ORGANS OF SENSE. 63 



through the internal auditory meatus, which, like the in- 

 tergal and middle ear, is situated in the hardest part of 

 the temporal bone. 



Vibrations of the air are collected by the concha and 

 directed to the external auditory meatus, gaining in force 

 by concentration. The tympanum takes up the vibrations 

 and transmits them to the chain of bones, by which they 

 are exactly reproduced on the oval window. Here they 

 impress the liquid in the labyrinth, and, reaching the 

 nerve filaments, produce perception. Such is the mech- 

 anism of audition, a mechanism so perfect that the 

 human ear can distinguish the most delicate shades of 

 sounds varying between thirty-two and seventy thousand 

 vibrations per second. 



Imperfect hearing usually depends on too great tension 

 of the tympanum, or a want of elasticity of that mem- 

 brane. In such cases hearing is improved by the use 

 of ear-trumpets which concentrate the vibrations. Deaf- 

 ness caused by paralysis of the auditory nerve or similar 

 accident is hopeless, but when due to obstruction of the 

 Eustachian tube or of the external meatus, disappears 

 under appropriate treatment. 



Sight. The sense of sight acquaints the brain with 

 luminous impressions, and gives us notions of the color, 

 form, and external condition of bodies. It is, of course, 

 dependent on the eye. 



Lodged in the upper and front portion of the face, 

 surrounded by muscles that turn it at will in any direc- 

 tion, the eye observes from far, and gathers impressions 

 of all that exists in the complete hemisphere embraced 

 by its survey. It rests on a fatty cushion at the bot- 

 tom of the bony cavity of the orbit, whose thick walls 

 protect it eifectually on the sides, while the eyebrows 



