SUBPHYLUM VERTEBRATA 337 



includes one chamber known as the sacculus, in connection with which is 

 the organ of hearing, and another known as the utriculus, in connection 

 with which is the organ of equihbrium. The two cavities — sacculus and 

 utriculus — communicate with one another through the endolymphatic 

 duct, and each has a potential opening into the middle ear. 



The sacculus is the ventrally situated chamber. In connection with 

 it is a spirally coiled canal known as the cochlea (Fig. 235). In the canal 

 of the cochlea is a sheet of sensory cells known as the organ of Corti, which 

 is the essential organ of hearing. Sound waves stimulating these cells 

 mechanically give rise to impulses which, transmitted by the auditory 

 nerve, are interpreted by the brain as sensations of sound. In connec- 

 tion with the utriculus, which is the more dorsal chamber, are three semi- 

 circular canals lying in three planes. At the end of each of these canals 



Sccf/cr 

 vesfibuli 



Vestibular J^^^^^ "'^m ..,^^^^^^^°"^ 

 membrane- 



Oangilor, ^^^l^~-^cociilear nerve 



Fig. 235. — Section of the cochlea showing the two scalae, which communicate at the 

 tip of the coils, and the essential organ of hearing, the organ of Corti, lying in a part of the 

 Bcala vestibuli separated from the rest by a thin membrane, called the vestibular mem- 

 brane, or membrane of Reissner. The mass of bone surrounding the cochlea is shown cut 

 away from the rest of the temporal bone, in which the whole auditory organ is contained. 



is a dilatation known as an ampulla, in which are sensory hairs which, 

 when the body is moved, are stimulated by waves of movement in the 

 lymph and give a sense of position or of equilibrium. The opening from 

 the utriculus into the middle ear, known as the fenestra ovalis, is closed 

 by the innermost of three bones known as the stapes, while the opening 

 from the sacculus into the middle ear, the fenestra rotunda, is closed by a 

 thin membrane. 



The middle ear is known as the tympanum. It is a cavity filled with 

 air across which sound waves are conducted by a chain of three bones 

 called, from without inward, malleus, incus, and stapes. The first two of 

 these are not present in amphibians, reptiles, and birds, instead there is one 

 bone, the columella. This cavity is in communication through the eusta- 

 chian tube with the cavity of the pharynx and thus opens to the outside. 



The outer ear is separated from the middle ear by a tympanic mem- 

 brane. Sound waves entering the outer ear set this membrane into vibra- 



