2. THE SENSE CAPSULES. Skeletal capsules occur in connection with each of the three special sense organs of sight, of hearing and of smell. The =Auditory capsules= and associated bones. Three bones, the =epi-otic=, =opisthotic= and =pro-otic=, together form the auditory or =periotic= capsule of each side. They are wedged in between the lateral 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 =epi-otic= similarly becomes united with the supra-occipital, the =pro-otic= (fig. 45, 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 ovalis=. The =tympanic cavity=, leading to the exterior by the =external auditory meatus= (fig. 44, 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. 45, VIII), while dorsally there is a wide opening which forms a communication through the roof of the brain-case with the tympanic cavity of the other side. On its posterior wall is the prominent foramen through which the facial nerve passes on its way to its final exit from the skull through the exoccipital, this foramen is bounded by the quadrate, squamosal, and exoccipital. The opening of the fenestra ovalis is in the fresh skull occupied by the expanded end of the auditory ossicle, the =columella=, whose outer end articulates by a concave facet with a trifid =extra-columellar= cartilage which reaches the tympanic membrane. The lower process of this extra-columella passes into a cartilaginous rod which lies in a canal in the quadrate and is during life continuous with Meckel's cartilage within the articular bone of the mandible. The columella and extra-columella are together homologous with the chain of mammalian auditory ossicles. * * * * * The =Optic capsules= and associated bones.