342 MANUAL OF ZOOLOGY. 



cally said to be " amphicoelous." The cup-like margins of the 

 vertebral bodies are united by ligaments, and the cavities 

 formed between contiguous vertebras are rilled with the gela- 

 tinous remains of the notochord. This elastic gelatinous sub- 

 stance acts as a kind of ball-and-socket joint between the bodies 

 of the vertebrae, thus giving the whole spine the extreme mo- 

 bility which is requisite for animals living in a watery medium. 

 The ossification of the vertebrae is often much more imperfect 

 than the above, but in no case except that of the Bony Pike 

 (Lepidosteus] is ossification carried to a greater extent than 

 this. In this fish, however, the vertebral column is composed 

 of "opisthocoelous" vertebrae that is, of vertebrae the bodies of 

 which are concave behind and convex in front. The entire 

 spinal column is divisible into not more than two distinct re- 

 gions, an abdominal and a caudal region. The abdominal 

 vertebrae possess a superior or neural arch (through which passes 

 the spinal cord), a superior spinous process (neural spine), and 

 two transverse processes to which the ribs are usually attached. 

 The caudal vertebrae (fig. 125) have no marked transverse pro- 

 cesses ; but, in addition to the neural arches and spines, they 

 give off an inferior or hczmal arch below the body of the verte- 

 bra, and the haemal arches carry inferior spinous processes 

 (haemal spines). 



The ribs of a bony fish are attached to the transverse pro- 

 cesses, or to the bodies of the abdominal vertebrae, in the form 

 of slender curved bones which articulate with no more than 

 one vertebra each, and that only at a single point. Unlike the 

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

 cavity, but are simply embedded 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. 



The only remaining bones connected with the skeleton of 

 the trunk are the so-called i?iterspinous bones (fig. 125, ii). 

 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. 



