AUDITORY ORGANS. 187 
and may have to do with the recognition of sound waves of different 
rapidity. It has recently been shown that the membrana tectoria is 
connected with the hairs of the hair cells. The fact that in birds, where 
pitch is certainly recognized, there.isno organ of Corti, renders 
all speculation doubtful. 
Fic. 187.—Organ of Corti of guinea pig, after Schneider. d, Deiter’s cells; 4c, Henson’s 
cells; th, inner hair cells; 7p, inner pillar cells; /s, limbus spiralis; mf, membrana tectoria; 7, 
nerve fibres; oh, outer hair cells; op, bates pillar cells; si, inner sulcus; st, scala tympani; t, 
tunnel; én, tunnel nerve. 
The Middle Ear or tympanum first appears in the anura. It con- 
sists of a cavity (cavum tympani) in front of and below the otic 
capsule, connected by a slender duct, the Eustachian tube, with the 
pharynx. Externally it is separated from the outer world by a thin 
partition, the tympanic membrane, from which a chain of bones, the 
ossicula auditus (p. 73), extends across the cavity to the fenestra ovale, 
and serves to transmit the sound waves to the inner ear. The tympanic 
cavity is the homologue of the spiracular cleft of the elasmobranchs 
(see respiration), which never breaks through. The tympanic mem- 
brane, covered externally with ectoderm, on the inner surface with 
entoderm, represents the imperforate wall of the cleft, while the Eusta- 
chian tube is the narrowed internal end of the spiracle. The chain of 
ear bones has already been described. It is to be noted that the 
chain consists of columella and stapes in anura and sauropsida, 
while in the mammals columella is replaced by incus and malleus. 
In the urodeles and gymnophiones, where no tympanic cavity is devel- 
oped, the quadrate articulates with the stapes. 
The External Ear.—In the anura and in many reptiles the tym- 
panic membrane is flush with the surface of the head, but in other rep- 
tiles and in birds it is at the bottom of a canal, the external auditory 
meatus, the simplest expression of an external ear. In the mammals 
