91 6 A MANUAL OF PHYSIOLOGY 



may be corrected by concave glasses, which render the rays 

 more divergent. It is to be noted that many cases of internal 

 squint in children are connected with myopia, the eyes neces- 

 sarily rotating inwards as they are made to fix an abnormally 

 near object. The treatment both of the squint and the myopia 

 in these cases is the use of concave spectacles (Fig. 393). Myopia, 

 although a condition that shows a distinct hereditary tendency, 

 is rarely present at birth ; the elongation of the antero- 

 posterior diameter of the eyeball develops gradually as the 

 child grows. 



In hypermetropia, or long-sightedness, the eye is, as a rule, 

 too short in relation to its converging power ; and with the lens 

 in the position of rest, parallel rays would be focussed behind 

 the retina. Accordingly, the hypermetropic eye must accommo- 

 date even for distant objects, while even with maximum accom- 



FIG. 394. HYPERMETKOPIC EYE. 



The image P' of a point P falls behind the retina in the unaccommodated eye. 

 By means of a convex lens L it may be focussed on the retina without accom- 

 modation (dotted lines). 



modation an object cannot be distinctly seen unless it is farther 

 away than the near-point of the emmetropic eye. The far- 

 point of distinct vision is at the same distance as in the emme- 

 tropic eye viz., at infinity the near- point is farther from the 

 eye. The delect is corrected by convex glasses (Fig. 394). 

 Hypermetropia, unlike myopia, is present at birth. 



Presbyopia, or the long-sightedness of old age, is not to be 

 confounded with hypermetropia. It is essentially due to 

 failure in the power of accommodation, chiefly through weakness 

 of the ciliary muscle, but partly owing to increased rigidity 

 and loss of elasticity of the lens. Images of distant objec s are 

 still formed on the retina of the unaccommodated eye with 

 perfect sharpness i.e., the far- point of v sion is not affected. 

 But the eye is unable to accommodate sufficiently for the rays 

 diverging from an object at the ordinary near- point ; in other 



