ACCOMMODATION AND CONVERGENCE 79 



Weiss 36 has shown that the anterior segment of the eye- 

 ball grows rapidly during the first year of life and attains 

 its full dimensions by the end of the second year. The 

 subsequent growth of the eye, which continues into the 

 twenties, is entirely due to an expansion of the posterior 

 segment. At birth the depth of the vitreous is 11 mm. and 

 in the adult eye 15.4 mm. 



This difference in the depth of the vitreous chamber in 

 different classes of mammals, and at different periods in 

 the course of development in man's eye, may be correlated 

 with the varying states of refraction met with. 



The refraction of the eyes of a large number of mammals 

 has been investigated by Lang and Barrett 37 and by Lindsay 

 Johnson. 3 Their investigations agree in showing that wild 

 ^species of mammals are always hypermetropic. Lindsay 

 Johnson says: "A slight degree of hypermetropia, i. e., 

 under 1 D. may be said to be the rule throughout the 

 higher mammals, while higher hypermetropia, i. e., 2 D. 

 to 5 D., is found in the wild species of rodents, the Edentata 

 and the Marsupials." 



Among domesticated mammals myopia is not uncommonly 

 met with. In a series of mammals Lang and Barrett found 

 it in the following proportions of the eyes examined : Rabbits, 

 5 out of 52; guinea-pigs, 5 out of 28; rats, no myopia, but 



1 mixed astigmatism out of 10; cows, no myopia in 10; 

 horses, no myopia, but 1 mixed astigmatism out of 6; cats, 



2 out of 12; dogs, 2 slight myopia out of 6. Lindsay Johnson 

 says : ' Myopia is frequently met with both with and without 

 astigmatism among domesticated animals, especially guinea- 

 pigs, rabbits and other rodents kept in confinement in small 

 hutches." He also states that he found jnyopia to be a 

 persistent characteristic in mandrils and baboons. Lang 

 and Barrett found that 2 monkeys out of 11 which they 

 examined were myopic. There is no note as to how long 



