1 12 ANIMAL MECHANISM. 



nate in the lens ; sometimes half way between the centre 

 and margin, but ordinarily in the centre. They are either 

 a peculiar deposition of opaque or milky matter, entirely 

 preventing the ingress of light, or an opacity of some of 

 the internal layers of plates, equally destructive to vision. 

 Nothing short of the actual introduction of the couching 

 needle within the globe, or a knife, promises any hope of 

 recovery. Many children are born with this affection ; 

 at all ages, they are liable to form : perhaps the habit of 

 gazing habitually on a strongly reflecting surface, may 

 have a tendency to generate the disease. To remove ca- 

 taracts by extraction, the operator slides a sharp, thin knife, 

 resembling a lancet, through the cornea, from one side 

 to the other, cutting one half from its natural attachment 

 leaving it, when the knife comes out, in the form of a 

 flap, thus : FIG. 5. 



Explanation of Figure 5. 



This plan represents an eye, surrounded by its natural appen- 

 dages with a knife passing through the anterior chamber of the eye. 

 A dotted line indicates the lower edge of the flap, made by cutting 

 oflfjust one half the cornea from its attachment with the sclerotica, in 

 ordei 1 to allow the crystalline lens to escape, whenever the knife is 

 withdrawn. 



As a matter of course, the aqueous humor escapes in a 

 twinkling, at the same moment, the capsule of the lens, 

 previously ruptured, ^designedly, by the point of the knife, 

 as it slides along, spasmodically acts upon the lens by spon- 

 taneous contraction, and protrudes it through the wound. 

 Undoubtedly, the grasp which the straight muscles have on 

 the ball, accelerates its escape. 



Thus, in taking away the obstruction to sight, the 

 whole lens is extracted. Perhaps the question may arise 



