358 MANUAL OF ZOOLOGY SECT. 



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 region in 

 which the membranous labyrinth is enclosed). 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 

 of the membranous labyrinth and affect the terminations 

 of the auditory nerve-fibres. In the lizard the tympanic 

 membrane is nearly on a level with the skin of the head, 

 and its position is conspicuously indicated by a brown 

 patch situated behind the eye. In the rabbit the tympanic 

 membrane is more deeply sunk, and a wide passage, the 

 passage of the outer ear, leads to it from the exterior. The 

 ear of the rabbit also differs from that of the lizard in the 

 presence of the prominent auricle or pinna of the ear to 

 which reference has been already made. 



The kidneys, or organs of renal excretion, though they 

 differ in form in the three examples are not widely different 

 in essential structure. Their function is the secretion of 

 urine, which consists of water containing various nitrogenous 

 waste matters in solution. Essentially the kidney is a mass. 



