EFFECTS OF RADIANT ENERGY ON THE EYE. 715 



Senile Cataract. 



The theory has been advanced (see page 780) that senile cataract 

 is due to exposure of the lens to daylight, particularly that from the 

 sky. This is based solely on the fact that the cataractous changes 

 usually begin in the lower part of the lens. It is undoubtedly true 

 that the changes do first appear below, but as a rule they are so far 

 below that they are in a portion of the lens completely shaded by the 

 iris. Thus it is most often necessary to produce artificial mydriasis 

 before incipient lens changes can be seen with the ophthalmoscope. 

 Moreover, if the cataract were due to exposure to light, the pupillary 

 area should be the first affected, since from such an extended source 

 as the sky it receives the greatest concentration of light, and since 

 the chief absorption must occur here. We must conclude, therefore, 

 that there is no sound evidence for this theory of cataract formation. 



A possible explanation of the fact that the lower part of the lens 

 is usually first affected in senile cataract is that the structure of the 

 lens ma.y normally be slightly diiferent here than elsewhere. From a 

 developmental standpoint this is indicated by the fact that coloboma 

 of the lens usually occurs below. Burge ^^'' has recently attempted 

 to supply an experimental basis for the view that ultra violet light is 

 responsible for cataract. He found that the rays from an unscreened 

 quartz merciny vapor lamp had almost no coagulating efl^ect upon 

 the lens protein even after an exposure of 72 hours at a distance 

 of 5 cm. but that when acting in the presence of weak solutions of 

 calcium chloride, sodium silicate, or dextrose, coagulation occurred. 

 Since in senile cataracts calcium, magnesium, and sometimes silicates, 

 are greatly increased, and in diabetic cataracts dextrose is presum- 

 ably present, Burge assumes that these cataracts are due to the action 

 of ultra violet light. That is, he assumes that these substance are 

 present in undue quantities in the lenses of certain individuals and 

 that this renders their lenses vulnerable to the short waves of daylight. 



This assumption is suflSciently controverted by the fact just men- 

 tioned that senile cataract usually begins at the periphery below. 

 But in addition, other serious objections to his argument may be 

 pointed out. In the first place, in traumatic cataracts and cataracts 

 due to inflammatory conditions, calcium salts, and no doubt mag- 

 nesium and other salts, are deposited in great abundance, and the 

 lens may even become completely calcified. In fact, the same thing 

 occurs in dead tissues anywhere in the bodv, so that the reasonable 



