94 



ANATOMY OF VERTEBRATES. 



lodging the brain and the organs of hearing; the 'orbital,' and ( nasal' 

 chambers ; the ' buccal ' and ' branchial ' canals. Some of these 

 cavities are not well denned. The exterior of the skull is traversed 

 by five longitudinal crests, intercepting four channels which lodge 

 the beginnings of the great muscles of the upper half of the trunk. 

 The median crest is developed from the superoccipital, figs. 75, 76, 3, 

 aud sometimes also from the frontal, fig. 75, n : the lateral crest is 

 formed by the parietal, fig. 76, 7, and paroccipital, ib. 4: the 



external crest by the postfrontal, ib. 12, 

 " 6 and mastoid, ib. 8. The lower border 



of the orbit, fig. 75, g, g, projects freely 

 downward. The hind border of the 

 operculum is produced into spines in 

 some species, fig. 82. 



In the analysis of the fish's skull 

 it is best to begin at the back part ; 

 for the segments of the skeleton de- 

 viate most from the archetype as they 

 recede in position toward the two ex- 

 tremes of the body. After a little 

 practice one succeeds in detaching the 

 bones which form the back part or 

 base of the conical skull, and which 

 immediately precede and join those of 

 the trunk ; we thus obtain a ' segment ' 

 or ' vertebra ' of the skull. If wo 

 next proceed to separate a little the 

 bones composing this segment, we 

 find those that were most closely in- 

 terlocked to be in number and ar- 

 rangement as follows : — Two single and symmetrical bones, 

 and two pairs of unsymmetrical bones, forming a circle; or, if 

 the lower symmetrical bone, which is the largest, be regarded as 

 the base, the other five form an arch supported by it, of which 

 the upper symmetrical bone is the key-stone, fig. 77. This 

 answers to the ' neural ' arch of the typical vertebra : the base- 

 bone is the ' centrum,' 1 ; the pair of bones, which articulated with 

 its upper surface and protected the hind division of the brain, 

 form the ' neurapophyses,' 2 ; the smaller pair of bones, projecting 

 outward, like transverse processes, are the ' diapophyses,' 4 ; the 

 symmetrical bone completing the arch, and terminating above in a 

 long crest or spine, is the ' neural spine,' 3. It will be observed 

 that the centrum is concave at that surface which articulates with 



Upper surface of cranium, Porch 

 (Percaflvmatilis) 



