608 



EMBRYOLOGY. 



older part and alone comparable with the remainder of the axial 

 skeleton, that the non-vertebral part, on the contrary, is a later acquisi- 

 tion and constitutes a new structure, which has been caused by the 

 forward extension of the fore-brain vesicle and by the development 

 of the organ of smell, to the enclosing of which (nasal capsule) it 

 contributes. 



The second method of division is based upon the different appear- 

 ance which the individual regions of the primordial cranium acquire 

 through their relations to the sense organs. The anterior end of 



Ik 



Fig. 330. Diagrammatic representation of the cartilaginous cranial capsule and the cartilaginous 

 visceral skeleton of a Selachian and of the larger nerve trunks of the head. 



2V, Nasal capsule (ethmoid region of the prin ordial cranium) ; Au, cavity for the eye (orbital 

 region) ; La, region of the labyrinth ; Oc, occipital region of the cranium ; O, palato-quad- 

 ratum ; U, lower jaw (mandibulare) ; Ik, labial cartilage ; zb, hyoid arch ; kb, first to fifth 

 branchial arches ; Tr, nervus trigeminus ; Fa, facialis ; 01, glosso-pharyngeus ; Va, vagus ; 

 rl, ramns lateralis of the vagus ; rb, rami branchiales of the vagus. 



the cartilaginous capsule (fig. 330) receives the organ of smell ; a 

 following portion contains depressions for the eyeballs ; in a third 

 are imbedded the membranous auditory labyrinths ; finally, a fourth 

 effects a union with the vertebral column. Consequently one may 

 distinguish an ethmoidal, an orbital, a labyrinthine, and an occipital 

 region. 



In addition to the cartilaginous primordial cranium, there are 

 developed in the head numerous cartilaginous pieces (which serve as 

 supports to the walls of the head-gut) in a manner similar, although 

 not directly comparable, to that in which the ribs (fig. 330) have 



