64 



VERTEBRATE SKELETON 



eyes. When the heighth of the head equals or exceeds its breadth, 

 and especially when the eyes are large, the trabeculae are forced 

 towards the middle line, leaving a small hypophysial fenestra, in 

 front of which the bars of the two sides are fused to a common tra- 

 becula which continues forwards into the nasal region. This is the 

 tropibasic cranium, occurring in many Teleosts, in all Sauropsida 

 (tig. 67) and in a modified form, in Mammalia. 



The great size of the eyes and the narrow head also affect the ali- 

 and orbitosphenoid cartilages, especially the latter, which may be so 

 pressed together that they form a thin vertical plate (interorbital 



septum), and only dorsal to the eyes do 

 they retain their individuahty as a pair of 

 diverging supraseptal plates (fig. 68). 

 This constriction of the cranial cavity also 

 restricts the brain largely or wholly 

 (mammals excepted) to the postorbital 

 part of the cranium, but a part of the 

 anterior cranial cavity can still be recog- 

 nized in the groove formed by the supra- 

 septal plates, which contains the olfactory 

 nerves (sometimes the olfactory lobes as 

 /, frontal bone; pi, pterygoid; -well) and which is closcd dorsally by the 



r, retina; s, interorbital septum; ., 



5c, sclera; 5/7, supraseptal plates; frontal and nasal bones. ine tropibasic 

 2, zygomatic. craniuiii and the greater development of 



membrane bones in the cranial wall are accompanied by a reduction 

 of the interorbital cartilages (ah- and orbitosphenoids) which become 

 fenestrated (fig. 162) and in some cases they fail to ossify. 



In platybasic crania the second to fifth nerves at first leave the 

 skull through the preotic foramen (f. lacerum) and through the 

 orbital fissure (fig. 66), but with the extension of the cartilage they 

 may become surroujided by ali- and orbitosphenoids in different 

 ways, it often happening that in the adult neither of the primitive 

 openings allows passage of nerves to the exterior. 



Cartilaginous Visceral Skeleton 



The visceral skeleton arises as a series of cartilage bars in the 

 walls of the anterior part of the digestive tract (mouth and pharynx). 

 At first these are continuous rods of procartilage, right and left, 

 their dorsal ends lying on either side of the cranium and anterior verte- 



FiG. 68. — Schematic section 

 through orbital region of Sauro- 

 psidan. rt, trabecula communis; 



