The Skeleton 73 



'Three bones, the epiotic, opisthotic, and pro- 

 otic, together form the auditory or periotic capsule 

 of each side. They are wedged in between the lat- 

 eral portions of the occipital 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 capsule. 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 epiotic 

 similarly becomes united with the supraoccipital, 

 the pro-otic (Fig. 23, 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 fcnestra 

 ovalis. 



" The tympanic cavity, leading to the exterior by 

 the external auditory meatus (Fig. 21, 16), is well 

 seen in a side view of the skull ; it is bounded on its 

 inner side by the periotic bones, posteriorly in 

 part by the exoccipital, and elsewhere mainly 

 by the quadrate. A large number of canals and 

 passages open into it. On its inner side opening 

 ventro-anteriorly is the fenestra ovalis, opening 

 ventro-posteriorly the internal auditory meatus 

 (Fig. 23, VIII), while dorsally there is a wide open- 



