426 HUMAN PHYSIOLOGY 



that the image is inverted. The same is true of the image on the 

 retina. By means of this image thrown on the sensory layer, the 

 rod and cone cells of the retina are stimulated and the image is 

 transmitted to the forebrain. We must remember that the optic 

 nerve crosses under the brain so that images formed in the right 

 eye are received by the left half of the forebrain, and vice versa, 

 '^ Effects of Drinks containing Alcohol upon the Eye. — Through 

 its influence upon the nerves and the muscles, the continued and 

 too free use of alcohol renders the eye unsteady and its adjustment 

 uncertain; the small blood vessels become dilated, and the eyes 

 are blood-shot and often inflamed. The optic nerve is frequently 

 affected, causing dimness of vision, and specific diseases of parts 

 of the eye may result, such as cataract and disorders of the retina. 

 The confirmed inebriate is the victim of diseased conditions in 

 which the sight becomes untrustworthy. He sees horrible visions, 

 frightful, venomous creatures appear to threaten him, and he is 

 haunted by specters. Under his imaginary suffering he may be- 

 come a raving maniac, and repeated attacks of the disease are 

 likely to prove fatal." — Macy, Physiology. 



The Blind Spot. — Although the eye is an accurate organ for sight, yet 

 one part of the retina is not provided with sensory cells. This area, called 

 the blind spot, marks the point within the eye where the optic nerve enters 

 and spreads out in a very thin layer over the surface of the retina. 



How THE Voice is Produced. — The voice box, or larynx, is well known to 

 every boy as his " Adam's apple." It consists of a box of movable cartilages, 



joined in such a way to each other and to 

 muscles so that by contraction of certain 

 muscles, the form of the larynx is changed. 

 Within the larynx are stretched two narrow 

 pads of elastic tissue called the vocal cords. 

 It is by the vibration of these cords that the 

 sounds of the voice are produced. The vocal 

 cords project into the larynx so that only a 

 ^j , J narrow slit is formed between the larynx and 



V ocs,l cords 



the mouth cavity. This slit is the glottis. 

 When the vocal cords are drawn into position for speech, they vibrate, the 

 vibration being caused by a current of air driven against them. This cur- 

 rent of air is caused by movement of the lips, cheeks, palate, and tongue. 

 The stronger the air current, the louder the voice. 



