554 STUDIES IN ADVANCED PHYSIOLOGY. 



by either one or both of the following causes: (a) The 

 ball may be too long from before backwards, and in this way 

 the retina too far behind the lens to receive the image. 

 Or, (b) the lens may be too convex. It may converge 

 the rays of light too much and so bring them to a focus 

 sooner than ought to have been done. When myopia is the 

 result of a long eyeball it is frequently a natural defect. 

 When, however, it is due to too great a convexity of the 

 lens it may be an acquired defect. An individual who has 

 been in the habit of looking closely at things held very near 

 the eye, has been obliged in doing so, to keep his lens 

 strongly curved, and the continuation of such a state of 

 things is liable to finally fix itself and so there arises short- 

 sightedness, which necessitates that all objects be held close 

 to the eye to be distinctly visible. 



In such acquired myopia there has occurred really no in- 

 herent change in the lens, the change has been in the mus- 

 cle itself, which, owing to its constant tension in looking 

 closely, has finally become somewhat fixed in this position, 

 and by this fixation of the muscle the lens remains, on ac- 

 count of its elasticity, in constant excessive convexity. 



The remedy for short-sightedness is obvious. As the 

 rays of light are bent too much a lens must be placed before 

 the eye which shall disperse them to such an extent that the 

 image shall be thrown on the retina. Concave spectacles 

 are therefore demanded, the extent of the concavity depend- 

 ing upon the degree of myopia. 



a --- 



Fig. 172. THE MYOPIC EYE AND ITS CORRECTION. 



The tendency to acquire myopia when the eye is re- 

 peatedly subjected to a close strain shows the necessity de- 

 volving upon teacher and parent to prevent the habit of 

 reading continuously with the book unnaturally close. It 



