256 



MAMMALIAN ANATOMY 



On each outer wall is a ridge which begins in front about half-way 

 down and runs backward and upward to the upper end of the median 

 partition on the superior border, thus forming a little oval pocket in 

 the superior external angle. To the ridge is attached a lower posterior 

 division of the ethmo-turbinal, or lateral ethmoid, which is known as 

 the spheno-turbinal. 



The interior of the presphenoid (Fig. 186) is almost entirely hollow, 

 and is divided by a longitudinal vertical partition into two sphenoidal 

 sinuses. The partition is often deflected from the middle line, in 

 which case one sinus is much wider than the other. The sinuses 



Posterior Wall of Right Frontal Sinus. \ 



FIG. 186. 



Frontals. 



Postorbital Process. 



Frontal Orbital Plate. - 

 Parietal, 



Orbitosphenoid. 



Part of Vertical Plate A 



of Palatine. 



Alisphenoid. 

 Orbitosphenoid and 

 Presphenoid. 



Cribriform Plate. 

 Line of Attachment of Lower Eth- 



moidal Scroll to Orbitosphenoid. 

 Septum between Sphenoidal 

 Sinuses. 



Lower Ethmoidal Scroll. 



Pterygoid. 

 THE SKULL. TRANSVERSE SECTION JUST BEHIND POSTERIOR NARES, SEEN FROM IN FRONT. 



are continued for a short distance only, or not at all, into the bases 

 of the small wings. Sometimes they do not extend all the way back, 

 but leave a small posterior part of the bone solid and composed of 

 cancellous tissue. They are narrowed by a transverse ridge in front 

 of the middle. The sinus in front of this constriction is occupied by 

 the spheno-turbinal. 



The posterior surface of the anterior sphenoid is small, circular, 

 and rough for attachment to the basisphenoid. 



The posterior sphenoid consists of two thin crescentic lateral 

 plates, the alisphenoids, or great wings of the sphenoid, which curve 

 upward, before and behind, and are united at the back part of their 



