EYE DISEASES 469 



total blindness. Removal of the eye lens completely, and the 

 wearing of special forms of spectacles, permits the spectacle lens 

 to act as a substitute for this eye lens and form images on the 

 surface of the retina as before. Since these glass lenses have no 

 power of accommodation, a different pair must be used for reading 

 and the examination of near objects than is used for distant 

 objects. They are of course a makeshift, but infinitely preferable 

 to blindness. 



Presbyopia. — In old age the lenses become stiffened and do not 

 bulge enough. Hence convergent lenses are necessary, as in far- 

 sightedness. Such stiffening is called presbyopia. 



Eye diseases. — Diseases of the eye include the formation of 

 ulcers or scars on the cornea, the failure of the muscles of the 

 iris to expand or contract, inflammation of the conjunctiva (pink 

 eye and granulated lids), and the wasting of the optic nerve. 

 Most are curable provided the optic nerve is not permanently 

 injured. 



