710 



SPECIAL SENSES. 



. The ciliary muscle arises from the circular line of junction of the cornea 

 and sclerotic, passes backward, and is lost in the tissue of the choroid, ex- 

 tending as far as the anterior border of the retina. Most of the fibres pass 

 directly backward, but some become circular or spiral. When this muscle 

 contracts, the choroid is drawn forward, with probably a slightly spiral 

 motion of the lens, the contents of the globe, situated behind the lens, are 

 compressed, and the suspensory ligament is relaxed. The lens itself, the 

 compressing and flattening action of the suspensory ligament being dimin- 



FIG. 256. Section of the lens etc., showing the mechanism of accommodation (Fick). 

 The left side of the figure (F) shows the lens adapted to vision at infinite distances. The right side of 

 the figure (N) shows the lens adapted to the vision of near objects, the ciliary muscle being con- 

 tracted and the suspensory ligament of the lens consequently relaxed. 



ished, becomes thicker and more convex, by virtue of its own elasticity, in 

 the same way that it becomes thicker after death, when the tension of the 

 ligament is artificially diminished. 



This is in brief the mechanism of accommodation. Near objects are 

 seen distinctly by a voluntary contraction of the ciliary muscle, the action of 

 which is perfectly adapted to the requirements of vision. In early life the 

 lens is soft and elastic, and the accommodating power is at its maximum ; 

 but in old age the lens becomes flattened, harder and less elastic, and the 

 power of accommodation necessarily is diminished. 



Changes in the Iris in Accommodation. The size of the pupil is sensibly 

 diminished in accommodation of the eye for near objects. Although the 

 movements of the iris are directly associated with the muscular effort by 

 which the form of the lens is modified, the contraction of the pupil is not 

 one of the essential conditions of accommodation. Helmholtz reported a 

 case in which the iris was completely paralyzed, the power of accommoda- 

 tion remaining perfect ; and he described another case, reported by Von 

 Oraefe, in which accommodation was not disturbed after loss of the entire 

 iris. 



It has already been noted that the pupil contracts when the eyes are made 

 to converge by the action -of the muscles animated by the third pair of nerves ; 

 and it is evident that convergence of the eyes occurs in looking at very near ob- 

 jects. It has been shown by Bonders, that increased convergence of the vis- 

 ual lines without change of accommodation makes the pupil contract, as is 

 easily proved by simple experiments with prismatic glasses, and that when 



