AYES. 



'255 



(3) Olfactory Organs. The olfactory sac on each side, which 

 communicates by an external naris with the exterior, and by an 

 internal nans with the mouth-cavity, is partly lined by olfactory 

 epithelium. This covers the projecting turbinal, the rolled shape 

 of which increases the surface. The olfactory nerve breaks up 

 below the olfactory epithelium to supply the olfactory cells, of 

 which this is largely made up. 



(4) Auditory Organs. External, middle, and internal ears are 

 present. The external ear consists of a short tube, the external 

 auditory meatus, opening below and behind the eye. It is 

 separated by the tympanic membrane from the middle ear or tym- 



Fig. 72. DIAGRAMS OF THE MEMBRANOUS LABYRINTH (from Haddon}. 

 Internal side of left labyrinth. A, Fish. B, Bird. (7, Mammal; 

 us, utriculus and sacculus ; u, utriculus ; 5, sacculus ; c, cochlea. 



panic cavity. This communicates with the mouth-cavity by an 

 ffustachian tube, while the columella stretches across it, having one 

 end attached to the tympanic membrane and the other inserted 

 into the fenestra walls, below which is the fenestra rotunda. The 

 internal ear is essentially composed of the membranous labyrinth 

 (Fig. 72), enclosed in a somewhat larger cavity (filled with 

 perilympfi) of closely corresponding shape. This cavity is con- 

 tained in the auditory capsule, and is surrounded by a thin, dense 

 layer of bone, forming the bony labyrinth. The utriculus and 

 sacculus are ill-marked off from one another. With the former 

 three semicircular canals are connected, arranged as in the frog, 



