HEARING — SEEING 



205 



so sound waves are transmitted by means of the fluid fiUing the 

 cavity to the sensory cells of the cochlea and thence to the brain 

 by means of the auditory nerve. 



The Character of Sound. — When vibrations which are received 

 by the ear follow one another at regular intervals, the sound is said 

 to be musical. If the vibrations come irregularly, we caU the 

 sound a noise. If the vibrations come slowly, the pitch of the 

 sound is low; if they come rapidly, the pitch is high. The ear 

 is able to perceive as low as thirty vibrations per second and as 

 high as almost thirty thousand. 



Seeing. — The organ of vision, the eye, is almost spherical, and 

 fits into a socket of bone, the orbit. A stalklike structure, the optic 

 nerve, connects the eye with the brain. Free movement is made by 

 means of six little muscles which are attached to the outer coat of 

 the eyeball, and to the bony wall around the eye. 



The wall of the eyeball is made up of three coats. An outer 

 tough white coat, of connective tissue, is called the sclerotic coat. 



..ReHna 



Bellows 



Sensitive 

 plate 



Shutter 

 ^.Openingin Shutter 

 Lens 



yfe ' ' "■ || s 



Diagram of section of the human eye. Compare it with a camera. 



In front, where the eye bulges out a little, this outer coat is replaced 

 by a transparent tough layer called the cor'nea. A second coat, 

 the choroid (ko'roid) , is supplied with blood vessels and cells which 

 contain pigments. The i'ris is part of this coat which we see through 

 the cornea as the colored part of the eye. In the center of the iris 

 is a small circular hole, the pwpil. The iris is under the control of 

 muscles, and may be adjusted to varying amounts of light, the hole 

 becoming larger in dim light, and smaller in bright light. The 

 mmost layer of the eye is called the ret'ina. This is, perhaps, the 



