ll4 ZOOLOGY. 



much like those of the olfactory cells : they are not nerve- 

 cells, however; but their base is arborized around by a fibre 

 of the eighth nerve. 



The second sac, or sacculus, has a spirally twisted out- 

 growth resembling a snail-shell in form, the cochlea, whose 

 epithelium resembles that of the ampullae. This last part 

 is distinctive of the mammalia, but the rest of the internal 

 ear is represented in all vertebrata, except Amphioxus. 

 The whole of the labyrinth is membranous, and contains 

 a fluid, the endolymph ; between the membranous wall 

 of the labyrinth and the enclosing bone is a space con- 

 taining the p&rilymph. The latter space is part of the 

 general lymphatic system. Strange as it may appear at 

 first, the entire lining of the internal ear is, at an early 

 stage, continuous with the general epidermis of the animal. 

 It grows in, just as a gland might grow in, and is finally 

 cut off from the .exterior ; but a considerable relic of this 

 former communication remains as the Y-shaped blind tube 

 already referred to, the ductus endolymphatictts or endo- 

 lymph-duct. The eighth nerve runs from the brain case into 

 the periotic bone, and is distributed to the several portions 

 of this labyrinth, especially to the ampullae and cochlea. 



6. Accessory Parts ol the Ear. In fishes this internal 

 ear is the sole and sufficient organ of hearing ; the sound- 

 waves transmitted by the water travel through the elastic 

 cranium and so reach and affect the nerve- endings, just 

 as we can hear the ticking of a watch that touches our 

 head when our outer ears are stopped up. But in all 

 air-frequenting vertebrates this original plan of an 

 ear has to be added to, to fit it to the much fainter 

 sound-vibrations of the compressible air. "A receiving 

 apparatus" is needed, and is supplied by the ear-drum, 

 middle ear, or tympanic cavity. In the mammal there is 

 also a collecting ear-trumpet, the ear commonly so-called, 

 or pinna, enclosing an external auditory meatus. A tightly 

 stretched membrane, the tympanic membrane, separates 

 this from the drum. A chain of small bones, the malleus, 

 incus, os orbiculare (a very small bone), and stirrup- 

 shaped stapes, collectively called the " auditory ossicles," 



