376 ZOOLOGY. 



by an opening : the double cavities resulting from the juxta- 

 position of the two vertebrae are filled with a soft elastic sub- 

 stance. The osseous ring formed by the processes of the dorsal 

 part of the column for the protection of the spinal marrow, is 

 repeated beneath the column in its caudal portion ; it lodges 

 the great artery of the trunk. 



In some the ribs are wanting, in others they are very com- 

 plete, and surround the trunk ; and in some they are con- 

 nected anteriorly with a chain of bones representing the 

 sternum. The ribs, moreover, often carry one or two small 

 spines, which are directed outwards, and penetrate the flesh. 

 Similar stylets also sometimes proceed from the bodies of the 

 vertebrae ; and these are in some very numerous, as in the 

 herring. Finally, in the median line of the body are found 

 the interspinal bones (Fig. 386, i) ; these rest on the spinous 

 processes of the vertebraa, and articulate by the other extre- 



Fig. 385. Common Swordfish (Xiphias Gladius). 



mities with the radii of the median fins (r) ; these radii vary 

 much in different fishes : sometimes they are only ossified at 

 the base, formed afterwards of a number of small articula- 

 tions. These last are called soft or articulated radii ; they 

 always form the caudal fin (Fig. 382), and sometimes there 

 are no others. 



480. The lateral fins, representing the limbs, are ter- 

 minated by radii similar to the dorsal, and analogous to the 

 fingers; in the pectoral fin, four or five small flat bones (Fig. 

 384 ca) represent the carpus, and these are supported by two 

 flat bones (ab), the radius and the ulna. The osseous girdle 

 supporting these is composed of a series of three bones, of 

 which one (h) represents the humerus, a second (o) the sca- 

 pula, and a third, composed of two pieces, may be called the 

 coracoid (co). The posterior limb (Fig. 382) is less com- 

 plex ; the radii of the ventral fin are supported only on a 

 single bone. 



481. In cartilaginous fishes, such as the skate and the 

 shark, the skeleton somewhat resembles that of the tadpole. 



