymal sac and is continued down into the nasal duct. The 

 crest ends below in the hamular process, turning forwards and outwards and resting 

 on the maxilla, and making the outer margin of the opening. In rare instances the 

 hamular process is found extending forward to the orbital margin ; on the other hand, 

 the process may be absent. The Tensor tarsi arises from the crest and passes forward 

 to the outer side of the nasal sac, which it is supposed to compress on its contraction. 



The bone is sometimes perforated, and is occasionally represented by several 

 ossicles. Its complete absence has been recorded. 



Ossification commences in membrane before the end of the third month, on the 

 surface of the nasal capsule. It reaches the frontal within a few weeks, but does not 

 extend back to articulate with the os planum until some months later, as the posterior 

 or ethmoidal portion of the bone is the last to be formed. 



VOMER. 



A thin bony plate, composed of two fused laminae, and situated in the lower part 

 of the nasal septum. Its upper end is thickened to make two alee, which are splayed 



FIG. 190. Vomer seen from the right, with outline of palate indicated by an interrupted line. 

 A., area of palate bones supporting vomer ; U., region where it is carried on maxilla;. The 

 lower front corner, shown as a separate piece, projects slightly into the anterior palatine 

 foramen. The small figure shows schematically how the alae articulate with the lower surface 

 of the sphenoid and fit in between it and the vaginal processes of the internal ptyergoid plates. 



out under the body of the sphenoid and fit in between this and the vaginal plates of 

 the internal pterygoid processes. The bone is covered by mucous membrane on both 

 sides, and has, between it and the membrane, the naso-palatine nerve running forward 

 and making the naso-palatine groove on it. Its posterior margin, thin but rounded, 

 makes the free back edge of the bony septum, but this does not quite correspond with 

 the edge of the complete septum, for the mucous membrane that forms the edge stands 

 back from the bone (Fig. 190) for a little distance. The lower border rests on the septal 

 crest made by the palate bones and maxillae. The antero-superior margin supports the 

 perpendicular plate of the ethmoid in its upper part, and is usually fused with it after 

 middle life, but in its front part its two plates are distinct and enclose between them the 

 margin of the septal cartilage (see Fig. 193). 



F A 16 



