THE EAR 95 



and upper border of the sacculus as a tube which 

 terminates in a large thin-walled sac. 



3. The semicircular canals are three tubular offsets of the 



vestibule, into which they open at both ends. They lie 

 in canals in the periotic cartilage, and are placed in planes 

 at right angles to one another ; each has at one end, close 

 to its opening into the vestibule, a dilatation or ampulla, 

 i. The anterior vertical canal has its ampulla at its 



anterior end. 



ii. The posterior vertical canal has its ampulla at its 

 outer end, while its inner end joins the posterior 

 end of the anterior vertical canal to open into 

 the vestibule by a common orifice. 

 iii. The horizontal or external canal has its ampulla 

 at the anterior end. 



4. The auditory 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 diverti- 

 cula, and to the ampullae of the semicircular canals. 



5. The accessory auditory apparatus. The essential organs 



of hearing i.e., the vestibule and its offsets, and the 

 auditory nerve are enclosed in the periotic cartilage, 

 which is deeply placed in the side of the head : the com- 

 munication with the surface is brought about by the 

 accessory apparatus, which consists of the following parts 

 (Fig. 10, p. 48). 



i. The Eustachian passage and tympanic cavity are 



formed in connection with the hyomandibular gill 



cleft of the tadpole. (See Chap. VIII., p. 126.) 



ii. The tympanic membrane closes the tympanic 



cavity on its outer side. 



iii. The columella is a rod of bone and cartilage, the 

 outer end of which is attached to the tympanic 

 membrane, while its inner end is inserted into a 

 hole in the outer wall of the periotic capsule (the 

 fossa fenestrse ovalis) so as to lie in close contact 

 with the vestibule. It serves to communicate 

 the auditory vibrations of the tympanic mem- 

 brane to the vestibule. 



