THE BONES OF THE FACE. 25 



sphenoidal region. In ordinary fishes and AmpJiibia, this 

 bone appears to replace the basisphenoid and presphenoid 

 functionally, while in the higher Vertebrata it becomes 

 .;onfoimded with the basisphenoid. The Vomer is a similar, 

 splint-like, single or double, membrane bone, which, in like 

 manner, underlies the ethmoid region of the skxill. 



In addition to the bones already mentioned, a prefrontal 

 bone may be developed in the prefrontal region of the nasal 

 capsule, and bound the exit of the olfactory nerve externally. 



A postfrontal bone may appear behind the orbit above the 

 alisphenoid. Sometimes it seems to be a mere dismember- 

 ment of that bone ; but, in most cases, the bone so named 

 is a distinct membrane bone. 



Furthermore, on the outer and upper surface of the 

 auditory capsule a membrane bone, the squamosal, is very 

 commonly developed ; and another pair of splint-bones, the 

 nasals, cover the upper part of the ethmovomerine cham- 

 bers, in which the olfactory organs are lodged. 



TJie Osseous Facial Apparatus. — The bones of the face, 

 which constitute the inferior arches of the skull, appear 

 within the various processes and visceral arches which 

 have been enumerated. Thus, the premaxilloe are two bones 

 developed in the oral part of the naso-frontal process, one 

 on each side of the middle line, between the external nasal 

 apertures, or anterior nares, and the anterior boundary of 

 the mouth. 



Ossification occurs in the palato-pterygoid cartilage at 

 two chief points, one in front and one behind. The ante- 

 rior gives rise to the palatine bone, the posterior to the 

 pterygoid. Outside these, several membrane bones may 

 make their appearance in the same process. The chief of 

 these is the maxilla, which commonly unites, in front, with 

 the premaxilla. Behind the maxilla there may be a 

 second, the jugal; and occasionally behind this lies a 

 thii'd, the quadrato-jugal. 



Between the maxilla, the prefrontal, and the premaxilla, 

 another membrane bone, called lachrymal, from its ordinary 



