288 



REPORT — 1846. 



Disarticulated mesencephalic arch, 

 Crocodile, 



viewed from behind :■ 



curved arrows H n and H in. The relations of the superior series of bones 

 as neural arches to the optic lobes and cerebrum are even less doubtful than 

 in many fishes, by reason of the much smaller degree of independent ossifi- 

 cation of the proper capsule of the acoustic labyrinth. Taking, then, the 

 bones forming the arch N n, we find them, viewed from behind, to present 

 the general arrangement shown 



in fig. 19. The hinder (basisphe- Fig. 19. 



noidal) portion of the bone 5 and 

 o forms the centrum, and imme- 

 diately supports the floor of the 

 mesencephalon, or lobe of the 

 third ventricle, being' excavated 

 for the pituitary prolongation of 

 that cavity : it also sends a pro- 

 cess downwards, repeating, like 

 the basioccipital, j the inferior 

 exogenous spine of the centrums 

 of the cervical vertebrae. The 

 bones o, 6 protecting the sides 

 of the mesencephalon, and notch- 

 ed for the transmission of the 

 trigeminal nerve, manifest the 

 neurapophysial characters of the 

 segment. As accessory func- 

 tions they contribute, like the corresponding bones in fishes, to the forma- 

 tion of the ear-chamber. They have, however, a little retrograded in posi- 

 tion (see fig. 9), resting below, in part, upon the occipital centrum, and sup- 

 porting more of the spine of that centrum (3) than of their own (7) ; which 

 is, however, formed of a single bone, and in so far manifests more of the 

 normal character of the element completing the neural arch, as its crown or 

 key-bone, than does the homologous divided and often divaricated bone in 

 fishes. This and other analogous facts show that although the lowest ver- 

 tebrate class adheres most, as a whole, to the archetype, yet that it can be 

 recognised clearly and unequivocally only by patient study of its modifica- 

 tions in all classes: for even the lowest have special exigencies arising out 

 of their sphere of existence calling for modifications of the type which are 

 not present in other and higher classes. We shall find, indeed, that the con- 

 nation of the basi- and pre-sphenoids ceases in mammals, and that they only 

 coalesce in that class, being primitively distinct ; so that the second cranial 

 centrum (5) may be removed with its neural arch, in the foetal quadruped 

 (fig. 24) or human subject (25), without doing violence to nature by the use 

 of the saw. The bones s, s, fig. 19, wedged between 6 and 7, here, also, ma- 

 nifest more of their parapophysial character than in fishes, inasmuch as they 

 are excluded from the inner walls of the cranium, whilst they retain and 

 manifest broadly their characters as outstanding processes for muscular at- 

 tachment. But, besides affording ligamentous attachment to the hyoid arch 

 (39, 40), they articulate largely with the proximal element (is) of the man- 

 dibular arch, whose backward displacement, in comparison with its more 

 normal position in the fish's skull (fig. 5), is as clearly illustrated in the meta- 

 morphosis of the anourous batrachia, as is that of the hyoidean or scapular 

 arches. 



Referring, then, to the side view of the cranial vertebrae of the crocodile 

 (fig. 22), we see the haemal arch of the second or parietal vertebra in the 

 hyoid (39,40, 41) retaining so much of its embryonic dimensions as is required 



