482 VEHTEBRATA. 



This bone (Jig. 219) is found to consist of a central portion 

 (a), and of sundry processes derived therefrom, some of which the 

 younger student of human anatomy would at once be able to call 

 by their appropriate names : to the body of the bone (a) he finds 

 appended the arch (6) which encloses the spinal cord, surmounted 

 by its spinous process (c), and with equal facility he recognises in 

 the lateral processes (d, d) the analogues of the transverse pro- 

 cesses of the human spine ; but here his knowledge fails him, inas- 

 much as he finds another arch (e) formed beneath the body of the 

 bone, and moreover an inferior spinous process (g), neither of 

 which have any representatives in the human body. 



It is evident, therefore, that the human vertebrae are imperfectly 

 developed bones, and do not possess all the parts or elements 

 met with in the corresponding portion of the skeleton of a fish. 



The question, therefore, to be solved is this, how many 

 elements exist in the most perfect vertebra known ? and this being 

 once satisfactorily settled, it is easy to detect the deficiencies of 

 such as are less completely developed. 



Taking the example F/g.219. 



above given as a speci- 

 men of a fully formed 

 vertebra, it has been 

 found to be divisible in- 

 to the following pieces, 

 all or only a part of 

 which may be present 

 in other vertebrae, even 

 belonging to the same 

 skeleton ; and these 

 parts are represented 

 detached from each 

 other in the diagram which accompanies the figure (Jig. 219, B). 

 They are 1st. the centre or body of the bone ; 2dly. two elements 

 (6, >), which embrace the spinal marrow ; 3dly. the superior spinous 

 process (c) ; 4thly. the two transverse processes (d) ; othly. two 

 elements forming the inferior arch, and enclosing the principal 

 blood-vessels (e) ; and 6thly. an inferior spinous process (g). 



With this key before us, we are able with the utmost ease to 

 comprehend the structure of any form of vertebra that may offer 

 itself. Thus, in different regions of the back of the same fish, 

 the composition of the vertebrae is totally different ; near the tail 



