112 



FISHES. 



muscles which bound the abdomen. Usually each rib gives 

 ofif a spine-like bone, wliich is directed backwards amongst 

 the muscles. Inferiorly the extremities of the ribs are free, 

 or are rarely united to dermal ossifications in the middle 

 line of the abdomen ; but there is never any breast-bone or 

 sternum properly so called. 



Fig. 4S7. — Skeleton of the common Percli {Perca fluviatilis). p. Pectoral fin ; r, One of 

 the ventral fins ; a. Anal fin, snpport«d upon interspinons hones (i) ; c, Caudal fin ; <?, First 

 dorsal fin ; d'. Second dorsal fin, hoth supported npon interspinons hones ; i, i, Interspinons 

 hones ; r, Rihs ; s. Spinous processes of vertehree ; h, Hsmal processes of vertehrse. 



The only remaining bones connected with the skeleton of 

 the trunk are the so-called interspinous hones (fig. 487, i, i). 

 These form a series of dagger-shaped bones plunged in the 

 middle Kne of the body between the great lateral muscles 

 which make up the greater part of the body of a fish. The 

 internal ends or points of the interspinons bones are attached 

 by ligament to the spinous processes of the vertebras ; whilst 

 to their outer ends are articulated the "rays" of the so-called 

 " median " fins, which will be hereafter described. As a 

 rule, there is only one interspinons bone to each spinous 

 process, but in the Flat-fishes (Sole, Turbot, &c.) there are 

 two. 



Besides the fins which represent the limbs (pectoral and 

 ventral fins), fishes possess other fins placed in the middle 

 line of the body, and all of these alike are supported by bony 

 spines or " rays," which are of two kinds, termed respectively 

 " spinous rays " and " soft rays." The " spinous rays " are 

 simple bony spines, apparently composed of a single piece 



