250 THE VERTEBRATE SKELETON. 



of the orbit and sends veiitralwards a process which meets 

 the palatine. 



The postfrontals (fig. 44, 6) are small bones lying at the 

 sides of the posterior part of the frontal. Each is united 

 with a number of bones, on its inner side with the frontal and 

 parietal, behind with the squamosal, and ventrally with the 

 alisphenoid. It also unites by means of a strong descending 

 process with an upgrowth from the jugal, and thus forms a 

 postorbital bar separating the orbit from the lateral tem- 

 poral fossa. The postfrontal forms also part of the outer 

 boundary of the supratemporal fossa. 



2. THE SENSE CAPSULES. 



Skeletal capsules occur in connection with each of the three 

 special sense organs of sight, of hearing and of smell. 



The Auditory capsules and associated bones. 



Three bones, the epi-otic, opisthotic and pro-otic, 

 together form the auditory or periotic capsule of each side. 

 They are wedged in between the lateral portions of the occi- 

 pital and parietal segments and complete the cranial wall in 

 this region. Their relations to the surrounding structures are 

 very complicated, and many points can be made out only in 

 sections of the skull passing right through the periotic cap- 

 sule. The relative position of the three bones is, however, 

 well seen in a median longitudinal section. The opisthotic 

 early becomes united with the exoccipital, while the epi-otic 

 similarly becomes united with the supra-occipital, the pro- 

 otic (fig. 45, 7), seen in longitudinal section to be pierced 

 by the prominent trigeminal foramen alone remaining 

 distinct throughout life. The three bones together surround 

 the essential organ of hearing which communicates laterally 

 with the deep tympanic cavity by the fenestra ovalis. 



The tympanic cavity, leading to the exterior by the 

 external' auditory meatus (fig. 44, 16), is well seen in a 



