SENSE ORGANS AND NERVOUS COORDINATION 



583 



cavity to a fenestra ovalis in the otic capsule. The fenestra ovalis com- 

 municates with the inner ear. 



The hearing apparatus of mammals is basically similar but much 

 more elaborate (Fig. 29.6 A). Most mammals have a well developed 

 external ear consisting of a canal, the external auditory meatus, and 

 an external flap, the pinna, which in some species helps funnel sound 

 waves into the meatus. The delicate tympanic membrane hes at the 

 internal end of the meatus where it is protected against injury. The 

 three auditory ossicles (the hammer-shaped malleus, the anvil-shaped 

 incus and the stirrup-shaped stapes, arranged in sequence) transmit 

 vibrations across the middle ear cavity to the fenestra ovalis, or oval 

 window. The stapes evolved from a part of the hyoid arch of fishes, 

 and the malleus and incus were derived from the posterior part of the 

 mandibular arch when a new jaw joint evolved in mammals anterior 

 to the former one. These three ossicles form a system of levers that 

 reduces the amplitude, but increases the force of the sound waves. The 

 movement of the loot plate of the stapes against the membrane within 

 the oval window is only about one half as extensive as the movement 



-Membrekxious labyrinth 



■Perilymph 



-Middle S3cr cavity 



"Inciis 



rPinria. 



-Malleus 



Craniad 

 cavity 



Canal for 

 acoustic nerve 



Cochlea 



External auditory meatus 

 ■Tympanic membrane 

 Stapes in oved -window 

 Rourid v7indo-w 

 ■Eustachian tube 



Cochlear 

 duct 



Basilar 

 meiTibrane 



Scalatympani Round window-' 



B C 



Figure 29.6. The mammalian ear. A, Schematic drawing of the outer, middle 

 inner ear of a human. B, Diagram of the cochlea as though it were uncoiled. C, 

 enlarged cross section through the cochlea. The cochlear duct and other parts of 

 membranous labyrinth are filled with endolymph. 



and 

 An 

 the 



