372 THE MECHANICAL FUNCTIONS. 



ber of vertebrsB is very various in different fishes : 

 in some they are multiplied exceedingly, as in the 

 shark, where there are more than two hundred. 



In the spine of fishes, we may trace a regular 

 gradation of developement from the simplest and 

 almost rudimental condition in which it exists in 

 the Myxine and the Lamprei/, to that of the most 

 perfect of the osseous tribes. Its condition, in the 

 former of these animals, presents a close analogy 

 with some structures that are met with in the mol- 

 luscous, and even in annulose animals. So near 

 is the resemblance of the spinal column of the 

 myxine, more especially, to the annular condition 

 of the framework of the Vermes, 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 

 amons: the Vertebrala are, indeed, but slender and 

 equivocal ; for, in place of a series of bones com- 

 posing the vertebral column, it has merely a soft 

 and flexible tube of a homogeneous and cartilagi- 

 nous substance, exhibiting scarcely any trace of 

 division into separate rings, but appearing as if it 

 were formed of a continuous hollow cylinder of 

 invertebral substance, usurping the place of the 

 vertebrse, which it is the usual office of that sub- 

 stance to connect together, and having in its axis a 

 continuous canal filled with gelatinous fluid. This, 

 however, 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 pro- 

 tection. The nervous matter here consists merely 



