290 THE MECHANICAL FUNCTIONS. 



that doubts have often arisen in the minds of naturalists 

 whether that animal ought not properly to be ranked among 

 this latter class. Its pretensions to be included among ihe 

 vertebrata are, indeed, but slender and equivocal; for, in 

 place of a scries of bones composing the vertebral column, 

 it has merely a soft and flexible tube of a homogeneous and 

 cartilaginous substance, exhibiting scarcely any trace of divi- 

 sion into separate rings, but appearing as if it were formed 

 of a continuous hollow cylinder of intervertebral substance, 

 usurping the place of the vertebras, which it is the usual oflSce 

 of that substance to connect together, and having in its axis 

 a continuous canal filled with gelatinous fluid. This, how- 

 ever, is not the channel intended for containing the spinal 

 marrow, for that nervous cord is on the outside of this 

 column. The cartilage, indeed, sends out no processes to 

 bend round the spinal marrow, and forms no canal for its 

 passage and protection. The nervous matter here consists 

 merely of two slender cords, which run parallel to one ano- 

 ther in a groove on the upper part of the spinal column; and 

 these cords are covered only by a thin membrane, the pre- 

 sence of which it requires very minute attention to detect. 

 The partial protection thus afforded to so important an 

 organ is not greater than that given by the cartilaginous 

 lamina of the cuttle-fish, which in form, texture, and situa- 

 tion, 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, appearing like beads strung together. 

 These rudimental bodies of vertebras have not yet completed 

 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 developments, we find, in 



the sturgeon and other cartilaginous fishes, a greater conden- 



# sation of substance produced by the deposition of granules 



of osseous matter; the central canal becomes divided into 



Jozenge-shaped compartments by the closing in of the sides 



