194 ZOOTOMY. 



seen by cutting away with a penknife the outer table of the cranial 

 bones in the auditory region, and then carefully removing the diploe ; 

 the canals are surrounded by a dense layer of bone, which, isolated 

 in the way described, forms the bony semicircular canals, precisely 

 corresponding in form and position with the contained membranous 

 canals. The anterior canal is considerably larger than the other two ; 

 the plane of the posterior canal is at right angles to that of the anterior, 

 and its ampulla is on its lower or outer ( = posterior) end ; owing to 

 this peculiarity in the position of the posterior canal, it crosses at right 

 angles the horizontal canal, which has the normal position (see descrip- 

 tion of cod's auditory organ, pp. 122, 123, § 1S2). 



55. In the young bird the main part of the auditory 

 capsule is formed by a single distinct bone, the prootic : 

 two other ossifications are however formed, one upper 

 and posterior, the epiotic, which early fuses with the supra- 

 occipital, another ventral and posterior, the opisthotic, 

 which becomes united with the exoccipital. 



56. A little below the swelling for the ampulla of the 

 anterior semicircular canal, on the inner surface of the skull, 

 are two small apertures, for the passage of the seventh 

 (facial) and eighth (auditory) cerebral nerves. 



57. Below these apertures,, and separated from them by a 

 ridge of bone, is a small oval fossa with an aperture (ix, x) 

 at the bottom of it, which leads directly on to the outer 

 surface of the skull, and serves to transmit the ninth 

 (glossopharyngeal), tenth (vagus), and eleventh (accessory) 

 cerebral nerves. 



58. A small aperture within and behind the above is the 

 condyloid foramen (xii) ; it also leads on to the exterior 

 of the skull and transmits the twelfth (hypoglossal) cerebral 

 nerve. 



59. Immediately below the optic fossa are two adjacent 

 apertures which lead on to the anterior surface of the 

 orbital plate, and serve for the passage of the fifth nerve 

 (trigeminal). 



