Motor System — Muscles and Skeleton 113 



Ribs and Sternum 



Ribs are elongated skeletal parts (cartilaginous or bony) attached 

 to the vertebral column and projecting laterally into the body-wall. 

 They strengthen the body-wall and provide attachment for muscles. 

 They develop in the connective tissue between myomeres or parts of 

 myomeres. They are at first cartilaginous but, except in sharklike 

 fishes, are more or less completely ossified in the adult. Two types of 

 ribs are distinguished, depending mainly on the relation of the rib to 

 the body-muscle. A dorsal (or "true") rib arises from a vertebra at 

 the level of the horizontal septum — i.e., between the epaxial and hy- 

 paxial parts of a myomere — and extends outward and downward in a 

 myoseptum — i.e., between two myomeres (Fig. 100B). A ventral (or 

 pleural or hemal) rib arises at a lower level on a vertebra and extends 



Fig. 111. Vertebrae and ribs of {left) ante- 

 rior and (right) posterior trunk region of Po- 

 lypterus. (h) Hemal rib; (p) pleural rib. (After 

 Gegenbaur. Courtesy, Kingsley: "Compara- 

 tive Anatomy of Vertebrates," Philadelphia, 

 The Blakiston Company.) 



downward at the intersection of a myoseptum with the peritoneum, 

 therefore internal to the hypaxial muscle and closely adjacent to the 

 peritoneum. A vertebra carries only one pair of dorsal ribs or one pair 

 of ventral ribs, but ribs of both kinds may occur on one vertebra. 



In the great majority of vertebrates, the ribs are of the dorsal type. 

 Of modern vertebrates, only the fishes possess ventral ribs, and they 

 are the characteristic ribs of most fishes. But the relatively short car- 

 tilaginous ribs of sharklike fishes are dorsal, and ventral ribs are lack- 

 ing. Some fishes (e.g., sturgeon and gar pike — "ganoid" fishes) have 

 ventral ribs only, while in many fishes both series of ribs are present 

 (Fig. 111). In many of the so-called "bony fishes," one or more pairs 

 of riblike intermuscular bones may be attached to a vertebra in 

 addition to its two pairs of ribs. These intermuscular bones, however, 

 usually develop as direct ossifications of myoseptal connective tissue 

 — i.e., they are membrane bones. With this multiplicity of ribs and 

 riblike appendages of the vertebrae, many fishes become excessively 

 "bony." 



