474 THE SPECIAL SENSES 



semicircular canals are about one third the diameter of 

 the bony canals which they occupy. One of these is hori- 

 zontal when the body is upright. The others are vertical, 

 but at right angles to one another. All three have swel- 

 lings at the front where they join the utriculus, called 

 ampulloe. At the back these swellings are absent, and the 

 vertical canals fuse before entering the utriculus. The three 

 ampullae and the walls of the utriculus and sacculus are 

 supplied with branches of the auditory nerves which pene- 

 trate the membrane and attach it at these points to the 

 bony walls. 



Into the bony cochlea projects from the central pillar a 

 thin shelf of bone dividing it partially into two sections. 

 Attached to the edge of this shelf is the membranous 

 cochlea, forming a triangular tube with the base of the 

 triangle against the opposite wall. It thus divides the 

 bony cochlea into three compartments. One of these 

 compartments (the vestibular passage) copimimicates 

 with the bony vestibule. Another, the tympanic passage, 

 has its base against the fenestra rotunda. The middle 

 passage, or membranous cochlea (sometimes called the 

 cochlear canal), communicates with the sacculus. The 

 vestibular and t3mapanic passages are filled with perilymph, 

 the cochlear canal with endolymph. Along a groove in 

 the bony shelf passes a branch of the auditory nerves 

 whose fibers penetrate the membranous cochlea and end 

 in special end organs known as the fibers of Corti. These 

 fibers are the seats of origin of sound impulses. 



Nature of sound stimuli and path of sound in the ear. — 

 Just as light stimuli are produced by waves of vibrating 

 ether, so sounds are produced by waves of vibrating air. 

 Any vibrating body can produce these waves, and if they 



