. Zoology : for students and general readers . Zoology. COMPOSITION OF THE SKULL. 397. extreme modification in form. At first all limbs arise as little pads, in which the skeletons subsequently develop, and in early life the limbs of all Vertebrates above the fishes are much alike, the mod- ifications taking place shortly before birth. Ac- cording to Gegenbaur and others, the limbs of Vertebrates have been probably derived from the pectoral and ventral fins of fishes in which the fin-rays are irrela- tively repeated. * In the fins of fishes there is a simple system of leverage ; in the limbs o
. Zoology : for students and general readers . Zoology. COMPOSITION OF THE SKULL. 397. extreme modification in form. At first all limbs arise as little pads, in which the skeletons subsequently develop, and in early life the limbs of all Vertebrates above the fishes are much alike, the mod- ifications taking place shortly before birth. Ac- cording to Gegenbaur and others, the limbs of Vertebrates have been probably derived from the pectoral and ventral fins of fishes in which the fin-rays are irrela- tively repeated. * In the fins of fishes there is a simple system of leverage ; in the limbs of higher air-breathing ^j^ ^gg _b^ aijâii. ^^ vertelirae ; c, sacrum, VorfoVirn tpa fnrmpd bv and «, its continuation (iii-ostyle) ;/, enprascap- V eiteuiauefe, loimeu uy ^j^. ^_ humerus; A, fore-arm bones; i, wrist Tvnli'i'no- rm bmrl a com- bones'(carpals and metacarpals); d, ilium; m, walking on lanu, a com ^^^.^j^ \{^^^^â) â ââ les bone (ulna); o, elongated â pound system of lever- first pair of ankle-bones (tarsals) ; ;), (/, foot â ^ -^ bones or plialanges.âAfter Owen. age (Wyman). The head of all Vertebrates above the lancelet is supported by a more or less perfect cartilaginous or bone framework, the skull (cranium), or brain-box (Fig. 386). It is a contin- uation of the vertebral column, and protects the brain, besides forming the support of the jaws, tongue-bone (hyoid bone), and branchial arches. The series of lateral (visceral or branchial) arches varies, but there may be nine ; the most anterior (if it be counted as the first one, Fig. 387, a, b, c) is formed by the labial cartilages {traleculm cranii); next comes the mandibular arch (o, n), which is suc- ceeded by the hyoid arch (II.) and the six branchial arches. In the embryos of all Vertebrates these visceral arches are * A modified form of this theory is advocated hy Balfour and J. K. Thatcher, who attempt to show that the limbs with their girdles were derived from a series of similar simple paralle
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
