86 THE VERTEBRATE SKELETON. 



B. THE RIBS. 



The ribs are slender, more or less cylindrical bones at- 

 tached to the postero-dorsal faces of the ventri-lateral pro- 

 cesses of all the trunk vertebrae except the first and second. 

 The earlier ones are thicker and more curved ; the later ones 

 thinner and more nearly straight. The ribs are homologous 

 with the distal parts of the haemal arches of the caudal 

 vertebrae. 



Associated with the ribs are a second series of rib-like 

 bones, the intermuscular bones. These are slender, curved 

 bones which arise from the ribs or from the ventri-lateral 

 processes at a distance of about an inch from the centra, and 

 curve upwards, outwards and backwards. In the anterior 

 region where the ventri-lateral processes are short they arise 

 from the ribs, further back they arise from the ventri-lateral 

 processes. 



C. THE UNPAIRED OR MEDIAN FINS. 



These are six in number, three being dorsal, one caudal 

 and two anal. 



The dorsal and anal fins each consist of two sets of 

 structures, the fin-rays and the interspinous bones. Each 

 fin-ray forms a delicate, nearly straight, bony rod which 

 becomes thickened and bifurcated at its proximal or vertebral 

 end, while distally it is transversely jointed and flexible, 

 frequently also becoming more or less flattened. 



The first dorsal fin has thirteen rays, the second, sixteen to 

 nineteen, the third, seventeen to nineteen. The first anal fin 

 has about twenty-two, the second anal fourteen. In each 

 fin the posterior rays rapidly decrease in size when followed 

 back. 



The interspinous bones of the dorsal and anal fins 

 alternate with the neural and haemal spines respectively, and 

 form short, forwarclly-projecting bones, each attached proxi- 

 mally to the base of the corresponding fin-ray. 



