22 THE ANATOMY OF VEETEBRATED ANIMALS. 



floor. The ossific deposit, nearest the occipital foramen, 

 becomes the hasi-occipital bone ; that which takes place' in 

 the floor of the pituitary fossa becomes the basisphenoid; 

 that which appears in the reunited trabeculse, in front of the 

 fossa, gives rise to the presphenoicl. Again, in front of, and 

 outside, the cranial cavity, the ethmoid may be represented 

 by one or more distinct ossifications. 



An ossific centre may appear in the cartilage on each 

 side of the occipital foramen, and give rise to the ex-occipital; 

 and above it, to form the supra-occipital. The four occi- 

 pital elements, uniting together more or less closely, com- 

 pose the occipital segment of the skull. 



In front of the auditory capsules and of the exit of the 

 third division of the fifth nerve, a centre of ossification 

 may appear on each side and give rise to the alisphenoid ; 

 which, normally, becomes united below with the basi- 

 sphenoid. 



In front of, or above, the exits of the optic nerves, the 

 orbitosphenoidal ossifications may appear and unite below 

 with the presphenoid. 



In front of the occipital segment, the roof of the skull is 

 formed by membrane ; and the bones which complete the two 

 segments of which the basisphenoid and presphenoid form 

 the basal parts, are membrane bones, and are disposed in 

 two pairs. The posterior are the parietals, the anterior the 

 frontals; and the segments which they complete are re- 

 spectively called pametal and frontal. Thus the walls of the 

 cranial cavity in the typical ossified skull are divisible into 

 three segments — I. Occipital, II. Parietal, III. Frontal — 

 the parts of which are arranged vrith reference to one 

 another, the sensory organs and the exits of the first, 

 second, fifth, and tenth pairs of cranial nerves (i., ii., t., 

 and X.), in the manner shown in the annexed diagram * : — 



* The names of the purely which are preformed in cartilage 



membrane bones in this diagram are in smaller type, as Basisphe- 



are in large capitals, as PARI- noid. 

 ETAL ; while those of the bones 



