418 THE MECHANICAL FUNCTIONS. 



situation is very analogous to the spine of the 

 myxine. 



As we ascend from this rudimental condition 

 of the spine, we find it, in the lamprey, more 

 distinctly divided into rounded portions, appear- 

 ing like beads strung together. These rudi- 

 mental bodies of vertebrae have not yet com- 

 pleted the cup-like hollows on their two ends, 

 but are shaped like rings, being perforated in the 

 centre, so as still to form a continuous canal 

 throughout the whole column. 



Proceeding to more advanced developements, 

 we find, in the sturgeon and other cartilaginous 

 fishes, a greater condensation of substance pro- 

 duced by the deposition of granules of osseous 

 matter ; the central canal becomes divided into 

 lozenge-shaped compartments by the closing in of 

 the sides of the body of each vertebra.* Fre- 

 quently the sides do not quite meet, and the 

 leaves, which are developed from the upper 

 surfaces of the vertebrae, now form arches over 

 the spinal cord, and are united above by spinous 

 processes. Yet the whole skeleton in these 



* A small aperture still remains, establishing a communication 

 between the cavities the whole length of the spine. This is sup- 

 posed to be designed to obviate the compression of the fluid in 

 the different cells or cavities during the motions of the spine. 

 The vertical sections, Fig. 189 and 190, of two contiguous ver- 

 tebrae in different fishes, will convey an idea of this gradation of 

 developernent. 



