78 EVOLUTION OF THE HUMAN EYE 



compensated for, as already mentioned, by increased 

 refractive power in the cornea. Further compensation is 

 effected by increase in the depth of the vitreous chamber 

 and displacement backward of the retina, on which the 

 image of the object looked at has to be focussed. The 

 following table gives the depth of the vitreous chamber 

 from the posterior pole of the lens directly backward to the 

 retina, and the diameters of the lens, in a series of mamma- 

 lian eyes. From this it will be seen that a decrease in the 

 sphericity of the latter is accompanied by an increase in 

 the depth of the former. 



Lens. 



Vitreous chamber. Antero-posterior. Lateral. 



Man 15.4mm. 4.5mm. 9 mm. 



Chimpanzee ... 11.5 " 4 7 



Horse .... 20 11 20 



Antelope . 12 14 17.5 " 



Pig . . 9 6 9 



Rabbit 6 7 9 



Thus, while in the rabbit and antelope the depth of the 

 vitreous chamber is less than the width of the antero- 

 posterior diameter of the lens, in the horse and pig it is 

 considerably greater. In man and the chimpanzee the 

 depth of the vitreous chamber is nearly three times as great 

 as the width of the antero-posterior diameter of the lens. 



A similar comparison may be made in the changes which 

 take place in the shape of the lens and in the depth of the 

 vitreous chamber during the process of development of the 

 human eye. The following table show^s the depth of the 

 vitreous chamber and the diameters of the lens of the devel- 



# 



oping human eye up to the time of birth. 



Lens. 



Age. Vitreous chamber. Antero-posterior. Lateral. 



Fourth month 4.5mm. 2.8 mm. 3.3 mm. 



Sixth month . 

 Seventh month 

 Ninth month . 

 At birth 



5 3.8 4 



7.5 " 4 4.5 



9.5 " 4.3 5.75 



U " 3.75 " 7 



