110 PHYSIOLOGY OF FARM ANIMALS [CH. 



of the temporal bone specially hollowed to receive it. Between 

 the membranous labyrinth and the investing bony or osseous 

 lab3rrinth is a fluid called the perilymph. 



The osseous labyrinth consists of three portions, the vestibule, 

 the cochlea and the semicircular canals (see p. 96) . The vestibule, or 

 middle cavity, presents several ai^ertures through which penetrate 

 divisions of the auditory nerve. The portion of the membranous 

 bag or labjrrinth lying within the vestibule forms two communi- 

 cating compartments, the utricle and the saccule. Into the utricle 

 open the semicircular canals, while the saccule communicates 

 with the canal of the cochlea by a small canal called the canalis 

 reuniens. The cochlear portion of the osseous labyrinth resembles 

 in shape a snailshell, the circular coil consisting of two and a half 

 turns, closed at its upper termination, but presenting openings at 

 the base for the fenestra ovalis, the fenestra rotunda and the 

 canalis reuniens. A section through one of the coils reveals the 

 fact that the coil is separated into three portions, the scala media 

 or canal of the cochlea, the scala vestibuli and the scala tympani. 

 The division of the cochlea into the scala vestibuli and scala 

 tympani is effected by a flat sill-like projection of bone from the 

 internal surface of the coil (the lamina spiralis), the outer edge 

 of this spiral being continued to the outer border of the coil by a 

 membrane called the basilar membrane. On this membrane rests 

 the canal of the cochlea. The scala vestibuli is closed at its basal 

 portion by the fenestra ovalis and communicates with the scala 

 tympani at its apical end by a small aperture called the helico- 

 trema. The scala tympani in its turn is closed at its basal portion 

 by the fenestra rotunda. 



Nerve distribution. The auditory nerve divides into two 

 branches, one of which supplies the utricle, saccule, and the semi- 

 circular canals, the other or cochlear branch, running up the centre 

 of the cochlea, giving off branches through the lamina spiralis, 

 these branches terminating in fine filaments which form special 

 end sense organs in the canal of the cochlea. These sense end 

 organs have a complicated structure and are known collectively 

 as the organ of Corti. 



Physiology of hearing. The transmission of sound waves to 

 the brain takes the following course. The wave-like concussions 

 caused by the vibration of air particles against the tympanic 

 membrane causes this tightly stretched membrane to vibrate 



