SKULL — ^CHONDROCRANIUM 6l 



preotic cranial nerves, of which the second and the fifth afford 

 important landmarks. With increase in age the two bars of a side 

 become connected by cartilage, at first in two places. The more 

 posterior hes just in front of the mandibular nerve and is, in part, 

 the homologue of the alisphenoid bone of the higher groups. In 

 some of the higher Vertebrates the union is very narrow and has been 

 called the preotic pillar (pila prootica). When complete, this ali- 

 sphenoid cartilage extends as far forwards as the optic nerve, in 

 front of which is a second (orbitosphenoid) cartilage connexion which 

 reaches to the ethmoid region. Thus the optic and some other nerves 

 pass from the brain through an opening, the primitive orbital fissure, 

 which is completed above and below by the sphenolateral and 

 trabecular cartilages. The mandibularis branch of the fifth, the sixth 

 and at least a part of the seventh nerves leave the cranium by the 

 gap between otic capsule {infra) and the preotic pillar. Later this 

 gap is closed dorsally by a growth of cartilage from the sphenolateral 

 to the capsule, while below the union of trabecula and basal plate 

 completes the wall of the opening (foramen lacerum) which trans- 

 mits the nerves to the exterior. Trabeculse, sphenolaterals and 

 orbito- and ahsphenoid cartilages form the side wall of the brain case 

 in the interorbital region. 



In some Vertebrates basal plate, trabeculae, sphenolaterals and ethmoid 

 plate form as a continuum with small foramina for the exit of the nerves. Usu- 

 ally the mandibularis ramus is eventually included in the alisphenoid cartilage, 

 its opening being the foramen ovale. So, too, the optic nerve is often included 

 in the orbitosphenoid (optic foramen) while the third, fourth nerves and other 

 branches of the fifth nerve vary in their place of exit, sometimes passing 

 through a foramen rotundum in the ahsphenoid cartilage. 



The parts thus outhned form most of the cartilaginous brain 

 case or chondrocranium, which is completed, not only by the formation 

 of a roof and a more complete floor, arising by growths from the parts 

 enumerated, but by the sense capsules — nasal, sclerotic and otic — 

 which are formed around the three principal organs of special sense. 



The trabeculae continue in front of the ethmoid plate as a pair of 

 diverging horns (cornua trabecularum), each cornu lying beneath 

 the corresponding nasal sac and forming the floor of the nasal cap- 

 sule. The two sacs are usually separated by a median nasal septum, 

 an upgrowth from the ethmoid plate. Each nasal capsule is con- 

 nected with the interorbital wall by a continuation (sphenethmoid 



