OK VERTEBRA. 45 



backward, can be detaclied from the foremost segment 

 without dividing any bone. It is then seen to consist, 

 lilvc the third and fourth segments, of two arches and a 

 common centre ; but the constituent bones have been 

 subject to more extreme modifications. The centrum, 

 called " presphenoid," No. 9, is produced far forwards, 

 slightly expanding; the neurapophyses, called "orbito- 

 sphenoids," No. 10, are small semioval plates, protecting 

 the sides of the cerebrum ; the neural spine, or key -bone 

 of the arch, called "frontal," No. 11, is enormously ex- 

 panded, but in the cod and most fishes is single; the 

 diapophyses, called " post-frontals," No. 12, project out- 

 wards from the hinder angles of the frontal, and give 

 attachment to the piers of the inverted hoamal arch. The 

 first bone of this arch is common in fishes to it and to 

 that of the last-described vertebra, being the bone called 

 "epitym panic," No. 28 (Fig. 9); this modification is called 

 for by the necessity of consentaneous movements of the 

 two inverted arches, in connection with the deglutition 

 and course of the streams of water required for the bran- 

 chial respiration. The ha3mal arch of the present seg- 

 ment — enormously developed — is plainly divided prima- 

 rily on each side into a pleurapophysis and hgemapophysis; 

 for these elements are joined together by a movable 

 articulation, whilst the bones into w^iich they are sub- 

 divided are suturally interlocked together. The pleura- 

 pophysis is so subdivided into four pieces; the upper 

 one, articulating with the post-frontal and mastoid — the 

 diapophyses of the two middle segments of the skull — is 

 called "epitympanic," No. 28, a; the hindmost of the two 

 middle pieces is the "mesotympanic," No. 28, h] the fore- 

 most of the two middle pieces is the "pretympanic," No. 

 28, c; the lower pioce is the hypotympanic, No. 28, (/; 



