THE VERTEBRATE ANIMAL — SUBPHYI.UM VERTEBRATA 403 



The ear structures provide most classes of vertebrates with facilities 

 for two functions: hearing and equilibrium. This organ consists of 

 an external ear, which serves in catching and directing sound waves 

 within, a middle ear or tympanum, containing ossicles, and the inner 

 ear, which contains the sensory cochlea with its organ of Corti for 

 hearing, and the semicircular canals, which are concerned with equi- 

 librium rather than hearing. The latter are common to all verte- 

 brates while the cochlea is limited to Amphibia and higher classes. 

 The external ear is still further limited to reptiles, birds, and mam- 

 mals. The middle ear is a space beneath a tympanic memhrane 

 which separates it from the external auditory canal. In this cavity 

 are three bony ossicles, the malleus, incus, and stapes, which transmit 

 the sound vibrations from the tympanic membrane to the membrane 

 over the fenestra ovalis leading into the internal ear. The mem- 

 branous labyrinth is the name often applied to the chambers of the 

 inner ear. Its ventral chamber is the sacculus connected with the 

 organs of hearing, and the dorsal portion is the utricidus which is 

 related to equilibrium. The two semicircular canals in simpler 

 forms and the three in higher, join the body of the utriculus in as 

 many different planes as there are canals. In the higher forms 

 there are two vertical canals, one anterior and one posterior, with 

 their planes at right angles to each other, and one horizontal canal. 

 At one end of each canal there is a bulblike swelling or ampulla 

 which contains sensory hairs. When the position of the head is 

 moved, the fluid in the canals changes its level and position to 

 stimulate the sensory hairs, thus giving a sense of position. The 

 sound waves which stimulate the sensory cells of hearing enter the 

 external ear and set up vibrations in the tympanic membrane. 

 These are in turn transmitted by the ossicles to the fluid endolymph 

 within the labyrinth. The vibrations of the fluid extend through 

 the cochlea, in which the sensory cells are supported on the organ 

 of Corti stretched across it. These cells are connected with the 

 brain by way of the auditory or eighth cranial nerve (Fig. 222). 



The sense of smell is centralized in the epithelial lining of the nasal 

 chamber. Special olfactory cells are stimulated by particles of mate- 

 rial from the air dissolving on this membrane and making contact 

 with the sensory cells. The sense of taste is similar except that it 

 is located in sensory cells in taste buds on the tongue, epiglottis, and 



