cjA STRUCTURE OF VERTEBRATES 



help in the stiffening of the sclerotic. The trabeculae fuse, but 

 leave a gap between their posterior ends for the hypophysis. 

 Where they meet only at the anterior tips, so that the fenestra 

 is large, the skull is said to be platybasic ; where they fuse down 

 most of their length, it is said to be tropybasic. In this con- 

 dition, which is found in most teleosts and in amniotes, the 

 trabecule grow up to form an interorbital septum which con- 

 tinues the nasal septum backwards, and so confines the brain to 

 the posterior part of the skull. The interorbital septum is less 

 strongly developed in mammals than in birds and reptiles. 



In summary, the floor of the chondrocranium is formed from 

 trabecule, parachordals and occipital elements ; its side walls 

 from the orbitals, the auditory capsules and the occipitals ; its 

 anterior end from the nasal capsules, while its roof is complete 

 only in the occipital region. 



It is logical to try to fit these cartilages into the segmental 

 scheme of the head which we have discussed above, but attempts 

 to do so have not been completely successful. The occipital 

 cartilages more or less clearly come from the sclerotomes of the 

 posterior three or more segments of the head, but the relationship 

 of the other cartilages to their segments is not clear. One scheme 

 is shown in Table VII. According to this view the trabeculae 

 and polars represent the pharyngeal elements of the preman- 

 dibular and mandibular arches respectively, and the parachordals 

 are formed from mesenchyme in segments three and four. 



THE BONY SKULL 



In all vertebrates above the Chondrichthyes there is at least 

 some replacement of the cartilage by bones formed in it and 

 gradually replacing it (pp. 519-20). In the reptiles, birds and 

 mammals the cartilage bones are much the same, although 

 there is often fusion which obscures their identity in the adult 

 animal. The Anura have much reduced cartilage bones, and the 

 bony fishes have some peculiarities. Taking the cranium topo- 

 graphically and beginning at the front end we find that the nasal 

 septum ossifies as the mesethmoid (Fig. 340). This spreads to 

 some extent over the nasal capsules, and in mammals its posterior 

 edge expands to form the cribriform plate, which forms the 

 anterior wall of the brain cavity, and is pierced by many holes 

 for the olfactory nerves. Behind the mesethmoid the side walls 



