vi . ] GENERAL VIE W OF INTERN A L SKELE TON. 229 



they so in the sense that each of the successive cranial arches 

 (each cranial segment) answers to the centrum neurapophyses 

 and neural spine of a vertebral arch (or spinal segment) i.e. 

 are they modified vertebrae ? To this it must be replied that 

 in development the parts which transitorily or permanently 

 represent in cartilage the bony brain-case of man are never 

 serially segmented, and thus differ fundamentally from the 

 cartilaginous predecessors of the bony vertebrae. Neverthe- 

 less, it cannot be denied that there is a singular and striking 

 reminiscence of vertebras in the three arches of the bony 

 skull. Certainly, if the essence of vertebrae consists in being a 

 series of bony rings fitted together and enclosing the nervous 

 centres, then it must be asserted that the skull is in part 

 composed of three bony vertebrae. Moreover the condition, 

 before described, of Bagrus (Fig. 60) shows how undoubted 

 vertebrae may become expanded, like the bony arches form- 

 ing the cranial walls. In their mode of development, how- 

 ever, these cranial segments certainly do not agree with true 

 vertebrae, though they have this singular secondary and in- 

 duced resemblance to such skeletal parts. 



It may be objected, however, that the number of the 

 splanchnapophysial arches does not correspond with that of 

 the cranial ones. . To this it may be rejoined, either (i) that, 

 as is very probably the case, more numerous primitive cranial 

 segments have coalesced or ceased to be developed, though 

 their hypaxial members still persisting, point to their former 

 existence ; or (2) that though the paraxial and hypapophysial 

 arches correspond in number with the vertebral segments 

 of their respective regions, yet that the splanchnapophysial 

 arches, belonging to a category by themselves, may really 

 answer to the existing epaxial arches, and this as follows: 



The trabeculae to the first arch ; 



The palatine arch to the sphenoidal arch ; 



The mandible to the occipital arch ; 



The hyoid and cornua to the atlas ; 



The hyoid and cornicula to the axis ; and 



The succeeding splanchnapophysial arches to the several 

 succeeding cervical vertebrae or their representatives in 

 lower animals. On the other hand, it may be that these 

 arches correspond with aborted vertebrae, all the centra of 

 which are, in existing animals, represented only by the basi- 

 occipital and basi-sphenoid. 



17. We may now review man's limb-bones (or appendi- 

 cular skeleton) in the most generalized way. 



