Cataract 89 



no part of the iris in contact with the capsule. The posterior surface 

 of the lens is more convex than the anterior. 



Structure. The lens is composed of transparent fibres which are 

 connected by a clear cement. Diminution of the transparency con- 

 stitutes cataract (the etymology of the word is uncertain). 



If the opacity be central, the eye is almost blind in a strong light, 

 as the contracted pupil admits light only over the opacity. Thus the 

 subject sees best when the pupil is dilated, as towards evening, or in 

 a fog or shade. In such circumstances the surgeon may content 

 himself with making an artificial pupil. 



The old operation for senile cataract was to dislocate the lens into 

 the vitreous, where, however, it was apt to set up inflammatory 

 disease. 



In young people an opaque lens is soft, and the surgeon treats it by 

 passing a needle through the cornea, tearing the capsule, and breaking 

 the front of the lens, so that the aqueous humour may further soften 

 it, and promote its complete absorption. In the adult, however, the 

 hard and opaque lens (which has a strong resemblance to an ' acid 

 drop ') must be removed by extraction. 



In the operation of extraction a preliminary iridectomy is sometimes 

 done in order that the risk of iritis may be diminished, and also 

 that there may be a more ready escape for the lens. The anterior 

 part of the capsule is then lacerated with a ' cystitome,' and the 

 lens is very gently squeezed out of the sclero-corneal wound by 

 pressure delicately applied on the sclerotic, below the cornea. If the 

 cystitome be used with too much force, the lens is apt to be dislocated 

 into the vitreous ; and if, after the use of the instrument, too much 

 pressure be made upon the eye-ball the vitreous may be extruded. 



The lens being removed, the rays of light meet at a focus very 

 far behind the retina, so that strong convex glasses are needed (p. 86). 



The shape of the lens is regulated by the extremely elastic capsule 

 which contains it. This elasticity is shown by the way in which, when 

 it is scratched in the operation of extraction, the scratch becomes a 

 tear, and the tear a rent, so that the cataractous lens is set free. But 

 to ensure the ready escape of the lens the cataract should be fully 

 ' ripe ' before its extraction is attempted, otherwise its escape may be 

 associated with that disagreeable phenomenon, the escape of the 

 vitreous. Moreover, if some of the * unripe ' cortical part of the lens 

 be left adhering to the capsule, it may in time become opaque, and 

 entail further operation for its removal. 



The vitreous body forms a transparent mould for the eye-ball, 

 the retina being spread over the greater part of its circumference, and 

 the lens being supported by it in front. Probably there is running 

 throughout it a delicate reticular structure ; it also contains corpus- 

 cular elements which usually float unobserved, but which, under 

 certain conditions, can throw shadows upon the retina like gnats 



