Mammalia: Nervous System 693 



labyrinth." The space between the two is filled by the perilymph, 

 similar in nature to the endolymph. The outer surface of the bony 

 otic shell is continuous with the adjacent spongy or cancellous part of 

 the petrosal bone. The cancellous bone, however, because of its loose 

 texture, is easily dissected away so that it is not difficult to make an 

 instructive separate preparation of the bony labyrinth. 



The bony cochlear tube is of much greater diameter than that 

 of the membranous cochlea within it and is divided into an upper and a 

 lower passage by a spiral partition, partly bony and partly of connec- 

 tive tissue, the lamina spiralis (Figs. 515, 516). On this lamina rests 

 that part of the membranous cochlear wall which contains the organ of 

 Corti. The outer (in relation to the vertical axis of the spiral) wall of 

 the membranous cochlea is closely attached to the adjacent wall of the 

 bony tube, and the third side of the triangular (in cross section) mem- 

 branous tube is unsupported by solid tissue and is therefore bounded 

 by endolymph below and perilymph above. The upper spiral passage 

 within the bony cochlea is called the scala vestibuli; the lower is the 

 scala tynipani. The lamina spiralis does not quite reach the apex of 

 the cochlea, so that there the two scalae are in communication. 



The bony cochlea of the human ear has a diameter of about 9 mm. 

 at its base and an axial height of 5 mm. Its spiral length is about 

 30 mm. 



The mammalian tympanic apparatus ("middle ear") possesses 



the same general structure as that of reptiles. The tympanic mem- 

 brane ("eardrum") is at the bottom of an external auditory pas- 

 sage (meatus), as it is in some reptiles and in birds, but deeper. The 

 meatus is lined by infolded skin which produces hairs (at least in the 

 outer region of the meatus) and ceruminous (wax) glands which are 

 modified sebaceous glands. The connection with the pharynx is by 

 way of an elongated and narrow Eustachian tube (Fig. 518), 

 which, in consequence of the development of a secondary palate, opens 

 not into the alimentary pharyngeal passage but into the posterior 

 region of the nasopharynx above the "soft palate." A thin bony wall 

 separates the tympanic cavity from the perilymph space of the internal 

 ear (Fig. 518). 



The mechanism for transmitting vibrations from the tympanic 

 membrane to the sensory internal ear — the function performed by 

 the simple rodlike columella of amphibians and sauropsids — consists 

 of a chain of three small bones articulated together (Figs. 468, 518, 

 519). Their names — malleus, incus, stapes, meaning respectively 

 "hammer," "anvil," "stirrup" — were suggested by their several 

 forms, especially as they appear in the human ear. The handle of the 



