IX.] OF THE DOG. in 



cranial segment, in which were inserted certain bones not 

 wt noticed. These bones form a definite group by them- 

 selves, at all events locally connected, though very different 

 in function and structure. 



In a mass of cartilage, in the position just indicated, 

 ossification takes place from several centres (three, called 

 respectively pro-otic, epiotic, and opisthotic, according to 

 Professor Huxley and others, in the human skull; 1 but the 

 process has not been accurately traced in other Mammals). 

 These very rapidly unite to form a single bone, which com- 

 pletely encloses the labyrinth or essential organ of hearing, 

 consisting of the vestibule, semicircular canals, and cochlea. 

 This bone is the periotic (Per). It is divided into two por- 

 .ions : an antero-internal, which forms a somewhat angular 

 >rojection within the cranial cavity, and is of remarkable 

 lensity — the petrous portion • and a postero-external, a sort 

 )f process from the former, smaller, less dense, and forming 

 i small portion of the wall of the cranium, appearing 

 jxternally just in front of the exoccipital — the mastoid 

 lortion. 

 The petrous is of course the more important, and has 

 Constant characters throughout the class, while the mastoid 

 s very variable, and sometimes can scarcely be said to exist, 

 t is in no case a separate bone ; and, although a portion of 

 may develop originally from a separate centre, it is always 

 before birth firmly united with the petrous, so that they 

 ill be spoken of here as one bone, under the name of 



>tic. 



j The essential characters of the petrous portion of the 



eriotic are, that it contains within it the internal ear ; 



it has on its inner or cranial side a foramen, through 



hich the facial and the auditory nerves leave the cranial 



1 Elements of Comparative Anatomy (1864), p. 148. 



