THE CRANIAL SKELETON. 23 



The first visceral arch bounds the cavity of the mouth be- 

 iiind, and marks the position of the mandible or lower jaw. 

 The cartilage which it contains is termed MeckePs cartilage. 



The cartilaginous rod contained in the second visceral arch 

 of each side is the rudiment of the hyoidean apparatus. Like 



«!. /lllii.. 



illlillli 



Fro, 8. — Tlnder-Tlew of the head of a Fowl at the Beventh day of incubation. — la^ the cere 

 bral hemispheres causing the integument to bulge ; a, the eyes; o', the olfactory sacs 

 k^ the fronto-nasal ])rocess ; I, the maxillary process ; 1, 2, the first and second yiscezal 

 arches ; (c, the remains of the first viBceral cleft. 



the preceding, it unites with its fellow in the ventral median 

 line, where the so-called " body " of t he hyoid arises. 



A ridge, continued forward from the first visceral arch to 

 the olfactory sac (Fig. 4, F ; Fig. 8, I), bounds the mouth on 

 each side, and is called the maxillary process. A cartilaginous 

 palato-pten'ygoid rod, developed in this process, becomes con- 

 nected with Meckel's cartilage behind, and with the prefirontal 

 cartilage in front. 



The maxillary process is at first separated by a notch cor- 

 responding with each nasal sac, from the boundary of the 

 antero-median part of the mouth, which is formed by the free 

 posterior edge of a, fronto-nasaZ process (Fig. 4, F ; Fig. 8, Jc). 

 This separates the nasal sacs, and contains the cartilaginous, 

 ethmovomerine, anterior termination of the skull. The notch 

 is eventually obliterated by the union of the fronto-nasal and 

 maxillary processes, externally ; but it may remain open in- 

 ternally, and then gives rise to the posterior nasal apertvre, 

 by which the nasal cavity is placed in communication vnth 

 that of the mouth. 



