146 



CHORDATE ANATOMY 



and not even in the 'higher forms have become completely integrated. 

 These are the appendicular skeleton of the four limbs with their girdles; 

 and the axial skeleton, which includes the skull with the jaws, and the 

 vertebral column, the sternum, and the ribs. The individual bones 

 number, in man, sixty-four for the shoulder girdle and the arms, sixty-two 

 in the pelvic girdle and the legs, twenty-three in the skull, twenty-six 

 in the backbone, and twenty-five for ribs and sternum, with six ear bones 

 besides, over two hundred in all. 



TARSALS 

 METATARSALS 

 PHALANGES 



Fig. 133. — A diagram of the vertebrate skeleton, showing the division of the skeleton 

 into axial, visceral, and appendicular. Membrane bones are shown in black, cartilage 

 bones stippled. 



THE AXIAL SKELETON 



Evolution of the Vertebral Colunin. Nothing like a vertebral column 

 appears in any invertebrate, so that the earlier portions of its history are 

 unknown; though, if Amphioxus gives the clue, it was once no more than a 

 medial dorsal fold of the ahmentary canal. Its first certain beginnings are 

 the notochord of the lower chordates, the Hemichorda, Urochorda, and 

 Cephalochorda. In the cyclostomes, the notochord is still the main 

 part of the axial skeleton. Since the cyclostomes have cartilaginous neural 

 arches, it is probable that neural arches are the earhest vertebral elements. 



Elasmobranchs, both fossil and modern, show a considerable advance 

 over the cyclostomes. Cartilaginous haemal arches and centra appear, 

 with both neural and haemal spinous processes. The anterior trunk 

 vertebrae of elasmobranchs have short lateral or "costal" processes which 

 extend between the myotomes and which suggest the future ribs of 

 mammals. Since in fossil and living forms two centra may occur in each 

 body segment, and since each centrum usually develops in ontogenesis 

 by the fusion of antero-posterior anlagen, it is possible that two centra 

 in each segment (diplospondyly) may have been the original arrange- 

 ment in vertebrates. Elasmobranchs, moreover, begin the long process 

 of vertebral differentiation, the vertebrae of the tail being unlike those 



