SKULL. 



117 



anterior or frontal region likewise consist of the basal praesphenoid, 

 the two lateral orbitosphenoids, and the two dorsal frontal bones, 

 which are membrane bones and complete the arch dorsally. 



The ethmoid may be regarded as representing the body of a fourth 

 or anterior vertebra ; it is covered above by the nasal bones and 

 below by the vomer. 



Finally, between these different bones other bones are intercalated, 

 e.g., the mastoid and petrosal between the occipital and sphenoidal. 



Recently essential objections to this vertebral theory have been 

 raised by Huxley and Gegenbaur ; and the.-e objections have proved 

 fatal to the theory. According to Gegenbaur, the skull is composed 



FIG. 575. Median longitudinal section of a sheep's skull seen from the inside. Ob, basi- 

 occipital ; O7, exoccipital ; Os, supraoccipital ; Pe, petrous bone ; Spb, basisphenoid ; 

 Px, praesphenoid ; Ah, alisphenoid ; On, orbitosphenoid ; Pa, parietal; Fr, frontal; 

 Sf, frontal sinus ; Na, nasal ; C, tnrbinal ; Ci, interior turbinal ; Ft, pterygoid ; Pal, 

 palatine; I'o, vomer ; MX, maxilla ; JW-, inter-maxi!la (pre-maxilla). 



of a much greater number of segments corresponding to the primary 

 visceral arches, and the resemblances between the cranial bones, 

 especially of the median and anterior regions of the skull, and the 

 parts of a vertebra are entirely secondary. 



The rest of the hard parts, which are more or less intimately 

 connected with the skull, consist of a number of arches lying one 

 behind the other, and surrounding the entrance into the visceral 

 cavity. 



The anterior of these the maxillo-palatine apparatus forms the 

 facial region. In its simplest form it consists of two moveable pieces 

 (palato- quadrate and lower jaw), which are attached by the hyoman- 

 dibular (the dorsal element of the second arch) to the auditory region 



