THE EN TIKE SKULL 483 



The Olfactory Fossa, for the olfactory lobes of the brain, is the 

 prolongation forward of the general cranial cavity over the sinus in 

 the body of the presphenoid, over the lower part of the nasal cavity, 

 under the sinus in the frontal bone, and under the upper part of the 

 nasal cavity (Fig. 392). It is quadrate, but compressed laterally, the 

 width being less by a quarter than the height. The greatest antero- 

 posterior length is slightly less than the height. 



The floor is faintly concave from side to side, and slopes slightly 

 downward in front (Fig. 390). It is formed by the coalesced pre- 

 sphenoid and orbitosphenoids. At its anterior end is the transverse 

 spheno-ethmoidal suture, and along each side is the anterior end of 

 the fronto-orbitosphenoidal suture. 



The lateral walls (Fig. 391) are almost flat, vertical, and parallel ; 

 they are formed by the vertical plates of the frontals. Along the 

 arcuate anterior edge of each wall is the jagged line of attachment of 

 the edge of the cribriform plate. Near the lower anterior angle of the 

 lateral wall are the internal openings of at least one of the ethmoidal 

 foramina from the orbit. 



The short roof is convex from before backward and concave from 

 side to side. In the middle line is a ridge formed by the lower edges of 

 the conjoined median plates of the frontals. The roof of the olfactory 

 fossa is also the floor of the frontal sinuses. 



The anterior wall is concave from above downward, and is pierced 

 by numerous foramina for the olfactory fibres. It is the cribriform 

 plate of the ethmoid, which has been already described (page 312). 



The Anterior Fossa occupies about the anterior third of the cranial 

 cavity (Figs. 390, 391, 392). It is easily distinguished from the 

 olfactory fossa in front and at its lower part from the middle fossa 

 behind, but at the upper part it appears to be confluent with it. The 

 posterior point on its roof, at the bregma, lies further forward than 

 the posterior point on the floor, and, as the anterior walls incline for- 

 ward, the whole fossa appears to be directed upward and forward. 



Its floor is on a higher plane than the floor of the succeeding 

 fossa3 ; it faces somewhat backward and upward, and is continuous in 

 front with the floor of the olfactory fossa. Its outline is that of the 

 sphenoid, namely, a hexagon, compressed from before backward, 

 bounded in front by the posterior limit of the floor of the olfactory 

 fossa and on each side by the fronto-sphenoidal sutures and behind 



