98 THE EYE AND EAR 
4, The auditory nerve: the auditory or eighth nerve leaves 
the cranial cavity through a hole in the inner wall of the 
periotic capsule, and divides into branches distributed 
to the sacculus and its diverticula, and to the ampulle 
of the semicircular canals. 
5, The accessory auditory apparatus. ‘he essential organs 
of hearing :—z.e., the vestibule and its offsets, and the 
auditory nerve—are enclosed in the periotic cartilage, 
deeply placed in the side of the head: the communi- 
cation with the surface is brought about by the accessory 
apparatus, which consists of the following parts (Fig. 
10) :-— 
i. The Eustachian passage and mpanic cavity: 
really the hyomandibular gill cleft, which having 
lost its original respiratory purpose persists with 
modified function as an accessory organ of hearing. 
ii. The tympanic membrane; closing the tympanic 
cavity on its outer side. 
iii. The columella: a rod of bone and cartilage, whose 
outer end is attached to the tympanic membrane, 
while its inner end is inserted into a hole in the 
outer wall of the periotic capsule so as to lie in 
close contact with the vestibule. It serves to 
communicate the auditory vibrations from the 
tympanic membrane to the vestibule. 
