48 INNERVATION. [CHAP. XVII. 



faculty of the eye is greatly impaired or altogether lost by extraction 

 of the lens, or by paralyzing the ciliary and iridial muscles by bel- 

 ladonna. Dr. Clay Wallace considers that the ciliary muscle advances 

 the lens by compressing the veins, and thus causing an erection 

 or lengthening, of the ciliary processes. 



It has long been observed that the pupil is very prone to contract 

 during near vision, and to dilate when the organ is adapted to view 

 remote objects; and it has been imagined that this change is the ne- 

 cessary condition of adaptation, and may affect the lens through the 

 ciliary body. In some persons, however, not at all deficient in the 

 adjusting power, the iris continues to oscillate for some time after 

 the eye is adjusted, without disturbing vision; and we have occa- 

 sionally found the pupil to remain dilated, though a near object is 

 being gazed at and the illumination remains the same. Moreover, 

 the action of light on the pupil has no effect on the adjustment of 

 the eye, since we can continue to see an object distinctly, whether 

 it be viewed by a strong or by a feeble light. 



These facts are sufficient to prove that the movements in the iris, 

 usually coincident with the act of adjustment, are not the cause of 

 that act. They seem rather to be of the nature of associate move- 

 ments, produced by the close connexion of the iris with the ciliary 

 muscle, and by the community of source from which both these derive 

 their motor nerves, viz. from the third pair, through the ophthalmic 

 ganglion. And it is an important circumstance, that certain con- 

 sensual movements of the eyeballs, performed through the medium 

 of the third pair, are likewise associated with the act of adjustment. 

 The movements of the iris chiefly minister to another function, the 

 regulation of the quantity of admitted light. 



The contraction of the pupil during near vision, by obstructing 

 more of the circumferential rays, answers the important purpose of 

 correcting the excessive aberration of sphericity which results from the 

 greater divergence of the rays entering the eye from near objects. 



Some persons have the power of adjusting the eye to distinct 

 vision at different distances, either to a very limited extent or not 

 at all; and we observe two states of vision connected with this de- 

 fect, which are generally dependent on certain physical conditions 

 of the lenses of the eye : these are shortsightedness, or myopia; and 

 longsightedness, or presbyopia. 



Myopia. Thus, we meet with many persons who cannot dis- 

 tinctly see a yard before them, who fail to recognize the features 

 of their acquaintances in the street. In reading, they are obliged 

 to bring the book close to their eyes : in looking at an object at all 



