CHAPTEE Y. 



MOVEMENTS OF THE IRIS ACCOMMODATION. 



Direct action of light upon the iris Action of the nervous system on the iris 

 Mechanism of the movements of the iris Accommodation of the eye to 

 vision at different distances Theories of accommodation Changes in the 

 crystalline lens in accommodation Action of the ciliary muscle Changes 

 in the iris in accommodation. 



THE movements of the iris are sufficiently simple, as well 

 as the physiological conditions under which they take place ; 

 and it is only when we come to study the exact mechanism 

 of the production of these movements through the nervous 

 system, that the subject becomes complex, and, to a certain 

 extent, obscure. As regards the movements themselves, the 

 simple facts are as follows : 



There are two physiological conditions under which the 

 size of the pupil is modified: The first of these depends 

 upon the amount of light to which the eye is exposed. When 

 the quantity of light is small, the pupil is widely dilated, so 

 as to admit as much as possible to the retina. When the eye 

 is exposed to a bright light, the retina is protected by con- 

 traction of the iris. The muscular action by which the iris is 

 contracted is characteristic of the smooth muscular fibres, as 

 can be readily seen by exposing an eye, in which the pupil is 

 dilated, to a bright light. Contraction does not take place 

 instantly, but an appreciable interval elapses after the expos- 

 ure, and a more or less gradual diminution in the size of the 

 pupil is observed. This is seen both in solar and in artificial 

 light. The second of these conditions depends, indirectly, 

 upon the voluntary action of muscles. We have already seen, 



