THE SKULL 273 



border for the passage of the lacrymal duct, 

 4 which is supported by a process of the bone 



extending forwards into the nose. 



The lacrymals correspond to the most 

 anterior of a series of small bones, placed 

 round the borders of the orbits in bony fishes. 



c. Bones in relation with the auditory capsules. 



i. The periotics are the bones which replace the 

 cartilaginous capsules surrounding the organs 

 of hearing in the embryo. Each ossifies from 

 three centres ; but the several bones pro -otic, 

 epiotic, opisthotic early unite to form an 

 irregular mass, which fits loosely into the side- 

 wall of the cranium, between the occipital 

 segment and the squamosal, and which 

 readily falls out from the dried skull. 



In each periotic there may be distinguished 

 a dense petrous portion, enclosing the essential 

 part of the organ of hearing ; and a porous 

 mastoid portion, which forms its outer and 

 posterior part, and is produced downwards 

 into the mastoid process, lying immediately in 

 front of the par-occipital process of the ex-occi- 

 pital bone. 



On the inner surface of each periotic, as 

 seen in a bisected skull, there are two depres- 

 sions. Of these the upper and larger one, 

 the floccular fossa, lodges the floccular lobe of 

 the cerebellum. The lower one is divided by a 

 ridge of bone into two, of which the anterior 

 one is the opening of the aqueductus Fallopii, 

 transmitting the facial nerve ; and the posterior 

 one is that of the meatus auditorius internus, 

 through which the auditory nerve enters the 

 internal ear. 



The anterior border of the periotic is pro- 

 duced inwards into a prominent crest of bone 



