206 THE SPECIAL SENSES. 



having the iris very dilatable. The size of the pupil affects 

 the lustre of the eye. When it is large, as it usually is dur- 

 ing 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 reflec- 

 tion of light from the front surface of the crystalline lens. 



66. Certain poisonous vegetables have the property of 

 causing 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 pur- 

 pose, 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 belladonna is "deadly 

 nightshade." (In the diagram on page 214 the shape and 

 relations of the iris are more accurately shown than in the 

 figure 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 ultimate filaments 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 eye-ball occasions a 

 flash of light ; and a sudden fall, or a blow upon the eye, 

 is often apparently accompanied by the vision of "stars." 



68. These phenomena are due to what is termed the 



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

 quences * 



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

 Sent" itiveness ? 



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



