. The chordates. Chordata. 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 "tru
. The chordates. Chordata. 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—, between the epaxial and hy- paxial parts of a myomere—and extends outward and downward in a myoseptum—, 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 (, 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 bo
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