COMPOSITION OF THE SKULL. 



377 



extreme modification in form. At first all limbs arise as 

 little pads, in which the skeletons subsequently develop, and 

 iu early life tlie 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 

 £ns 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 

 Vertebrates, formed by 

 walking on land, a com- 

 pound system of lever- 

 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 (Pig. 381). 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. 

 382, a, b, c) is formed by what are called the labial carti- 

 lages; 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 modifled 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 folds of the 

 adult lancelet (Amphioxus). 



Fig. 377.— o, skull ; i, vertebrse : c, aacrnra, 

 and e, itB continuation (urostyle) ; /, eui)ra9cap- 

 nla ; g. humerus ; h, fore-arm bones : i, wrist 

 bones (carpala and metacarpals) : d, ilium ; m. 

 thigh ^femur) ; n, leg bone (tibia); 0, elongated 

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

 bones or phalanges.— After Owen. 



