THE SKELETON OF THE TURTLE. SENSE CAPSULES. 227 



2. THE SENSE CAPSULES. 



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 com- 

 pletely 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 acces- 

 sory 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 

 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 longi- 

 tudinal section of the skull is a large opening constricted in the 



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