Skull and Hyoid 249 



palate shows cell walls, and the middle turbinate articulates with its lower and inner 

 part. 



The roof of the nasal fossa is highest in the ethmoidal region and slopes down in 

 front of and behind this. It is made (Fig. 193) from behind forwards by the sphenoid, 

 the cribriform plate of the ethmoid, nasal area of frontal, nasal bone, and finally by the 

 junction between the cartilaginous alae and septum. The student should follow the 

 formation of the roof with care, especially in the region of the frontal bone : the nasal 

 area of this bone is situated beside the nasal spine (see Fig. 199). The nasal nerve 

 comes through the roof of the cavity between the frontal and ethmoidal parts of the 

 roof, so that the nerve runs down below the frontal and nasal bones, to emerge once 

 more between this last bone and the cartilaginous part of the roof. The olfactory fibres 

 pierce the ethmoidal (cribriform) part of the roof. 



The floor, made by the horizontal processes of palate and maxilla, is concave from 

 side to side, and also in some degree from before backwards, owing to the higher level 

 of the premaxillary region ; the curve of the floor is continuous with that of the inferior 

 meatus at the side. The general level of the cavities of the nose and the maxillary 

 sinus is about the same, but the front part of the nasal floor may be rather higher than 

 the front part of the floor of the sinus. 



The posterior nares are the hinder openings of the fossae ; they are separated from 

 each other by the posterior edge of the vomer. Each opening has the vomer as its 

 inner boundary, the internal pterygoid plate for its outer wall, the horizontal plate of the 

 palate below, and the body of the sphenoid above. But it must be noticed that the 

 ala of the vomer and the inner pterygoid plate meet below the sphenoid and are therefore 

 concerned in forming the roof of the opening. 



The anterior opening is a common one for both fossae unless the septal cartilage 

 is in place. It is often termed the " pyriform aperture " owing to its shape, and is 

 bounded laterally and below by the maxillae and by the nasals above ; its lower edge 

 is produced centrally into the (anterior) nasal spine. 



A slight ridge may be visible just behind the margin at the infero-lateral angle, 

 more apparent when the proper margin turns down, as it sometimes does, to be lost 

 on the front surface of the premaxilla ; the ridge thus seen lies over the course of the 

 anterior dental (incisive) nerve, and here minute branches can pierce the bone to 

 supply the front end of the inferior meatus. The fossette (variable in size and defini- 

 tion) between the ridge and the true margin is seen well in lower races. The supero- 

 lateral edges have the lateral cartilages attached to them by fibrous tissue. The lower 

 edge is free and covered by the tissues of the lower margin of the narial aperture. 



The width of the aperture varies considerably in different races ; compared with 

 the height it is least in Europeans and greatest in certain Negro races. It is pro- 

 portionately broader in the infant than in the adult, owing to the shallowness of the 

 maxilla at birth. 



Sphenoidal Sinus. Variable in extent in different individuals and usually on different sides in 

 any one individual. The variations, and the situations of the different incomplete bony septa 

 found within the cavities, may be largely explained by reference to developmental points. There 

 are paired centres for the presphenoid, situated in front of paired centres for the basisphenoid : the 

 centres for the linguhc lie outside these last and separate them from the inner parts of the great 

 wings, which, however, come into relation, in front of the lingulae, with the presphenoidal centres. 

 Each sinus, growing back from the spheno-ethmoidal region, comes first into relation with the pre- 

 sphenoidal part and hollows this out, the condensed bone between the two ossific areas remaining 

 as a median vertical septum. If now each cavity extends further back, equally on the two sides, 

 deeper sinuses result : these involve the postsphenoid centres, and, being equal growths, still main- 

 tain a median antero-posterior septum in their deeper parts, formed by the condensation between 



