106 SPECIAL SENSES. 



which he supposed, with Leeuwenhoek, to be muscular, was 

 not understood. 



The changes in the eye, by which accommodation is ef- 

 fected, are now known to consist mainly in an increased con- 

 vexity of the lens for near objects ; and the only points in 

 dispute are a few unimportant details in the mechanism of 

 this action. The simple facts to be borne in mind in study- 

 ing this question are the following : 



"When the eye is accommodated to vision at an infinite 

 distance, the parts are passive. 



In the adjustment of the eye for near objects, the con- 

 vexities of the lens are increased by muscular action. 



In accommodation for near objects, the pupil is con- 

 tracted ; but this action is merely accessory and is not essen- 

 tial. 



The ordinary range of accommodation varies between a 

 distance of about five inches and infinity. 



Changes in the Crystalline Lens in Accommodation. It 

 is important to determine first the extent and nature of the 

 changes of the lens in accommodation ; and, by the ingenious 

 experiments of the German physiologists, particularly those 

 of Helmholtz, these changes have been accurately measured 

 in the living subject. As the general result of these meas- 

 urements, it was ascertained that the lens becomes increased 

 in thickness in accommodation for near objects, chiefly by an 

 increase in its anterior curvature, by which this surface of the 

 lens is made to project toward the cornea. As the iris is in 

 contact with the anterior surface of the lens, this membrane 

 is made to project in the act of accommodation. The poste- 

 rior curvature of the lens is also increased, but this is slight as 

 compared with the increase of the curvature of its anterior 

 surface. The distance between the posterior surface of the 

 lens and the cornea is not sensibly altered. It is unnecessary 

 to describe minutely the methods employed in making these 

 calculations, and it is sufficient for our purposes to state that 



