Skeletal structures occur in connection with each of the three special sense organs of hearing, sight, and smell.
The Auditory capsules.
The auditory or periotic capsule of the turtle is rather large and its walls are well ossified, epi-otic, pro-otic and opisthotic bones being present.
The epi-otic (fig. 39, 13) is the more dorsal of the three bones, and in the adult is completely ankylosed with the supra-occipital.
The opisthotic (fig. 39, 8) is the ventral posterior element. On its inner side it is united to the supra-occipital above, and to the exoccipital below; it sometimes becomes completely fused with the exoccipital. In front it meets the pro-otic, and on its outer side the squamosal and quadrate. Its anterior portion is hollowed out by the cavity in which the auditory organ lies, it gives off also a process which is separated from the exoccipital by an oval foramen through which the glossopharyngeal, pneumogastric, and spinal accessory nerves leave the cranial cavity.
The pro-otic is the anterior element; it meets the supra-occipital and opisthotic posteriorly, while anteriorly it is separated from the alisphenoidal plate of the parietal and pterygoid by a large oval foramen through which the maxillary and mandibular branches of the trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out posteriorly by the cavity in which the auditory organ lies, and its inner wall as seen in longitudinal section is pierced by a foramen through which the external carotid artery and facial nerve leave the cranial cavity,—the nerve finally leaving the skull through a small oval foramen on the anterior face of the pro-otic near its junction with the quadrate.
Between the pro-otic and opisthotic as seen in a longitudinal section of the skull is a large opening constricted in the middle. This is the internal auditory meatus (fig. 39, VIII.). Through it the auditory nerve leaves the cranial cavity and enters the ear. The ramus vestibularis leaves through the dorsal part of the hole, the ramus cochlearis through the ventral.
The cavity of the auditory or periotic capsule communicates with the exterior by a fairly large hole, the fenestra ovalis, which lies between the opisthotic and pro-otic, and opens into a deep depression, the tympanic cavity, which is seen in a posterior view of the skull lying just external to the exoccipital. The cavity communicates with the exterior by a large opening, the external auditory meatus (fig. 38, 22).
Several other openings are seen in the tympanic cavity; through one at the extreme posterior end the pneumogastric and spinal accessory nerves leave the skull, and through another, a little further forwards, the glossopharyngeal.
The auditory ossicles consist of a long bony columella, whose inner end fits into the fenestra ovalis, while the outer end is attached to a small cartilaginous plate, the extra-columella, which is united to the tympanum.