FISHES. 



309 



ribs of the higher Vertebrates, the ribs do not enclose a thoracic 

 cavity, but are simply imbedded in the muscles which bound 

 the abdomen. Usually each rib gives off a spine-like bone, 

 which is directed backwards amongst the muscles. Inferiorly 

 the extremities of the ribs are free, or are rarely united to der- 

 mal ossifications in the middle line of the abdomen \ but there 

 is never any breast-bone or sternum properly so called. 



Fig. 275. Skeleton of the common Perch (Perca fluviatilis). p Pectoral fin ; v One 

 of the ventral fins ; a Anal fin, supported upon interspinous bones (z) ; c Caudal fin ; 

 d First dorsal fin ; d' Second dorsal fin, both supported upon interspinous bones ; */ 

 Interspinous bones ; r Ribs ; ^ Spinous processes of vertebrae ; h Haemal processes of 

 vertebras. 



The only remaining bones connected with the skeleton of 

 the trunk are the so-called interspinous bones (fig. 275, i z). 

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

 middle line 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 interspinous bones are attached 

 by ligament to the spinous processes of the vertebrae ; 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 interspinous bone to each spinous process, 

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



Beside 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 "spi- 

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

 bony spines, apparently composed of a single piece each, but 

 really consisting of two halves firmly united along the middle 

 line. The " soft rays " are composed of several slender spines 



