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 



tins 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 



fnvniprl 

 lOimeU 



g Fig 383 __ a< 8knn . 6> vertebra ; c, sacrum, 



> verera ; , , 



(urostyle) ; /, Buprascap- 



ore-arm bones; t, wrist 



nn 

 On 



_ 



and e, its continaation 



u , a . ^ humerus; h, fore- 



COm- bones' (carpals and metacarpals) ; d, ilium ; m, 

 CU thjgh (fm ^ lr) . M( leg bone (ulna) . Of elongated 



iinnnd svstem of lever- first pair of ankle-bones (tarsals) ; p, q, foot 

 'v b ' bones or phalanges.- After 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, o) is formed by the labial cartilages (trabeculcc 

 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 by Balfour and J. K. 

 Thatcher, who attempt to show that the limbs with their girdles were 

 derived from a series of similar simple parallel rays, and that they 

 were originally a specialization of the continuous lateral folds or fins of 

 embryo fishes, and probably homologous with the lateral foils of the 

 adult lancelet (Amphioxus). 



