424 THE MECHANICAL FUNCTIONS. 



vestige of ventral fins, nor are any pelvic bones 

 provided for their support. This is the case 

 with the Eel, the Gymnotus, &c. In a few species 

 there is also a total absence of pectoral as well 

 as ventral fins. 



The dorsal fins are supported by a series of 

 slender bones (D Fig. 184), which are joined to 

 the spinous processes of the vertebrae, and are 

 formed 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 subdivided 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 allow of a greater de- 

 gree of flexibility. These rays assume branched 

 forms from the further subdivision of their parts, 

 and when, for the purpose of adding strength to 

 the fin, it becomes necessary to multiply the points 

 of support, intermediate bones are developed, 

 serving as the basis of the rays. Convenience 

 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 inter- 

 spinous bones; and when a still greater length 

 of osseous 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 



