66 THE ORGANS OF SPECIAL SENSE. 



and the ordinary person cannot hear more than 16,000 

 vibrations a second. Different sounds can be distin- 

 guished when they follow each other as closely as by 

 one one-hundredth of a second. 



All sound does not come through the canal of the 

 ear. The bones of the head vibrate and carry sound. 

 So there are instruments for the deaf which are put in 

 the ear and others which are placed between the teeth. 



The semicircular canals are not essential to hearing 

 but have something to do with a person's power of 

 maintaining his equilibrium. Injury to them may 

 cause dizziness and loss of equilibrium. 



The Eye. One more feature, perhaps the most ex- 

 pressive, remains to be described, the eye. The senses 

 are all modifications of the original cutaneous sensi- 

 bility and the nerve of sight is no more sensitive to light 

 than any other nerve. It therefore needs an end 

 organ that is sensitive to the motions of the ether in 

 order to give impressions of light. This organ is pro- 

 vided in the eye, which is not only itself capable of be- 

 ing moved in every direction, but is placed in the most 

 movable part of the body, the head, which can be 

 turned in almost a complete circle. The eyeball is 

 spherical and lies in the cavity of the orbit upon a cush- 

 ion of fat, where it has a large range of sight but is 

 securely protected from injury by its bony surroundings. 

 The sunken eyes following protracted illness are due to 

 the using by the system of the fat on which the eye- 

 ball ordinarily rests. 



Each orbital cavity is formed by the juncture of some 

 seven bones and communicates with the cavity of the 

 brain through the optic foramen and through the sphe- 

 noidal fissure. Above the orbits are arched eminences 

 of skin, the eye-brows, from which several rows of 

 short hairs grow longitudinally and which serve to pro- 

 tect the eyes and to limit the amount of light to a 

 certain extent, as in frowning. 



Still further protection is afforded by the eyelids, 



