THE EYE. 379 



and is apparently the only one that is found in the centre of the 

 lens. 



The two substances can be isolated from an aqueous extract of the 

 lens by saturating the solution with magnesium sulphate at a 

 temperature of 30 C. The precipitate is then dissolved in water, 

 dialyzed, and the resulting solution precipitated with acetic acid, 

 which throws down the a-body, while the ^9-crystalline remains in 

 solution. A small amount of the /?-substance, it is true, is also pre- 

 cipitated by the acetic acid, but can be separated from the a-body by 

 a repetition of the process. 



Both substances are precipitated from their neutral solutions by 

 carbon dioxide, but in the case of the /3-cry stall ine this precipitation 

 is never complete. The latter coagulates at 63 C., and the a-crys- 

 talline at 72 C. The /9-substance further differs from the a-body 

 in containing much more sulphur, 1.27 per cent., as compared with 

 0.56 per cent., which is, moreover, in part at least, present in a 

 loosely combined form, while the entire quantity that is found in the 

 a-crystalline is firmly combined. 



That these bodies are intimately concerned in the concentration of 

 the light cannot be doubted. The refractive index of the inner 

 layer of the lens, in man, is given as 1.407, while that of the 

 central portion is 1.456. 



Of the significance of the fats, lecithins and cholesterins in the 

 lens, nothing is known, but it appears that the amount of the two 

 latter, at least, is much increased in senile cataract, while the 

 quantity of the albumins, as a whole, is diminished. The albumoid, 

 however, is then possibly increased. 



The Vitreous Body. The vitreous body of the eye is a jelly- 

 like material, which consists of a fine framework of collagen, enclos- 

 ing the liquid portion of the body proper. This presents an alka- 

 line reaction, and contains only a very small amount of solids. Its 

 general composition is seen below : 



Pro mille. 



Water 989.00 



Solids . . . . - 11.00 



Albumin 0.70 



Urea 0.64 



Paralactic acid traces. 



Glucose traces. 



Mineral salts 9.00 



Among the albumins present Morner claims to have found a 

 hyalomucoid, which is closely related to the corneal mucoid, but 

 contains 12.27 per cent, of nitrogen and 1.19 per cent, of sulphur, 

 as compared with 12.79 per cent, of nitrogen and 2.07 per cent, of 

 sulphur in the case of the latter. 



The Retina. A general idea of the chemical composition of the 

 retina may be formed from the following analyses, which are takon 

 from Calm : 



