. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. â â ^v^Gastralia In Lacertilia (Fig. 463) and Crocodilia (Fig. 464) there is an oblong, cartilaginous plate continuous with which are usually two hornlike, posterior processes. To the anterior end of the sternum the pectoral girdle is attached. Ribs connect with the sides and with the posterior horns. These ani- mals may have been the first to have the ribs attached to the sternum, although some palaeon- tologists have suggested the possibility that the ri
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. â â ^v^Gastralia In Lacertilia (Fig. 463) and Crocodilia (Fig. 464) there is an oblong, cartilaginous plate continuous with which are usually two hornlike, posterior processes. To the anterior end of the sternum the pectoral girdle is attached. Ribs connect with the sides and with the posterior horns. These ani- mals may have been the first to have the ribs attached to the sternum, although some palaeon- tologists have suggested the possibility that the ribs of Stegocephalia reached such a midventral struc- ture. Snakes and turtles are without sterna. Birds have a large thin sternum of replacing bone that affords attachment for the pectoral muscles of flight. It is spread out flat in running birds (ratites), but in all flying birds (carinates), a carina, or keel, projects at right angles to the breastbone proper, furnishing considerable addi- tional surface for muscle attachment. The pec- toral girdle and bony sternal divisions of the ribs attach to the sternum. A comparison of the keels on the breastbones of an albatross, a pigeon, and a hummingbird (drawn to the same scale in Figure 465) shows how large a relative sur- face for muscle attachment is necessary to enable the tiny hummingbird to maintain its marve- lous vibrating wing movements, which are so rapid that the wings them- selves, like the spokes of a swiftly rotating wheel, blend out of sight. A carina is also a characteristic of extinct flying reptiles (pterodactyls) as well as of flying bats, and in burrow- ing moles among mammals, although in this latter case it is probably not homologous with the carina of birds. In mammals the sternum typically consists of a series of six to nine sternebrae, composed of replacing bone, followed by a small cartilaginous process (Fig. 466). To the anterior- most of these elements, called the man- ubrium, is attached the first pai
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