OR VERTEBRA. 47 



In consequence of this extreme modification, in rela- 

 tion to tlie offices of seizing and acting upon the food, 

 the pair of hasmapophyses of the present segment of the 

 skull have received the name of "lower jaw," or "man- 

 dible" {mandibula). The entire segment is called the 

 "frontal vertebra." 



The first segment, forming the anterior extremity of 

 the neuroskeleton, like most peripheral parts, is that 

 which has undergone the most extreme modifications. 

 The obvious arrangement, nevertheless, of its constituent 

 bones, when viewed from behind, 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. The 

 neural arch plainly exists, but is now reduced to its essen- 

 tial 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," No. 13. The neura- 

 pophyses are notched (in the cod) or perforated (in the 

 sword-fish), by the crura or prolongations of the brain, 

 which expand into its anterior divisions, called "olfactory 

 lobes;" the special name of such neurapophysis is "pre- 

 frontal," No. 14. The neural spine is usually single, 

 sometimes cleft along the middle ; it is the " nasal," No. 15. 



The h^mal arch is drawn forwards, so that its apex, 

 as well as its piers, are joined to the centrum (vomer) and 

 usually also to the neural spine (nasal), closing up ante- 

 riorly the neural canal. The pleurapophyses are simple, 

 short, sending backwards an expanded plate ; they are 

 called " palatines," No. 20. The h^emapophyses are sim- 

 ple, and their essential part, intervening between the 



