XXI] 



SKULL 



531 



is ossified by a large bone, the pro-otic, which in fully adult 

 specimens becomes confluent with the exoccipital. The hinder 

 visceral arches in the adult are present in a very degenerate con- 

 dition. Traces of three remain (Fig. 256). 



It is usual to speak of the hinder visceral arches of Amphibia 

 and higher Vertebrataas the hyoid apparatus, or simply as the 

 hyoid. The name suggests a misleading comparison with the 

 second visceral arch of Fish ; it is distinctly to be remembered that 

 the hyoid bone of even Man contains more than this second arch ; 

 a good definition of the hyoid of Amphibia and higher animals 

 would be "the degenerate remains of 

 the hinder visceral arches." 



Turning now to the dermal bones of 

 the skull, we find that it is roofed by 

 three pairs, viz., the nasals, frontals 

 and parietals. The nasals, of course, 

 roof in the nasal sacs. In the palate 

 there is one median bone, the para- 

 sphenoid, and three pairs of lateral 

 bones, viz., the vomers in front of the 

 posterior nares, the palatines fused 

 with them and running along the edges 

 of the parasphenoid, and lastly the 

 pterygoids underlying the pterygoid 

 process. Some of these bones are 

 actually built up by the formation of a 

 network of bone around the bases of 

 minute conical teeth in the larva. The vomers and palatines 

 retain their teeth in the adult, whilst the parasphenoid loses them, 

 but in other genera of Urodela the parasphenoid may retain its 

 teeth throughout life. When we were describing the skull of a 

 Teleostean fish, bones named palatine and pterygoid (viz., ecto- 

 pterygoid, entopterygoid and metapterygoid) were mentioned, but 

 these were regarded as cartilage bones, i.e., as bones which arise 

 in the connective tissue surrounding cartilage, and which sub- 

 sequently eat into the cartilage, destroy it, and replace it. 



Dermal bones, on the contrary, as the name implies, arise in 

 the dermis or the connective tissue underlying the ectoderm. Now 

 there is one situation where the dermis teuds.to become the "peri- 

 chondrum," or connective tissue surrounding cartilage, and that 

 is the stomodaeum or buccal cavity, whose lining membrane tends 

 to be tightly stretched over the supporting visceral arches. Hence 



342 



FIG. 256. Visceral arches of 

 Molge cristata. The ossified 

 parts are slightly shaded, the 

 cartilage is white. From 

 Parker. 



2. Hyoid arch. 3. First 

 branchial arch. 4. Second 

 branchial arch. 8. Copula, 

 i.e. the median piece connect- 

 ing successive arches. 



