ix.] OF THE DOC. 131 



cranial segment, in which were inserted certain bones not 

 yet 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, ebiotic, 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 (Pfr). It is divided into two por- 

 tions : an antero-internal, which forms a somewhat angular 

 projection within the cranial cavity, and is of remarkable 

 density the petrous portion ; and a postero external, a sort 

 of process from the former, smaller, less dense, and forming 

 a small portion of the wall of the cranium, appearing 

 externally just in front of the exoccipital the mastoid 

 portion. 



The petrous is of course the more important, and has 

 constant characters throughout the class, while the mastoid 

 is very variable, and sometimes can scarcely be said to exist. 

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

 it may develop originally from a separate centre, it is always 

 before birth firmly united with the petrous, so that they 

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

 periotic. 



The essential characters of the petrous portion of the 

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

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

 which the facial and the auditory nerves leave the cranial 



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

 K 2 



