SEC. 3. THE MECHANISM OF ACCOMMODATION AND 

 THE MOVEMENTS OF THE PUPIL. 



534. How is this increase in the curvature of the ante- 

 rior surface of the lens during accommodation for near objects 

 brought about ? 



It has been supposed to be due to a compression of the cir- 

 cumference of the lens by a contraction of the sphincter muscle 

 of the iris, which, as we shall see, is the cause of the narrowing 

 of the pupil attendant upon accommodation for near objects ; 

 but this is disproved by the fact that normal accommodation 

 may take place in eyes from which the iris is congenitally absent 

 or has been wholly removed by operation. It has also been 

 attributed to vaso-motor changes, to increased fulness of the 

 vessels of the iris or ciliary processes, surrounding and pressing 

 upon the lens ; but this also is disproved, not only by the fact 

 just mentioned but as well by the fact that accommodation may 

 be effected, after death, in an eye which is practically bloodless, 

 by stimulating the ciliary ganglion or short ciliary nerves with 

 an interrupted current or by other means ; as we shall see, these 

 nerves govern the accommodation mechanism. The real nature 

 of the mechanism seems to be as follows : 



The lens is a body of considerable elasticity. When the 

 curvature of the anterior surface of the lens is determined, as 

 may be done by appropriate means (by measurements of images 

 seen by reflection from it), in its natural position in the eye at 

 rest, and then again determined, after the lens has been removed 

 from the eye, the anterior surface is found to be more convex in 

 the latter than in the former case. There seems to be in the 

 eye in its natural condition at rest some agency at work, keep- 

 ing the anterior surface of the lens somewhat flattened. All 

 that is needed is some means of counteracting this agency, and 

 thereby allowing the lens through its elasticity to assume its 

 natural form. And the arrangements of the suspensory liga- 

 ment described in a previous section afford an explanation of 

 what is the agency in question, and how it is counteracted. 



The cavity of the eyeball behind the suspensory ligament is 

 filled with the vitreous humour. If this is sufficiently abundant 



854 



