360 BODY CONTROL AND HABIT FORMATION 



with a fluid. It is believed that somewhat as a stone thrown into water 

 causes ripples to emanate from the spot where it strikes, so sound waves 

 are transmitted by means of the fluid filUng 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. Under the sclerotic 

 coat, in front, the eye bulges outward 

 a little. Here the outer coat is con- 

 tinuous with 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 adjusted 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 body. Despite the 

 fact that the retina is less than ^ of an inch in thickness, there are 

 several layers of cells in its composition. The optic nerve enters the 

 eye from behind and spreads out to form the surface of the retina. 

 Its finest fibers are ultimately connected with numerous elongated 

 cells which are stimulated by light. The retina is dark purple in color, 

 this color being caused by a layer of cells next to the choroid coat^ This 



■Muscle 



Sclerotic Coat 



Longitudinal section through 

 the eye. 



