ii4 ELEMENTARY ANATOMY. [LESS. 



The infra-orbital foramen may be multifold, as in most 

 Apes, or may be enlarged into an enormous aperture, trans- 

 mitting part of the masseter muscle, as is the case in the Por- 

 cupine and some other Rodents. This great opening may 

 exist, and yet the true infra-orbital canal be denned by a bony 

 lamella, as in the genus Lagostomus. 



The palatine plate of the maxilla exists in all Mammals 

 and in Crocodiles, yet even in Mammals it may occasionally 

 be excessively reduced, as in the Hare. In the lower classes 

 it is wanting. 



JKK 



pm' 



FIG. 107. SIDE VIEW OF THE SKULL OF A PORCUPINE (Hystrix crisiata). 

 a, angle of mandible ; c, occipital condyle ; cr, coronoid process of mandible ; cy, 

 condyle of mandible ; f, frontal , z, incisor teeth ; j, ascending branch of 

 maxilla enclosing the enormous infra-orbital foramen, the course of the 

 masseter muscle through which is indicated by an arrow ; in, molar teeth ; 

 7/?_r, maxilla ; n, nasal ; pin, premolar teeth ; pm', paramastoid process ; px, 

 premaxilla ; t. tympanic bone ; z, zygomatic arch the part formed by the 

 malar. 



The maxilla may acquire an enormous size, and overlap 

 and almost entirely conceal the frontals, as in Cetaceans. 



Sometimes, as in Chiromys, the maxilla is shut out from 

 articulating with the nasal bone by the extension upwards 

 to the frontal of that separate element of the human 

 maxillary bone which is next to be noticed. The latter 

 element may, sometimes, be separated by an interval from 

 the part corresponding to the rest of the human maxilla, as 

 in the Three-toed Sloth. 



Occasionally (as in some specimens of Macacus nemes- 

 trinus, and in the Gavial) the maxillary bone may join its 

 fellow of the opposite side above the nasals, thus separating 

 the latter from the frontals. 



Occasionally, as in some Chameleons, the maxilla may 

 project freely forwards as a great bony horn at the front part 

 of the face. 



