294 



THE MECHANICAL FUNCTIONS. 



farther corroboration of the analogy which the cranial bones 

 have to vertebrae. 



In the ray and the shark tribes, both the anterior and pos- 

 terior extremities are sup- 

 ported by arches of bones, 

 formino; a sort of belt. This 

 structure is an approach to 

 that which obtains in many 

 reptiles, and indicates a farther step in the regular progress 

 of development. This belt in the ray is shown in Fig. 193. 

 In examining that part of the skeleton of fishes which 

 corresponds to the posterior extremity, we observe the total 

 absence of both femur and tibia; but the bones of the toes 

 are attached to a set of small bones, which appear to act the 

 part of a pelvis, but which, in consequence of their not being 

 connected with the spine, have no determinate situation, and 

 are found at various distances from the head in different 

 fishes. They appear emancipated from the restraints to 

 which they would have been subjected had they been fixed 

 to a sacrum, or to any particular part of the spine: and we 

 find them, accordingly, often placed considerably forwards; 

 and in some instances, as in the Subhrachieni, even anteri- 

 orly to the pectoral fins, which are the true arms of the ani- 

 mal. But in one whole order of fishes, the ^djjodes, there 

 is not even a 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. 1S4,) which are joined to the spinous processes of 

 the vertebras, and are formed from distinct centres of ossi- 

 fication. 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 filajiient of bone 

 would have been exposed, and also to allow of a greater de- 

 gree of flexibility. These rays assume branched forms from 



