THE NERVOUS SYSTEM 



251 



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 

 vestibuli (Figs. 18 and 120). 



The internal ear, or labyrinth, consists of three parts — the 

 vestibule, cochlea, and semicircular canals (Figs. 19 and 120). 

 All of these parts are of membrane and lie in cavities of corre- 

 sponding shape within the petrous bone. A lymphoid, periotic 



Fig, 121. — Section of the Cochlea of the Calf. X 10. {From Ellenberger, 



after KoUiker.) 

 a, Modiolus; c, scala tympani; v, scala vestibuli; hn, lamina spiralis; pt, portion 

 of the petrous, bone; r, scala media, or ductus cochlearis. 



fluid, the perilymph, floats the delicate membranous internal 

 ear within its bony cavity, while within the membrane is a 

 similar fluid, the endolymph. The vestibule is a small sac adja- 

 cent to the tympanum, and may be seen by looking through 

 the fenestra vestibuli. From the dorso-caudal aspect of the 

 vestibule, arch three semicircular canals at nearly right angles, 

 to one another. The external semicircular canal is in a hori- 

 zontal plane and surrounds a small fossa almost caudad of the 



