MUSCULAR SYSTEM OP FISHES. 



295 



the farther subdivision of their parts, and when, for the pur- 

 pose of adding strength to the fin, it becomes necessary to 

 multiply the points of support, intermediate bones are de- 

 veloped, serving as the basis of the rays. Convenience re- 

 quires that they should be detached from the ends of the 

 spinous processes, which is their usual position, and placed 

 between them: wdieh in this situation, they bear the name 

 of interspinoiis 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 ad- 

 ditional pieces, joined end to end, and carrying out the in- 

 terspinous bone, and the ray which terminates it, to a con- 

 siderable distance. This structure is distinctly seen in the 

 small dorsal fins of the Mackeral. The anal fins, which 

 are situated on the lower side of the body, in the vertical 

 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 man- 

 ner than those of the animals of the higher classes. Those 

 which appear immediately underneath the integuments are 

 shown in Fig. 194, where m, m are the great lateral muscles, 



producing the flexion of the body and tail: d is the dorsal fin, 

 which is raised by the muscle d; the pectoral fin, expanded 

 by the muscle p: v, the ventral fin, moved by the mu.scles 

 situated at v: a, the anal fin, in like manner moved by nms- 

 cles at its base a: and c, the caudal fin, the muscles for 

 moving which arc seen at c: o is the operculum, or flap. 



