264 report— 1846. 



Summary of modifications of corporal vertebra. — To sum up the kind and 

 degree of modification to which the several elements of the primary segments 

 of the endoskeleton of the trunk are subject, without masking their general 

 homology, we may commence with the centrum; and first, as to its existence. 

 It is wanting, as an ossified part, in the atlas of the wombat and koala*, in 

 which it remains permanently cartilaginous : in the petaurists, kangaroos, 

 and potoroos, ossification extends from the bases of the neurapophyses into 

 this cartilage, but the neural arch or ring long remains interrupted by a me- 

 dian fissure below. In man the rudimental body of the atlas is sometimes 

 ossified from two or even three distinct centres f- The centrums at the oppo- 

 site extremity of the vertebral column in homocercal fishes are rendered by 

 centripetal shortening and bony confluence fewer in number than the per- 

 sistent neural and haemal arches of that part. The centrums do not pass 

 beyond the primitive stage of the notochord in the existing lepidosiren, and 

 retained the like rudimental state in every fish whose remains have been found 

 in strata earlier than the permian oera in Geology, though the number of 

 vertebrae is frequently indicated in Devonian and Silurian ichthyolites by the 

 fossilized neur- and haem-apophyses and their spines %. The individuality of 

 the centrums is sometimes lost by their mutual coalescence without short- 

 ening. 



Although the normal form of the centrum is cylindrical, it may be cubical, 

 conical, hour-glass shaped, like a longitudinal bar, like a transverse bar, like 

 a depressed or a compressed plate, like a ploughshare, &c. The co-adapted 

 terminal surfaces of the centrum may be flat, slightly concave, deeply con- 

 cave, cupped or conical, concave vertically and convex transversely at one 

 end and the reverse at the other end§ ; or the fore-end may be concave and 

 the hind-end convex|| ; or the reverse^ ; or both ends may be convex**; 

 or both ends produced into long pointed processes with intervening deep fis- 

 sures, so as to interlock together by a deeply dentated sutural surfaceft- 



The centrum may be quite detached from its neural arch (atlas of siluroid 

 and many fishes), and from its haemal arch (atlas of most fishes). 



The centrum may develope not only parapophyses but inferior median 

 exogenous processes, either single, like those of the cervical vertebrae of 

 saurians and ophidians (which in Deirodon scaber perforate the oesophagus, 

 are capped by dentine, and serve as teeth \X); or double (atlas of Sudis gigas§§ 

 and the lower cervical vertebrae of many birds) ; or the fibrous sheath of the 

 notochord may develope a continuous plate of bone beneath two or more nuclei 

 of centrums, formed by independent ossification in the body of the notochord ; 

 these nuclei being partially coherent to the peripheral or cortical plate. The 

 vertebral centrum often shows the principle of vegetative repetition by its 

 partial ossification in the form of two or three bony rings, which answer to a 

 single neural arch (Hepta?ichus\\\\) } or by three osseous discs, one for each 



* Art. Marsupialia, Cyclopaedia of Anatomy and Physiology, vol. iii. p. 277, fig. 99. 



t Meckel, Archiv fiir Physiol, i. taf. vi. fig. 1. 



J See the admirahle Monograph by Agassiz, Sur les Poissons Fossiles du Systeme De- 

 vonien, 4to, 1846. § Most birds. 



|| Existing saurians and ophidians. 



If Extinct saurian called ' Streptospondylus ;' existing Salamandra, Lepidosteus. 



** 4th cervical of Emys, Bojanus, Anat. Test. Europ., tab. xiv. fig. 51, 4. 1st caudal of 

 crocodile. 



+f Cervicals or anterior trunk-vertebrae of Fistularia. 



X+ Jourdan, cited in Cuvier's Lecons d'Anat. Comparee, ed. 1835, p. 340, and ' Odonto- 

 graphy,' p. 179. 



§§ Agassiz in Spix, Pisces Brasilienses, 4to, 1829, p. 6, tab. B, fig. 8. 



I! || Miiller and Agassiz, in Recherches sur les Poissons Fossiles, t. iii. tab. 40 b , fig. 1. 



