300 ON THE STRUCTURE OF THE VERTEBRATE SKULL. 



cartilaginous plate, which results from the coalescence and out- 

 growth, beyond the boundaries of the brain-case, of the trabecular 

 cranii ; partly, by outgrowths from the superior and inferior 

 edges of that plate ; and partly, by a prolongation outwards 

 of the front part of the outer wall of the brain-case, into an 

 antorbital process, between the orbit and the nasal sac, on each 

 side. Cartilage bone developed in the septum gives rise to the 

 ethmoid ; in the antorbital processes, to the prefrontals ; in the 

 superior, or inferior, lateral prolongations of the side w T alls, to 

 the turbinal bones. Membrane bones developed upon the roof 

 of these olfactory capsules give rise to nasals ; beneath the 

 septum, to vomers. 



13. The ethmoid and its dependencies are developed within 

 the median " fronto-nasal ' process, which grows out from the 

 front wall of the embryonic skull, between the rudimentary 

 nasal sacs ; and the inferior, broad, free edge of which bounds 

 the mouth. It is in this free edge that the preniaxillae are 

 developed, and they are, at first, perfectly distinct from the 

 maxillae. The latter, together with the palatine and pterygoid 

 bones, are formed within the maxillary processes, which bound 

 the sides of the primitive oral cavity, and run, parallel with one 

 another, along the base of the fore-part of the embryonic cranium, 

 uniting, behind, with the first visceral arch, but being, at first, 

 completely separated, anteriorly, from the fronto-nasal process. 

 Clearly therefore, if the premaxillse and maxillae, &c, are to be 

 regarded as constituents of inferior arches of the skull, they are 

 not parts of one arch, but of, at least, two distinct arches. 



14. Of the first and second visceral arches, which He imme- 

 diately behind the mouth, the former, which gives rise to the 

 mandible and quadrate bone, passes into the skull under the 

 front part of the auditory capsule ; while the root of the latter, 

 in which a greater or smaller part of the hyoidean apparatus is 

 developed, underlies the hinder part of that capsule. It is 

 therefore impossible that the mandibular and hyoidean arches 

 should be dependencies of any other parts of the skull than 

 those which lie immediately in front of, or behind, the auditory 

 capsules ; and in the completely ossified skull we never, as a 

 matter of fact, meet with these arches in any other position. 



