THE NERVOUS SYSTEM. 



225 



of the stapes is attached. The ventral one is the fenestra 

 rotunduni, also closed by a membrane in the recent state. 

 In the dry skull the fenestra ovalis opens into the first or 

 basal whorl of the cochlea, and the fenestra rotundum opens 

 into the vestibule of the internal ear. The Eustachian tube 

 (Fig. 18), whose opening may be seen craniad of the audi- 

 tory bulla, connects the middle ear with the posterior nares 

 and thus admits air to the tympanic cavity. 



To demonstrate further the anatomy of the middle ear, 

 one should clean the flesh from a 

 fresh or preserved head and care- 

 fully cut away the ventral walls 

 of both chambers of the auditory 

 bulla (Fig. 17). 



In the middle, ear are three 

 bones, the malleus, incus, and 

 stapes, commonly called hammer, 

 anvil, and stirrup. They form a 

 crooked chain across the cavity. 

 The long process of the malleus is 

 fastened throughout nearly its 

 whole length to the inner surface 

 of the membrana tympani, and its 

 enlarged extremity articulates 

 with the body of the incus. The 

 latter has two legs, to one of 

 which the stapes is attached. The 

 base of the stapes is inserted in 

 the membrane closing the fenestra ovalis (Figs. 17 and 



no. 



The internal ear, or labyrinth, consists of three parts 

 the vestibule, cochlea, and semicircular canals (Figs. 18 

 and in). All of these parts are of membrane and lie in 

 cavities of corresponding shape within the petrous bone. 



FIG. in. DIAGRAM OF THE 

 MAMMALIAN EAR. 



The internal ear is repre- 

 sented removed about a 

 centimeter from the mid- 

 dle ear and slightly ro- 

 tated to the left. The 

 base of the stapes, s, in 

 nature, covers the fenes- 

 tra ovalis, ov, c, basal 

 whorl of the cochlea ; ea, 

 external auditory meatus ; 

 eu, opening of the Eu- 

 stachian tube ; i, incus ; in, 

 malleus; sc, semicircular 

 canals; ve, vestibule; t, 

 tympanum. 



