356 MANUAL OF ZOOLOGY SECT. 



rays of light proceeding from an object must fall in order 

 to produce the sensation of sight, is a soft grey layer, the 

 retina (fi.), lining that part of the cavity of the eye which 

 lies within the iris. The rays of light are brought to a 

 focus on the retina mainly by means of the crystalline lens 

 (Z.), a firm, glassy body situated within the iris. The 

 cornea also assists in this, as does a gelatinous substance, 

 the vitreous humour ( V.H.\ which fills the part of the cavity 

 of the eyeball internal to the lens. 



The ear in the Dog-fish is imbedded in the cartilage of 

 the posterior part of the skull (auditory region). It con- 

 sists of a somewhat complicated structure termed the 

 membranous labyrinth, with soft walls and an internal 

 epithelium in which the fibres of the auditory nerve termi 

 nate. Contained in the interior of the labyrinth is a fluid, 

 the endolymph, in which there are suspended particles of 

 carbonate of lime the otoliths. In the Lizard and Rabbit 

 there are superadded to this the essential part of the ear, 

 certain accessory parts. The most important of these is 

 the tympanum or drum of the ear. This is a cavity to the 

 outside of the auditory region of the skull. The tym- 

 panum communicates with the pharynx through a passage 

 known as the Eustachian passage. Externally the cavity of 

 the tympanum is closed by a tense, drum-like membrane, 

 the tympanic membrane. The tympanic membrane is set 

 in vibration by the waves of sound, and the vibrations are 

 transmitted across the tympanic cavity by a slender rod of 

 bone (in the Lizard) or a chain of minute bones (in the 

 Rabbit). The inner end of the rod or chain of bones is 

 inserted into a membrane covering over a small aperture in 

 the outer wall of the auditory region of the skull which 

 forms the inner wall of the tympanic cavity, and by this 

 means the vibrations are communicated to the endolymph 



