692 Comparative Morphology of Chordates 



membrane contains a system of close-set, straight, parallel fibers which 

 extend transversely to the longitudinal axis of the tube. One theory of 

 hearing assumes that these basilar fibers vibrate "in sympathy " with 

 external sound-vibrations and thus stimulate the adjacent "hair cells." 

 Suspended like a canopy in the endolymph, and with its under surface 

 in close proximity to the "hairs" of the sensory cells, is a membranous 

 structure (membrana tectoria). This membrane is noncellular and 

 of doubtful histologic nature. All of these various sorts of structures 

 are arranged, in the same relation to one another, in a continuous spiral 

 series extending from base to apex of the cochlea, but with the one 

 very significant difference that the width of the organ of Corti and the 

 length of the basilar fibers progressively increase from base to apex. 

 This fact, in itself, justifies the belief that it is the organ of Corti which 

 enables the animal to discriminate sounds of varying pitch. 



In the endolymph of each cavity, utriculus and sacculus, is a soft 

 conglomerate mass of a gelatinous substance containing numerous 

 minute crystals of calcium carbonate (otoconia). Substance of similar 

 nature occurs in close relation to the crista of each ampulla. These 

 bodies apparently serve to intensify the impact of vibrations or move- 

 ments of the endolymph on the "hairs" of the sensory cells. 



The eighth cranial ("acoustic") nerve distributes to six locali- 

 ties in the mammalian otic sac. In the wall of each of its main divi- 

 sions, the utriculus and sacculus, is a thickened spot (macula) oc- 

 cupied by sensory "hair cells" (Fig. 182). A branch of the nerve passes 

 to the macula acustica utriculi and another to the macula acus- 

 tica sacculi. In the wall of the ampulla of each semicircular canal is 

 an elongated sensory ridge (crista). A small nerve goes to each of these 

 three cristae acusticae. A major division of the nerve passes up the 

 axis of the cochlea, giving off a continuous spiral whorl of small 

 brandies which pass to the organ of Corti and terminate in close 

 relation to its sensory cells (Fig. 517). 



The early embryonic otic sac is enclosed in a cartilaginous otic 

 capsule. This later ossifies to produce the several otic bones which 

 eventually fuse to form the mammalian petrosal bone. In most 

 mammals fusion of the petrosal with the neighboring squamosal and 

 tympanic bones results in the complex temporal bone. Ossification 

 immediately adjacent to the otic sac produces a relatively thin layer of 

 extremely compact and hard bone which closely follows the contour of 

 all parts of the sac, but at a little distance so that nearly everywhere 

 space intervenes between the sac and the bone. Thus the complex form 

 of the otic sac is repeated by its bony case which, accordingly and in 

 distinction from the membranous labyrinth, is called the "bony 



