228 THE VERTEBRATE SKELETON. 



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 communi- 

 cates 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- 

 eolumella, which is united to the tympanum. 



The Optic capsules. 



The skeletal structures developed in connection with the 

 optic capsule do not become united to the skull. They con- 

 sist of : 



(a) the sclerotic, a cartilaginous sheath investing the eye 

 and bearing 



(6) a ring of ten small bony scales. 



There is no lachrymal bone. 



The Olfactory or Nasal capsules. 



The basicranial axis in front of the basisphenoid remains 

 cartilaginous, neither presphenoid nor mesethmoid bones are 

 developed, and the orbits in a dry skull communicate by a 

 wide space through which the second, third, fourth, and sixth 

 cranial nerves pass out. Separate nasal bones also do not 

 occur, the large prefrontals extending over the area usually 

 occupied by both nasals and lachrymals. 



