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 fenestra 
ovalts. 
‘“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- 
