ORGAN OF HEARING IN REPTILES. 347 



to the fish, and to the mode in which they are transmitted to the 

 organ ; in like manner as the eyeballs are expanded, in order to 

 take in the utmost possible amount of light. The contracted 

 encephalon harmonises with and suffices for the sensations and 

 volitions, and the simple series of ideas daily repeated in the 

 monotonous existence of the scaled inhabitants of the waters. 



67. Organ of Hearing in Reptiles. The otocrane is large in 

 proportion to the cranium, in the Perennibranchs, but is distinct 

 from it : it is chiefly excavated in the alisphenoid and exoccipital. 

 It includes a vestibule, three semicircular canals (Proteus, Axelotes), 

 and the otolithic sac containing a cretaceous matter, which has 

 more shape and consistency in the Axolotl than in the Newts and 

 Salamanders. The external orifice of the acoustic capsule can 

 now be recognised as a ( vestibular ' one, or ( fenestra ovalis,' and 

 it is closed by a cartilaginous plate representing the base of the 

 stapes, connected with a slender ossicle in the Axolotl : but as yet 

 there is no trace of tympanic cavity. The analogy of the ear to 

 the eye, by the absence of the tympanic and conjunctivo-lacrymal 

 forecourts in the respective organs of Fishes, is still kept up in the 

 fish-like Reptilia. 



The tympanic adjunct to the organ of hearing makes its first 

 appearance, simultaneously with the developement of eyelids and 

 lacrymal organs, in the Batrachia which have quitted their aquatic 

 for an aerial existence. Beyond the vestibular foramen is 

 continued a short but wide passage outward, or ( meatus,' closed 

 from the external air by a thin transparent vibratile membrane- 

 the ' tympanum ' or ear-drum. From the gristly plate closing 

 the vestibular foramen a slender bony style is continued across 

 the ( tympanic cavity ' to a drum, to which it is attached by a 

 capitular or spatulate cartilage, in which a small muscle is inserter !. 

 A wide vertical passage from the tympanic cavity to the fauces 

 preserves the equilibrium between the air in that cavity and the 

 atmosphere outside : it is called ' eustachian tube.' This tube 

 will be found to bear relation to the size and exposed condition of 

 the ear-drum, and perhaps, also, to its form, which, in the Frog 

 and other air-breathing Ovipara, is convex externally. In the 

 Pipa the bony meatus is long and tubular in shape, the eustachian 

 tubes terminate by a single median and minute orifice on the 



/ O 



palate : the ear-drum is concealed by a partial covering of skin. 

 It is less conspicuous externally in all Toads than in Frogs : a small 

 muscle acts on the cartilage connected with the ear-drum, and a 

 second longer muscle is attached to the discoid piece closing the 

 vestibular orifice. The labyrinth consists of alveus communis, 



