118 CONSCIOUS NERVOUS OPERATIONS 



the external rectus is too short, that eye will turn outward 

 and defective vision will result. If the external rectus is 

 paralyzed, the person will be cross-eyed, or squint-eyed. 

 Paralysis of either of the nerves distributed to the muscles 

 of the eye will result in abnormal action of those muscles. 



161. Astigmatism. Another defect, known as astigma- 

 tism, is due to the irregular curvature of the cornea or the 

 lens or both. The eye may be more convex on one merid- 

 ian than on others, so that rays of light falling upon one 

 part of the cornea (which is most frequently affected) are 

 brought to a focus at a different spot from the rays which 

 pass through other parts. Thus the image on the retina 

 is indistinct and the vision is blurred. 



162. Color Blindness. Some persons are unable to dis- 

 tinguish certain colors from certain other colors. They 

 are said to be color blind. Usually they differ from 

 persons of normal sight in their inability to distinguish 

 red from green. Sometimes only one eye is color blind, 

 the other being normal. The reasons for these phenom- 

 ena belong to the abstruse subject of color sensation, 

 which is beyond the scope of this work. 



The power to distinguish colors accurately is of great 

 importance to those engaged in certain occupations, for 

 example, to those employed upon railways, who are re- 

 quired to undergo examinations which test their eyes in 

 that particular. 



163. Training of the Eye. The eyes of the young may 

 be easily trained by practice under a teacher's guidance 

 to see quickly and accurately, and to judge correctly as to 

 the size of objects, distances, etc. Such training is of 

 great value in all circumstances of life. It is indeed 

 more strictly a training of the mind than of the eye, 

 but niay be fitly mentioned in this connection. 



