ANATOMY OF VERTEBRATES. 99 



these qualities seem to have been required in consequence of the 

 presence of a complex and largely developed diverging appendage, 

 which forms the framework of the principal flap or door, called 

 ( operculum,' figs. 81, 84, 34-37, that opens and closes the branchial 

 fissures on each side. The appendage in question consists of four 

 bones ; the one articulated to the tympanic pedicle is called ( pre- 

 opercular,' ib. 34 ; the other three are, counting downward, the 

 ' opercular,' ib. 35 ; the f subopercular,' ib. 36 ; the interopercular,' 

 ib. 37. The hremapophysis is subdivided into two, three, or more 

 pieces, in different fishes, suturally interlocked together ; the most 

 common division is into two subequal parts, one presenting the 

 concavo-convex joint to the pleurapophysis, and called ( articular,' 

 ib. 29 ; the other, bifurcated behind to receive the point of 29, and 

 joining its fellow at the opposite end, to complete the haemal arch : 

 it supports a number of the hard bodies called ' teeth,' and hence 

 it has been termed the ( dentary,' ib. 32. In the Cod there is a 

 small separate bone, below the joint of the articular, forming an 

 angle there, and called the e angular piece,' fig. 75, 30. 



In consequence of this extreme modification, in relation to the 

 offices of seizing and acting upon the food, the pair of ha3ma- 

 pophyses of the present segment of the skull have received the 

 name of ( lower jaw,' or ( mandible ' (mandibula). The haemal 

 arch is, hence, called ' mandibular : ' the neural arch ( prosen- 

 cephalic : ' the entire segment is called the ' frontal vertebra.' 



The first segment, forming the anterior extremity of the neuro- 

 skeleton, like most peripheral parts, is that which has undergone 

 the most extreme modifications. The 

 obvious arrangement, nevertheless, of its 

 constituent bones, when viewed from be- 

 hind, after its detachment from the second 

 segment, affords one of the most conclu- 

 sive proofs of the principle of adherence 

 to common type which governs all the 

 segments of the neuroskeleton, whatever 



Offices they may be modified tO fulfil. Disarticulated rhinencephalic arch, 

 , -I 1 r i i Cod (Morrhua vulgarly 



Hie neural arch, ng. 80, is plainly mani- 

 fested, but is now reduced to its essential elements viz., the 

 centrum, the neurapophyses, and the neural spine. The centrum 

 is expanded anteriorly, where it usually supports some teeth on 

 its under surface in fishes; it is called the ' vomer,' ib. 13. The 

 neurapophyses are notched (in the Cod), or perforated (in the 

 Sword-fish), by the crura or prolongations of the brain, which 

 expand into its anterior division, called rhinencephalon, or 



H 2 



