THE SKULL THE CEANIUM 257 



inner edges by a central, flattened, transverse piece, the basisphenoid. 

 From each alisphenoid, at its point of union with the basisphenoid 

 in front, a plate-like horizontal process bearing secondary smaller 

 processes projects downward and forward ; it also projects under the 

 anterior sphenoid, and is, in part, united with it : this is the ptery- 

 goid : process. The posterior sphenoid can be resolved into its parts, 

 but usually the three pieces are found firmly united. The basisphe- 

 noid is the central triangular piece. The smaller front margin lies 

 between the pterygoid processes, and the posterior margin is composed 

 of as much of the posterior margin of the bone as lies between the 

 small notches lateral to the large notches ; the lateral borders, there- 

 fore, are directed from in front obliquely backward and outward. 



In many animals the pterygoids persist throughout life as distinct 

 bones, but in the cat their union with the alisphenoid takes place so 

 early that, except in very young specimens, it is impossible to find 

 even sutures or lines which will indicate their boundaries. In the 

 following description, therefore, they will be treated as processes of 

 the alisphenoids. 



The posterior sphenoid presents a superior and an inferior surface, 

 and anterior, posterior, and lateral borders. 



The superior surface (Fig. 187) comprises the superior surfaces 

 of the basisphenoid and of the alisphenoids. It consists of two parts, 

 a larger, central, flattened portion, principally basisphenoidal, and a 

 curved, concave, lateral portion, wholly alisphenoidal. At the middle 

 of the anterior border the superior surface is elevated into a transverse 

 tuberosity which in front, by its quadrate, roughened, anterior surface, 

 articulates with the back of the presphenoid just within the anterior 

 clinoid processes. The posterior surface of this elevation slopes down- 

 ward, backward, and on each side slightly outward to the plane of the 

 general superior surface. It is concave in the centre longitudinally 

 and transversely. At about the middle of the entire surface is a 

 second transverse plate-like elevation which is slightly wider than the 

 olivary eminence. The anterior surface of this elevation is convex 

 from side to side, except just in the middle, where there is a slight 

 depression. It is strongly concave from above downward. The upper 

 edge overhangs the surface, which thus faces downward and forward. 

 This elevation is called the dorsum sellae. The fossa included between 



1 From (Gr.) pterux, a wing, and eides, like. 



17 



