Sauropsida : Class Reptilia 477 



lacking except that the development of the snake's eye gives evi- 

 dence that the adult eye is covered by a nictitating membrane which is 

 immovably attached and has become quite transparent. 



The sensory internal ear of the reptile differs in one significant 

 particular from that of the amphibian. In all vertebrates except 

 cyclostomes and a few fishes, the ventral chamber (sacculus) of the 

 main ear sac bears a more or less elongated hollow outgrowth, the 

 lagena (Fig. 180), containing an area of sensory cells (papilla acus- 

 tica lagenae) to which a branch of the auditory nerve is related. In 

 fishes the lagena is so small as to be sometimes hardly noticeable, and 

 its function is doubtful. In mammals, in conjunction with a closely 

 adjacent sensory area (the papilla basilaris), it becomes greatly 

 elongated and curved into a close spiral forming the so-called "coch- 

 lea." The complex sensory mechanism (organ of Corti) of the cochlea 

 is the essential organ of hearing. The keenness and range of the sense 

 of hearing in mammals are directly related to the degree of elongation 

 of the cochlea. In amphibians the lagena is somewhat more prominent 

 than in fishes. In reptiles it is more or less elongated — least so in turtles 

 and snakes, more so and highly variable in lizards. In crocodilians it 

 attains greatest elongation, its axis is curved, and its internal sensory 

 mechanism is closely similar to the mammalian organ of Corti (Figs. 

 180, 517). The name "cochlea" is commonly applied to the elaborated 

 lagena of the crocodilian. It is highly probable that the degree of 

 development of the lagena is a fair index of the efficiency of the sense 

 of hearing in reptiles. 



In all modern reptiles except snakes and legless lizards, the ear 

 possesses a more or less well-developed tympanic apparatus similar 

 to that of frogs and toads (Fig. 353). The embryonic pharyngeal 

 pouches of the most anterior pair fail to perforate to the exterior and 

 give rise to the tympanic cavities, each covered externally by a 

 tympanic membrane. In turtles and some lizards the membrane 

 lies at the level of the surrounding skin. In other lizards it is more or 

 less depressed below the surrounding surface so that it lies at the 

 bottom of a pocket which, except for its shallowness, resembles the 

 deeper external auditory passage (meatus) of a mammal (Fig. 518). 

 In crocodilians the tympanic membrane is fairly deep and the external 

 auditory cavity is nearly covered by folds of skin, one above and one 

 below (resembling upper and lower eyelids), with only a narrow slit 

 between them. The passage whereby the tympanic cavity opens into 

 the mouth is longer and narrower than that in the amphibian, resem- 

 bling more the Eustachian tube of mammals. 



The columella extends from the tympanic membrane across the 

 tympanic cavity and is joined to a small bony plate, the stapes, which 



