I20 



VERTEBRATE SKELETON 



general account of the bones of the skull (p. 68). In the existing 

 Amphibian orders, while many primitive features persist, there is 

 considerable modification and even degeneration. Nothing is known 

 of the chondrocranium of Stegocephals; in other groups it largely 

 persists in the adult, and in all the tenth nerve is the last to leave the 

 cranium, a marked contrast to the Amniotes with twelve cranial 

 nerves, and even to Elasmobranchs and many Teleosts where spino- 

 occipital nerves pass through the cranial walls. The Amphibia are 



poor in cartilage bones, their number being 

 less than in most Teleosts and paralleled, 

 among Ichthyopsida, by some Ganoids and 

 Dipnoi. With this poverty of cartilage 

 bones, the skull is largely a structure of 

 membrane bones and cartilage, the former 

 numerous in Stegocephals, fewer, both by 

 fusion and by absolute loss, in the living 

 groups (C/. figure 126, where lost dermal 

 bones are stippled). 



The general features of the cartilage skull are, 

 first, its platybasic character, the cranial cavity 

 extending to the ethmoid region, with no inter- 

 orbital septum. The notochord, which reaches 

 the hypophysial fenestra, is bordered on either 

 side by a parachordal (often fenestrated) which 

 joins the otic capsules. Each capsule has large 

 foramina on the medial side for vessels and nerves, 

 while the lateral side has the vestibular fenestra. 

 Two postotic vertebrae enter the occipital region, 



A 



Fig. 126. — Cranium of £ryo^.^ 

 (Gregory, '20); bones lacking in 

 recent Amphibia stippled, do, 

 dermoccipital; /, frontal; if, in- 



terfrontal; /, lacrimal; m, max- _ . , , • , 1 



ilia; WW, manubrium of malleus; the posterior just behmd the vagus nerve. 

 M, nasal; p, parietal, pf, post- gynotic tectum may arise from upgrowths from 



otic capsule and vertebra?, but usually there is no 

 roof in front of this until the ethmoid region is 

 reached, the fontanelle being enormous. The 

 prechordal parts, which appear before the chordal, 

 consist of trabecula and sphenolateral developing 

 as a continuum, the latter part forming the 'trabecular crest,' the nerve 

 foramina indicating the line between the two elements in the orbital region. 

 Posteriorly the crest connects with the otic capsule by a process dorsal to the 

 exit of the V-VII nerves (foramen lacerum). 



The fenestra hypophyseos is very large at first, later it is closed by cartilage in 

 Anura, but not in Urodeles and Gymnophiona. The floor of the nasal capsule is 

 formed by the trabecular cornua and growth from the ethmoid plate. It is 



frontal; po, postorbital; prf, pre- 

 frontal; qj, quadratojugal; sq, 

 squamosal; st, supratemporal; 

 t, tabulare, s, zygomatic. (The 

 zygomatic of Caecilians is prob- 

 ably a post-orbital.) 



