112 PHYSIOLOGY CHAP. 



amphibia it is much more developed, and in reptiles it develops 



3V. 



0. c 



vtr> 



Fio. 45. Membranous labyrinth of left side seen from without. (Merkel.) Co., cochlea; D.c. 

 ductus cochlearis; 'Sac., saccule ; Utr., utricle; s., superior; e., external (or lateral); p., 

 posterior semicircular canal ; a. r., aqueduct of vestibule; C.r., canalis reuniens. 



Fio. 46. Diagram of entire human auditory organ. (Debierre.) 1, auricular lobe; 2, external 

 auditory mcatus ; 3, tympanic membrane ; 4, stapes attached to base of fenestra vestibuli ; 

 5, bony part of Eustachian tube ; 6, its cartilaginous parts ; 6', mouth of tube ; 7, vestibular 

 cavity filled with perilymph ; 8, semicircular canals and utricle; 9, promontory; 10, fenestra 

 cochleae (the arrow indicates the tympanic opening of the cochlea) ; 11, tympanic cavity filled 

 with air; 12, cochlear duct filled with endolymph, united to saccule of vestibule by a narrow 

 junction canal ; 13, scala vestibuli ; 14, scala tympani terminating in fenestra cochleae ; 

 V>, apex of cochlear canal, where the two walls unite at 15' ; 16, cochlear aqueduct; 17, vesti- 

 bular aqueduct ; 18, endolymphatic sac ; 19, parotid region. 



progressively from the chelonians and ophidians to the saurians 

 and crocodiles. It is only in these last and in birds that the 



