202 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



other like the rafters of a roof , in an A-like manner (fig. 209, ip, 'op). 

 As the A's diminish in width from apex to base of the cochlea, this 

 part has been thought to play a part in the recognition of pitch. 

 There is also a cuticular structure, the membrane tectoria, which ex- 

 tends from the medial wall out over the hair cells, and this may be the 

 intermediate organ of stimulation and may have to do with the recog- 

 nition of sound waves of different rapidity. It has recently been 

 shown that the membrana tectoria is connected with the hairs 0/ the 

 hair cells. The fact that in birds, where pitch is certainly recognized, 

 there is no organ of Corti, renders all speculation doubtful. 



Fig. 210. — Diagram of mammalian ear. a, ampullae of semicircular canals; an, 

 acustic nerve; en, cochlear nerve; em, external auditory meatus; eu. Eustachian tube; 

 fi, fenestra tympani; i. incus; m, malleus; p, perilymph space (black); pd, perilymph 

 duct; ph, pharynx; s, stapes; sc, sacculus; sg, spiral ganglion; sm, st, su, scalae media, 

 tympani et vestibuli; t, tympanic cavity; tm, tympanic membrane; u, utriculus; m, 

 vestibular nerve. 



The Middle Ear or tympanum (fig. 210) first appears in the 

 anura. It consists of a cavity (cavimi tympani) in front of and below 

 the otic capsule, connected by a slender duct, the Eustachian or 

 auditory tube, with the pharynx. Externally it is separated from the 

 outer world by a thin partition, the tjrmpanic membrane, from which 

 a chain of bones, the ossicula auditus (p. 80), 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), but it never breaks 

 through. The tympanic membrane, covered externally with ecto- 



