THE NERVOUS SYSTEM. a yy’ 
The internal ear, or labyrinth, an irregular cavity in the 
solid part of the temporal bone, and separated from the 
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 float 
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. 
Now, the essential organ of hearing is the labyrinth, 
which is, substantially, a bag filled with fluid and nerve- 
filaments. Fishes generally have but little more. In 
Reptiles there are added a tympanum, chain of bones, 
cochlea, and Eustachian tube; the tympanum being ex- 
ternal. Birds have, extra to Reptiles, an auditory passage, 
opening on a level with the surface of the head, and sur- 
rounded by a circle of feathers. Mammals only have an 
external ear.™ 
Sight is the perception of light." In all animals it 
depends upon the peculiar sensitiveness of the optic nerve 
to the vibrations of ether. But while in Vertebrates this 
nerve comes from the middle mass of the brain, in Inver- 
tebrates it is derived from a ganglion. Many animals are 
utterly destitute of visual organs, as all the Protozoans, 
and the lower Radiates and Mollusks, besides intestinal 
Worms and the blind Fish of Mammoth Cave. Around 
the margin of the Jelly-fish, and at the end of the rays of 
certain Star-fishes and Sea-urchins, are colored spots, sup- 
posed to be rudimentary eyes; but as a lens is wanting, 
there is no image; so that the creature can merely dis- 
tinguish light from darkness and color without form. 
Such an eye is nothing but a collection of pigment gran- 
ules on the expansion of a nervous thread, and the percep- 
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