NERVES. 



751 



modation lies in the most anterior root-bundles of the oculomotorius. Stimulation 

 of the posterior part of the floor of the third ventricle causes accommodation ; if a 

 part lying slightly posterior to this be stimulated, contraction of the pupil occurs. 

 On stimulating the limit between the third ventricle and the aqueduct, there results 



Fig. 534. 



Scheme of accommodation for near and distant objects. The right side of the figure represents 

 the condition of the lens during accommodation for a near object, and the left side when 

 the eye is at rest. The letters indicate the same parts on both sides ; those on the right 

 side are marked with a dash ; A, left, B, right half of the lens ; C, cornea ; S, sclerotic ; 

 C.8., canal of Schlemm ; V.K., anterior chamber ; J, iris ; P, margin of the pupil ; V, 

 anterior surface ; H, posterior surface of the lens ; R, margin of the lens ; F, margin of 

 the ciliary processes ; a and b, space between the two former ; the line Z, X, indicates the 

 thickness of the lens during accommodation for a near object ; Z, Y, the thickness of the 

 lens when the eye is passive. 



contraction of the internal rectus muscle, while stimulation of the other parts around 

 the iter causes contraction of the superior rectus, levator palpebrse, rectus inferior, 

 and inferior oblique muscles. 



Proofs. That the lens undergoes an alteration in its curvature, during accommodation, is 

 proved by the following facts : 



1. Purkinje-Sanson's Images. If a lighted candle be held at one side of the eye, or if light 

 be allowed to fall on the eye through two triangular holes, placed above each other and cut in 

 a piece of cardboard, in the latter case the observer will see three pairs of reflected images [in 

 the former, three images]. The brightest and most 

 distinct image (or pair of images) is erect and is 

 produced by the anterior surface of the cornea (fig. 

 535, a). The second image (or pair of images) is 

 also erect. It is the largest, but it is not so bright 

 (b), and it is reflected by the anterior surface of 

 the lens. (The size of a reflected image from a 

 convex mirror is greater, the longer the radius of 

 curvature of the reflecting surface.) The latter 

 image lies 8 mm. behind the plane of the pupil. 

 The third image (or pair of images) is of medium 

 size and medium brightness it is inverted and lies 

 nearly in the plane of the pupil (c). The posterior 

 capsule of the lens, which reflects the last image, 

 acts like a concave mirror. If a luminous object be 

 placed at a distance from a concave mirror, its inverted, diminished, real image lies close to the 

 focus towards the side of the object. If the images be studied when the observed eye is passive, 

 i.e., in the phase of negative accommodation, on asking the person experimented upon to accom- 

 modate his eye for a near object, at once a change in the relative position and size of some of 

 the images is apparent. The middle pair of images reflected by the anterior surface of the lens 

 diminish in size and approach each other (&), which depends upon the fact that the anterior 

 surface of the lens has become more convex. At the same time, the image (or pair of images) 

 comes nearer to the image formed by the cornea {a t and c y ) as the anterior surface of the lens lies 

 nearer to the cornea. The other images (or pairs of images) neither change their size nor posi- 



Fig. 535. 

 Sanson -Purkinie's images, a, b, c, during 

 negative, and a n b n c positive accommo- 

 dation. 



