ON THE INFLUENCE OF SCHOOL-BOOKS UPON EYESIGHT. 273 
ing towards it. In short, the whole visual apparatus 
is still unfinished, and is therefore more liable than 
at a later age to injury by over-use. 
Over-use of the eyes is chiefly to be feared in 
such occupations as reading, writing, and sewing, 
not in viewing distant objects. During near work 
the head is usually bent forward, and the blood- 
vessels of the eyes tend to become fuller; the 
focus of the eyes is shortened by a muscular effort 
which alters the form of the crystalline lens; the 
visual axes, which in distant vision are nearly 
parallel, are held in a position of convergence, 
and if the work be reading, they are also moved 
continuously from side to side. It is near work, 
therefore, that makes the greatest demand upon 
the eyes, and the nearer the work the greater the 
strain. Moreover it is chiefly in near work that 
continuous mental effort is required. 
Children who do too much close eye-work 
suffer in various ways. Some simply from fatigue, 
showing itself by inattention, mental weariness, 
temporary dimness of sight, or aching of the eyes 
and head. Some from congestion of the eyes, as 
shown by redness, watering, and frequent blinking. 
A certain number, in circumstances which pre- 
dispose them to the disorder, develop strabismus, or 
squint. Some others—and these cases are perhaps 
the most important of all—develop progressive 
myopia. 
Myopia, or short sight, commonly depends on 
undue elongation of the eyeball. It is never, or 
hardly ever, present at birth. It is rare at five 
years of age. It usually begins during school life, 
and increases more or less from year to year during 
the period of growth. It sometimes continues to 
increase after growth is completed. It is not 
necessarily, or always, associated with over-use of 
1913. ch 
