180 
COMPARATIVE ZOOLOGY. 
middle ear by a bony partition, which is perforated by 
two small holes. The labyrinth consists of the vestibule , 
or entrance; the semicircular canals , or tubes; and the 
cochlea , or spiral canal. While the other parts are full of 
air, the labyrinth is filled with a liquid, and in this are 
the ends of the auditory nerve. The vibrations of the 
air, collected by the external ear, are concentrated upon 
the tympanum, and thence transmitted through the chain 
of little bones to the fluid in the labyrinth. 
The essential organ of hearing is the labyrinth, which 
is, substantially, a bag filled with fluid and nerve-fila¬ 
ments. Fishes generally have but little more. In Am¬ 
phibians and Reptiles there are added a tympanum, a 
single bone, connecting this with the internal ear, the 
cochlea, and the Eustachian tube; the tympanum being 
external. Birds have, besides, an auditory passage, open¬ 
ing on a level with the surface of the head, and surround¬ 
ed by a circle of feathers. Mammals ou\y have an exter¬ 
nal ear. 92 
Sight is the perception of light. 93 In all animals it de- * 
pends upon the peculiar sensitiveness of the optic organ to 
the luminous vibrations. In Vertebrates the optic nerve 
comes from the middle mass of the brain, in Invertebrates 
it is derived from a ganglion. Many animals are utter¬ 
ly destitute of visual organs, as the Protozoa, and the 
lower Radiates and Mollusks, besides intestinal Worms 
and the blind Fishes and many cave-animals. Around the 
margin of the Jelly-fish are colored spots, supposed to be 
rudimentary eyes; but, as a lens is wanting, there is no 
image; so that the creature can merely distinguish light 
from darkness and color without form. Such an eye is 
nothing but a collection of pigment granules on the ex¬ 
pansion of a nervous thread, and the perception of light 
is the sensation of warmth, the pigment absorbing the 
rays and converting them into heat. 
