THE SKELETON. 41 



tomy) is said to be the centrum or body of the occipital or posterior 

 cranial vertebra, its general homology is enunciated. "When it is said 

 to repeat in its vertebra, or to answer to the basi-sphenoid in the parie- 

 tal vertebra, or to the body or centrum in the atlas, dentata, or any other 

 of the vertebral segments of the skeleton, its serial homology is indi- 

 cated : when the essential correspondence of the basilar process of the 

 occipital bone in Man with the distinct bone called " basi-occipital " 

 in a Crocodile or Fish is shown, its sjjecial homology is determined. 



LECTURE III. 



THE VERTEBRA, AND VERTEBRAL COLUMN IN FISHES. 



To understand the fundamental type of the vertebrate skeleton its 

 study must be commenced, not in the highest species, — not in that 

 skeleton where irrelative repetition is least, and where modification 

 of each part in mutual subserviency to another is greatest, — but in 

 the lowest Class where, conformably with the law enunciated in the 

 previous Course *, vegetative uniformity most prevails, and the pri- 

 mitive type is least obscured by teleological adaptations. 



Such conditions are best displayed in the skeletons of fishes : fishes 

 form, however, but one branch of the vertebrate stem, which, like 

 other primary branches, ramifies in diverging from the common 

 trunk. We should miss our aim, therefore, and be led astray from 

 the detection of the true general type of the vertebrate skeleton, 

 were we to confine our observations to fishes only. A comparison of 

 their skeletons with those of the higher classes teaches that the na- 

 tural arrangements of the parts of the endo-skeleton in Vertebi'ata, 

 like that of the exo-skeleton in Articulata, is in a series of segments 

 succeeding each other in the axis of the body. I do not find these 

 successive segments composed of precisely the same number of bones 

 in all Vertebrata ; rarely, indeed, in the same animal. Yet certain 

 constituent parts of each segment do preserve such constancy in their 

 existence, relative position, and offices throughout the body, as to 

 enforce a conviction that they are homologous parts, both in the con- 

 secutive series of the same individual skeleton, and throughout the 

 entire series of vertebrate animals. 



* Lectures on Invertebrata, 8vo. 1843, p. 364. 



