278 REPORT— 1846. 



development from the under part of the centrum of the second cervical ver- 

 tebra of the siluroid fish (p. 260) were removed, with that segment, from the 

 atlas, the atlantal neural arch would still be completed by the rudimental body, 

 beneath which the ossification from the succeeding vertebras had extended 

 itself. 



Whether, however, we divide or not the bone s, 9, those which rest upon 

 its posterior or basisphenoidal part present, after the removal of the petro- 

 sals, when viewed from behind, and slightly disarticulated from each other, 

 the arrangement exhibited in fig. 2. The bones 6, 6 support and defend 

 the lobe of the third ventricle or the mesencephalic segment of the brain ; 

 they give exit to the trigeminal nerves (tr), and thus, as well as by their con- 

 nections with the other bones of the arch, repeat the neurapophysial characters 

 of the bones 2, 2 in the occipital segment. The bones s, s, by their more ex- 

 ternal position, by affording an articular surface to the haemal arch (zsa, 

 H 11), and their development of a strong transversely and backwardly pro- 

 duced process for muscular attachments, obviously repeat the parapophysial 

 characters of the bones 4, 4 in the occipital vertebra. 



The arch is not completed above in the cod-fish ; the bones 7, 7 being se- 

 parated at the mesial line by the interposition of the produced spine of the 

 occipital vertebra 3, which joins with 11. In some other fishes, however, 

 e. g. carp and pike, the bones 7, 7 do come in contact and join each other by 

 a ' sagittal ' suture, thus completing the neural arch. It will afterwards be 

 seen, by tracing the homologues of these bones in other animals and their 

 homotypes in other segments, what value may be assigned to the objection to 

 their general homology as the crown or haemal spine of the mesencephalic 

 neural arch, founded upon the median division and occasional divarication of 

 the two halves of no. 7 in osseous fishes. I may so far anticipate the discus- 

 sion as to remark that, even in the present group of vertebrates, the spine of 

 the occipital vertebra (3) is divided b} r a median suture in the lepidosteus ; so 

 that the condition of the epencephalic arch in that fish is precisely that of 

 the mesencephalic arch in the carp, and essentially the same as that in fig. 2, 

 and in most other osseous fishes. 



The remainder of the second or parietal segment of the skull, H n,repeats the 

 expanded modification of the haemal arch of the occipital vertebra, and even 

 approaches nearer to the character of the thoracic vertebrae of the higher 

 animals, by the development of single symmetrical bones at the crown of the 

 inverted arch. But the principle of vegetative repetition is still more mani- 

 fested in this arch than in the occipital one. If we regard the posterior half 

 of the epitympanic, 2so, as the proximal piece of the parieto-haemal arch, 

 which has coalesced with the corresponding piece of the fronto-haemal arch, 

 then the pleurapopkysis of the parieto-haemal arch will consist, in bony fishes, 

 of two pieces, im and 38, like the pleurapophysis of the occipito haemal arch, 

 soandsi. Thebones, 39 and 40, represent the hcemapophysis of the parieto-haemal 

 arch. The two pairs of small bones (41) with the single median anterior (42) 

 and posterior (43) appendages, represent a still more subdivided spine or key- 

 bone of this inverted arch. 



Beneath this mask of multiplication of bony centres, the broad characters 

 of the inverted arch suspended to the parapophyses of the parietal vertebra, 

 as the haemal complement of that natural segment of the skull, stand boldly 

 out : it encompasses, sustains and protects the branchial organs — the ana- 

 logues of lungs — the next great development of the vascular system anterior 

 to the heart ; and the subdivision of the piers of this expanded arch relates to 

 the necessity for a combination of strength, with flexibility and elasticity, in 

 the execution of the movements producing the respiratory currents. 



