387 



On examining a section of the cranium of a sheep, made either 

 along a vertical and longitudinal, or a transverse and horizontal 

 plane, a more or less completely ossified mass is observed in the 

 middle line below, which forms part of the floor of the cranial cavity, 

 but extends beyond it. This may be termed the ' craniofacial axis.' 

 Posteriorly it is a broad plate flattened from above downwards, and 

 is nearly parallel with the long axis of the cranial cavity ; but from a 

 point immediately behind the sella turcica, it becomes thicker and is 

 compressed from side to side, so that, at the anterior boundary 

 of the sella turcica, the craniofacial axis is much deeper than wide, 

 and assumes the form of a vertical plate. From the anterior boundary 

 of the cranial cavity onwards, or in its facial portion, the axial plate 

 is very deep and very thin, and a line drawn through its longitudinal 

 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 sepa- 

 rated 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. 

 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 

 spinal cord passes out behind the posterior margin of the basioc- 

 cipital. The olfactory nerves leave the skull on each side of the 

 ethmovomerine division of the craniofacial axis. 



The walls of the cranial cavity are formed by a number of bones, 

 which are divisible into two series, a superior and a lateral. Of the 

 latter, four pairs of bones, separated by natural lines of demarcation, 

 or sutures, are distinguishable, three of which abut directly upon the 

 cranio-facial axis, while the fourth pair are only indirectly connected 



