SE-NSE OF SIGHT. 201 



spring, when they are turned out to the green pasture, where 

 their diet is changed from dry to succulent, and where also 

 the head is held near the ground to collect the food, a con- 

 gestion of the aqueous humour is apt to occur, sufficient to 

 produce temporary blindness. 



20. The Crystalline Lens.-^-This is placed immediately 

 behind the aqueous humour, a short distance back of the 

 pupil, and is a double convex lens, perfectly transparent. In 

 shape it closely resembles the common burning glass ; its 

 posterior surface is, however, a little more convex than the 

 exterior, and it approaches nearer to a sphere in infancy 

 than in old age* When the crystalline lens is first removed 

 from an eye* it looks like a mass of transparent crystal, 

 without any trace of organization. Near the surface it is 

 much softer than at the centre, which is harder and more 

 compact. It is also softer in the young than in the old. As 

 it consists chiefly of albumen, when exposed to heat, as of 

 boiling water, alcohol or acids, it becomes white, like the 

 white of an egg. If we examine it in this state, we shall 

 find that it consists of an immense number of concentric 

 plates or Lamella. More than two thousand of these have 

 been counted, disposed in the form of layers like the coat of 

 an onion, each layer consisting of an infinite number of 

 very minute filaments, wound round in different directions, 

 from various centres. The arrangement of these fibres 

 differs in various animals, but is uniform in every individual 

 of the same species. In fishes the lens is nearly spherical ; 

 in reptiles it is less so, and in birds and mammalia, it is still 

 more flattened. In amphibious animals, as turtles or frogs, 

 or those whose vision is adapted both for air and water, as 

 seals and whales, the lens is more convex than in those which 

 live entirely in the water. When this lens becomes so 

 opaque as to obstruct the passage of the light, either par- 

 tially or entirely, a person is said to have a cataract. This 

 can only be cured by a surgical operation. 



