THE SPECIAL SENSES 295 



pupil expands and admits more light, we are enabled to distin- 

 guish the surrounding objects. Animals of the cat species, 

 and others which prowl around after nightfall, are enabled to 

 see in the dark by having the iris very dilatable. The size of 

 the pupil affects the lustre of the eye. When it is large, as it 

 usually is during youth, the eye appears clear and brilliant ; 

 while in old age the pupil is small and the eye is dull. The 

 brilliancy of the eye -is in part, at least, dependent upon the 

 reflection of light from the front surface of the crystalline 

 lens. 



66. Certain poisonous vegetables have the property of caus- 

 ing the pupil to dilate, and have been used in small doses to 

 increase the beauty of the eye. One of these drugs has been 

 so largely used by the ladies for this purpose that it has 

 received the name belladonna, from the Italian words meaning 

 " beautiful lady." This hazardous practice has resulted more 

 than once in the death of the person desiring thus to increase 

 her personal attraction. The common English name for bella- 

 donna is " deadly nightshade." (In the diagram on page 303 

 the shape and relations of the iris are more accurately shown 

 than in the figures referred to above.) 



67. The Retina constitutes the third and inner coat of the 

 globe of the eye. This, the important part of the eye that is 

 sensitive to light, is a kind of nervous membrane, formed by 

 the expansion of the optic nerve. Its texture is soft, smooth, 

 and very thin ; it is translucent and of an opaline, or grayish- 

 white color. It is sensitive to light alone ; and if any form of 

 mechanical irritation be applied to it, the sensations of touch 

 and pain are not experienced, but flashes of fire, sparks, and 

 other luminous appearances are perceived. Thus an electric 

 shock given to the eyeball occasions a flash of light; and a 

 sudden fall, or a blow upon the eye, is often apparently accom- 

 panied by the vision of " stars." 



68. These phenomena are due to what is termed the " specific 



66. Means used to increase the beauty of the eye ? The injurious consequences ? 



67. What part does the retina constitute ? How formed ? Its texture ? Color ? Senst 

 tiveness ? 



68. Specific energy of the optic nerve ? Trial in Germany f 



