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 only have an exter- 

 nal ear. 92 



Sight is the perception of light. 98 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 Ms 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. 



