378 THE MECHANICAL FUNCTIONS. 



from distinct centres of ossification. These rays^ as 

 they are called, are sometimes destined to grow to 

 so considerable a length, as to require being sub- 

 divided into many pieces, in order to lessen the 

 danger of fracture, to which a very long filament 

 of bone would have been exposed, and also to admit 

 of a greater degree of flexibility. These rays as- 

 sume branched forms from the further subdivision 

 of their parts ; and when, for the purpose of adding 

 strength to the fin, it becomes necessary to mul- 

 tiply the points of support, intermediate bones are 

 developed, serving as the basis of the rays. Con- 

 venience requires that they should be detached 

 from the ends of the spinous processes, which is 

 their usual position, and placed between them : 

 when in this situation, they bear the name of iuter- 

 spinous hones; and when a still greater length of os- 

 seous support is wanted, new centres of ossification 

 are developed at their extremities, giving rise to a 

 series of additional pieces, joined end to end, and 

 carrying out the interspinous bone, and the ray 

 which terminates it, to a considerable distance. 

 This structure is distinctly seen in the small dorsal 

 fins of the Mackerel. The anal fins, which are 

 situated on the lower side of the body, in the ver- 

 tical plane, and next to the tail, are, in like manner, 

 supported by rays, having the same parallel, or 

 fan-like arrangement as the preceding. The caudal 

 fin, or terminal expansion of the tail, has also a 

 similar structure. 



The muscles of fishes compose a large portion of 

 the bulk of the body ; but they are arranged in a 

 less complex manner than those of the animals of 

 the higher classes. Those which appear imme- 



