SKELETON. 



153 



urodeles, crocodiles, lizards, and many teleosts no such carti- 

 laginous cranial floor is formed, the ventral wall of the skull 

 being formed by membrane bones to be described later. 



Cartilage walls are also found in the optic and olfactory or- 

 gans. Since motion is necessary in the eye, the optic capsule 



FlG. 161. Dorsal and lateral views of the chondrocranium of Amphiurna. 

 anp, antorbital process; ap, ascending process of quadrate; c, cornu trabeculae; 

 e, ethmoid plate ; ef, foramen for ductus endolymphaticus ; /, jugular foramen ; 

 /, lamina cribrosa; w, Meckel's cartilage; n, notochord ; oc, foramen for oculomotor 

 nerve; ocp, occipital process (vertebra); of, foramen for optic nerve; /, para- 

 chordal ; pal, foramen for palatine nerve ; //, foramen for ductus perilymphaticus ; 

 </, quadrate ; s, stapes; s/>, stapedial process of quadrate; t, trabecula; trc, trabecular 

 crest ; V, VII, VIII, foramina for V, VII, and VIII nerves. 



(sclerotic, p. 83) never participates in the formation of the cra- 

 nium. The nasal capsules, on the other hand, unite with the 

 anterior ends of the trabeculae and with the cornua. They are 

 frequently extensively fenestrated. In the vertebrate series a 

 general law may be observed. The more completely the adult 

 skull is ossified, the less developed is the chondrocranium. 



