THE NERVOUS SYSTEM AND ORGANS OF SENSE 409 



are transmitted by means of the fluid filling the cavity to the sensory cells of 

 the cochlea (collectively known as the organ of Corti) and thence to the brain 

 by means of the auditory nerve. 



The Character of Sound. When vibrations which are received by the 

 ear follow each other at regular intervals, the sound is said to be musical. 

 If the vibrations come irregularly, we call the sound a noise. If the vibra- 

 tions 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. The ear can be trained to 

 recognize sounds which are unnoticed in untrained ears. 



The Eye. The eye or organ of vision is an almost spherical body which 

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

 connects the eye 

 with the brain. 

 Free movement is 

 obtained by means 

 of six little muscles 

 which are attached 

 to the outer coat, 

 the eyeball, and to 

 the bony socket 

 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; this 

 coat is lacking in 

 the exposed part of 

 the eyeball, but may 

 be seen by lifting 

 the eyelid. Under 

 the sclerotic coat, in 

 front, the eye bulges 

 outward a little. Here the outer coat is replaced by a transparent 

 tough layer called the cornea. A second coat, the choroid, is supplied 

 with blood vessels and cells which bear pigments. It is a part of 

 this coat which we see through the cornea as the colored part of 

 the eye (the iris}. In the center of the iris is a small circular hole 

 (the pupil). The iris is under the control of muscles, and may be ad- 

 justed to varying amounts of light, the hole becoming larger in dim 

 light, and smaller in bright light. The inmost layer of the eye is 

 called the retina. This is, perhaps, the most delicate layer in the entire 



Longitudinal section through the eye : Sc, sclerotic coat ; 

 Ch, choroid ; O, optic nerve ; C, cornea ; /, iris ; Con, 

 conjunctiva ; R, retina ; Y, yellow spot ; L, lens ; A, an- 

 terior chamber, filled with aqueous humor ; V, posterior 

 chamber, filled with vitreous humor. 



