VISION. 795 



behind the retina. In this way circles of diffusion form, which 

 would prevent the appearance of a distinct image if a special appar- 

 atus did not exist for the purpose of modifying the degree of refrac- 

 tion. This modification is what is understood by the term accommo- 

 dation. 



Mechanism of Accommodation. The ciliary muscle, when it 

 contracts, causes the zone of Zinn to advance, and thus diminishes 

 the tension exercised by the latter upon the capsule of the crystalline 

 lens. The lens, left to itself, assumes the form which the elasticity 



Fig. 342. Scheme of Accommodation for Near and Distant Objects. 

 (LANDOIS, after HELMIIOLTZ.) 



The right side of the figure represents the condition of the lens during 

 accommodation for a near object and the left side -when at rest. The letters 

 indicate the same parts on both sides; those on the right side are marked with 

 a stroke (or minute mark). A, Left half of lens. B, Right half of lens. G, 

 Cornea. 8, Sclerotic. OS, Canal of Schlemm. VK, Anterior chamber. J, Iris. 

 P, Margin of pupil. V, Anterior surface. H, Posterior surface of lens. R, 

 Margin of the lens. F, Margin of ciliary processes, a, 6, Space between the 

 two former. The line Z-X indicates the thickness of the lens during accom- 

 modation for a near object. Z-Y, the thickness of the lens when the eye is 

 passive. 



of its fibers naturally gives it and becomes more convex, especially 

 at its anterior surface. When the action of the oculomotor nerve 

 ceases, the ciliary muscle is relaxed, the ciliary processes become 

 tense and make traction on the zone of Zinn, which in turn flattens 

 the lens by exerting upon it a traction in the direction of its equator. 

 The retina follows along with the choroid in the movement of 

 accommodation. When the traction of the ciliary muscle ceases 

 relaxation of accommodation in this way, the border of the retina, 

 being closely attached with the choroid, is stretched and irritated by 

 the sudden relaxation of accommodation until the lens flattens. 



These locomotor changes of the choroid may generate a choroid- 

 itis, especially in the production and progress of myopia. Atropine, by 



