SEC. 4. THE MECHANISM OF ACCOMMODATION AND 

 THE MOVEMENTS OF THE PUPIL. 



721. We have seen ( 711) that the essential change in the 

 eye during accommodation for near objects is an increase in the 

 curvature of the anterior surface of the lens. How is this 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 accommo- 

 dation mechanism. The real nature of the mechanism seems to 

 be as follows : 



The lens, as we have said, 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, keeping 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 ligament described in a 

 previous section afford an explanation of what is the agency in 

 question, and how it is counteracted. 



