2 20 PHYLUM CHORD ATA 



the greater part of the roof of the skull and the medial walls of 

 the orbits. Immediately back of the frontal bone on each side 

 is the small, irregular parietal. 



The auditory capsule is formed of an intricate complex of bones 

 at the posterolateral corner of the skull. From the hinder end 

 of the capsule two prominent spinelike processes project ; these 

 processes on the two sides, together with the median supra- 

 occipital spine, form the five prominent projections at the hinder 

 end of the skull. 



Of the two lateral processes the more dorsal is the epiotic 

 process ; it is formed by the small epiotic bone, which lies lateral 

 to the supraoccipital and behind the parietal. The more ventral 

 process is the parotic process. It is formed by two bones, the 

 more dorsal being the pterotic, the more ventral the opisthotic ; 

 the latter contains the foramen of the glossopharyngeal nerve. 

 Directly in front of the opisthotic is the prootic bone, through 

 the anterior border of which go the trigeminal and facial nerves, 

 and directly above which is the sphenotic. These five otic bones 

 form the auditory capsule. 



Just in front of the prootic and sphenotic are two bones, the 

 orbitosphenoid and the alisphenoid, the former being the more 

 ventral of the two ; they belong to the cranium proper, lie in the 

 lateral wall of the brain case, and are not seen from above. 



In front of the frontals and forming the anterior end of the 

 cranium are the paired nasals, which form the roof of the nasal 

 capsule. 



Exercise 26, Draw the dorsal aspect of the cranium on a scale of 2. 



Study the ventral aspect. Three bones form the medial por- 

 tion of the ventral wall of the cranium : the basioccipital at the 

 hinder end, the vomer at the forward end, and the long, slender 

 parasphenoid, or parabasal, between. The vomer bears teeth and 

 forms the ventral wall of the nasal capsule. 



In front of the orbits and directly above the vomer are the me- 

 dian ethmoid and the two lateral ethmoids, which form the 

 anterior end of the cranium proper and the hinder end of the 

 nasal capsules. 



