ON THE THEORY OF THE VERTEBRATE SKULL 



543 



axis would cut that of the cranial cavity at a very considerable angle. 

 The craniofacial axis then is naturally divisible into three regions ; 

 a middle thick part, lodging the sella turcica, and composed of the 

 basisphenoid behind and presphenoid in front, the two being separ- 

 ated by a suture ; a posterior, lamellar, horizontally-flattened part, 

 forming in the young animal a distinct bone, the basioccipital, bound- 

 ing the occipital foramen behind and uniting with the basisphenoid 

 in front ; and an anterior laterally compressed portion, composed of 

 the bony " lamina perpendicularis " of the ethmoid above and behind, 

 united by the cartilaginous septum narium to the bony vomer below. 



Eig. I. — Longitudinal section of the Skull of a Sheep. In this and the following sections of 

 Crania the letters have the same meaning. 



A. S. Alisphenoid. Foramina for nerves . 



O.S. Orbitosphenoid. 



Pf. Prefrontal. 



Sq. Squamosal. 



Ep. Epiotic. 



S.O. Supraoccipital. 



Pa. Parietal. 



F. Frontal. 



B.O. Basioccipital. 

 B.S. Basisphenoid. 

 P.S. Presphenoid. 

 Eth. Ethmoid (lamina per- 

 pendicularis). 

 E.O. Exoccipital. 

 M. Mastoid. 

 P. or P.S. Petrosal. 

 P.M. Petromastoid. 



Olfactory ; 2. optic ; 3 & 

 4. oculomotor and pathetic 

 nerves ; 5. third division of 

 trigeminal ; 7. portio dura and 

 mollis ; 8. pneumogastric ; 

 Epiph. Pineal gland, or 

 epiphysis cerebri. 



This anterior division of the axis may be termed its ethmovomerine 

 portion. Its posterior edge helps to close the anterior outlet of the 

 cranial cavity, from which it is otherwise completely excluded. 



The sella turcica lodges the pituitary body, and the synchondrosial 

 union between the basisphenoid and presphenoid is situated so far 

 forwards that the anterior wall of the fossa is almost wholly formed 

 by the rostrum-like anterior prolongation of the basisphenoid. The 



