THE CRANIAL SKELETON. 



23 



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

 l)ind, and marks the position of the mandible or lower jaw. 

 The cartilage which it contains is termed MeckeVs cartilage. 



The cartilaginous rod contained in the second visceral arch 

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



Fig. 8. — TTncicr-v-lew of the head of a Fowl at the seventh day of incubation. — 7<7, the cere 

 bral hemispheres causing the integument to bulge ; a, the eyes ; g^ the olfactory sacs 

 ^•, the fronto-nasal process ; Z, the maxillary process ; 1, 2, the fii-st and second visceral 

 arches ; cc, the remains of the first visceral cleft. 



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

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



A ridge, continued forward from the first visceral arch to 

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

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

 palato-pterygoid rod, developed in this process, becomes con- 

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

 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 2i fronto-nasal process (Fig, 4, F ; Fig. 8, k). 

 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 aperture^ 

 by which the nasal cavity is placed in communication with 

 that of the mouth. 



