THE ORGANS OF SENSATION 185 



rived from the cartilaginous auditory capsule. In fishes, the 

 auditory apparatus is still quite primitive, being at its simplest 

 in Cylostomata, where it consists of one or two semicircular 

 canals with membranous vestibules : in higher fishes there is a 

 differentiation of the labyrinth, from which the semicircular 

 canals proceed into utricle and saccule. 



The further the development of the ear advances the more 

 deeply is it situated in the skull ; the labyrinth even in amphi- 

 bians is completely enclosed in the cranial wall ; in reptiles, 

 birds and mammals it is, as in man, deeply imbedded in bone. 

 The development of the internal ear is a subject of great 

 interest. Starting from a prolongation of the vestibule 

 (Sacculus), the cochlea grows out by degrees until it attains 

 in mammals its characteristic form. But the final and most 

 important metamorphosis comes from the amalgamation of 

 portions of the visceral arches with the ear. For instance, 

 in all orders from the amphibians upwards, the first visceral 

 cleft goes to form the tympanic cavity, the Eustachian tube, 

 fenestra ovalis, and fenestra rotunda : while in the Anura, a 

 tympanic membrane appears attached to a little bony process 

 the columella, an arrangement which gradually proceeds through 

 higher developments in lizards, crocodiles, turtles and birds up 

 to the mammals, including man. The human ear exhibits the 

 highest differentiation, for in addition to the stapes which de- 

 velops from the columella, two more ossicles appear, the malleus 

 and incus, which are formed from the bone of the first arch. 



Histologically, certain parallels can be traced between man 

 and some of the mammals. The bony wall of the tympanum 

 is compact in sheep, goats and dogs, just as in man ; spongy, 

 with tympanic cells, in horses, cattle and pigs ; double, con- 

 taining a hollow space,, in cats. 1 The number of turns in the 

 cochlea varies : 



In man there are not quite 3 turns 



horses ) 



rabbits/ " 



oxen 3* 



pigs ,, nearly 4 



carnivora 3 



1 Ellenberger, loc. cit., p. 562. 



