6o2 ON THE THEORY OF THE VERTEBRATE SKULL 



arches. The intercentra correspond in fishes, as in Amphibia and birds, to the 

 middle of each somatome ; and it does not appear that they are, to any appreciable 

 extent, produced by the metamorphosis of the notochord. 



When ossification takes place, the' diaphysis appears as a ring, or part of a 

 ring, in immediate contact with, or very close proximity to, the notochord, which 

 it usually embraces completely, though in the rare case of some Amphibia, only 

 partially. The diaphysis then increases inwards so as to constrict the notochord, 

 and outwards, so as to invade the centrum more and more. A distinct ossifica- 

 tion is commonly formed in each neural arch, and one or more others in each 

 htemal arch. 



In the higher or abranchiate oviparous Vertebrata there would seem to be no 

 other centres of ossification in the vertebra than the five just mentioned, except 

 those of the terminal epiphyses. In fishes, on the other hand, a distinct centre — 

 which might be termed Xh^ paraphysis — is occasionally found in the paraphysial 

 portion of the centrum. 



The dorsal vertebrae of a young carp exemplify this structure remarkably well- 

 The diaphysis is represented by an annular osseous ring, which surrounds the 

 notochord and gives off a vertical median process or plate, and two inferolateral 

 plates, which unite the hollow bony cones into which the osseous ring dilates 

 in front and behind. The ossified neurapophyses expand into wedge-like lower 

 ends, which embrace the vertical plate of the diaphysis, and whose apices come 

 into contact with its annular part. 



A thick cuneiform mass is interposed between the base of the neurapophysis 

 and the inferolateral plate of the diaphysis on each side. The outer surface 

 forms part of the general contour of the vertebra, and is not produced into a 

 distinct process, though it represents a parapophysis, and gives attachment to 

 the broad head of the distinctly ossified rib. The outer half of the mass is 

 ossified as a distinct paraphysis ; the inner in the young carp is still cartilaginous- 

 In the adult the whole wedge-like paraphysial portion of the centrum is ossified.;, 

 but, instead of becoming united with the diaphysis and neurapophysis, it is 

 anchylosed with the rib, and seems to form its head. In the pike, the paraphysis, 

 more or less produced into a parapophysis, remains distinct from both rib' 

 and diaphysis, and the latter occupies a very much larger share of the whole 

 vertebra. 



As I have said above, no distinct ossific centre appears to be developed in the 

 paraphysial region of the centrum in any of the abranchiate Vertebrata ; but it 

 becomes ossified partly by the encroachment of the neurapophysial, and partly by 

 that of the diaphysial, ossifications. 



These two ossifications may coalesce so as to leave no trace of their primitive 

 distinctness, as in Ophidia, Lacertilia, and birds ; or as in mammals. Crocodilian 

 Chelonia, and many extinct reptiles, they may remain for a long time, or 

 permanently, separated by a suture, which may be termed the " luiirocentral 

 suture.'' 



It is very commonly assumed that this neurocentral suture is a sort of morpho- 

 logical landmark, and that it always indicates the boundary between the. neura- 

 pophysis on the one hand, and the diaphysis or osseous centrum on the other ; so 

 that any process which is given off from the vertebra above the suture, is supposed 

 to arise from the neurapophysis, while those given off below it only, are said to. 

 arise from the centrum. 



It is only necessary to cite a few facts, which may be readily verified, however,, 

 to show that the neurocentral suture is of no value as a test of the nature of the 

 parts above and below it. 



