Phonoreception 475 



which is divided internally by membranes to form three long spiral canals. 

 Detection of motion is accomplished within a group of three osseous semi- 

 circular canals, more or less at right angles to each other, each of which con- 

 tains a membranous canal (Ch. 14, p. 517). These two osseous labyrinths 

 (cochlear and semicircular canals) open into a cavity, known as the vestibule, 

 which is separated from the middle ear partly by bone and partly by the 

 oval window. 



Within the vestibule are two membranous sacs, the sacculus and the 

 utriculus, which open into each other. 1 he utriculus also opens into both 

 ends of the three membranous semicircular canals, and the sacculus is also 

 connected to the cochlear duct or scala media, the middle of the three spiral 

 canals of the cochlea. The sacculus, utriculus, membranous semicircular 

 canals, and the cochlear duct together form a complete membranous laby- 

 rinth, which may be considered as a loosely fitting lining of the osseous 

 labyrinth. The cavity of the membranous labyrinth is filled with a fluid, 

 the endolymph, and the space between the membranous labyrinth and the 

 bone is filled with another fluid, the perilymph. The two fluids are not in 

 direct contact at any point; however, the space between the membranous 

 labyrinth and the bone is connected by means of the perilymphatic duct with 

 the subarachnoid space of the brain, and the cavity of the membranous laby- 

 rinth is connected by means of the endolymphatic duct which ends blindly 

 in the subarachnoid space. 



STRUCTURE OF THE COCHLEA. In general appearance the cochlea resem- 

 bles a snail shell with two and three-quarters turns to the spiral. It is about 

 5 mm, in diameter at the base and measures about 9 mm. from base to apex. 

 There is a central bony structure, the modiolus, around which the spiral 

 cavity is wound. This cavity is separated into three spiral canals by two mem- 

 branes: (1) the basilar membrane, broadest at the apex and narrowest at 

 the base and held under some tension; and (2) Reissner's membrane, a very 

 thin, delicate structure which spirals on the apical or upper side of the basilar 

 membrane. These two membranes extend throughout most of the length 

 of the spiral but join each other and then stop just short of the apex, thereby 

 lea\'ing the upper and lower spirals joined by a small opening. 



A cross section of one of the spirals of the cochlea is shown in Fig. 155, A. 

 Ihe middle canal contains a relatively complex formation of sensory and 

 supportive cells, known as the organ of Corti, attached to the basilar mem- 

 brane. Above the organ of Corti is the tectorial membrane which, in the 

 living animal but usually not in fixed material, is probably closely applied, 

 and perhaps loosely attached, to hairlike processes of the sense cells which 

 lie just below. I'here are four rows of sense cells, one inner row, toward the 

 modiolus, and three outer rows. The organ of Corti consists of the sense cells 

 plus their ner\'e connections and supportive structure. There is a firm support- 

 ing framework above the sense cells, consisting of two pillars lying between 

 the inner and outer rows of hair cells to which is attached a semirigid but 

 porous membrane, the reticular lamina. 



Vibrations are transmitted by the stapes to the perilymph of the vestibule 

 through the membrane of the oval window and from there to the perilymph 

 of the lower canal. Reissner's membrane is very thin and delicate and offers 



