75o 



ACCOMMODATION. 



from such a distance are practically parallel, and when they enter the eye, are in 

 the passive normal eye (emmetropic) brought to a focus on the retina. When 



looking at a distant object, a distinct image is 



formed on the retina without the aid of any 



muscular action. 

 That distant objects are seen without the aid of any 



muscular action is shown by the following considera- 

 tions : (1) With the 

 normal, or emmetropic 

 eye, we can see distant 

 objects clearly and dis- 

 tinctly, without expe- 

 riencing any feeling of 

 effort. On opening the 

 eyelids after a long period 

 of rest, the objects at a 

 distance are at once dis- 



Fig. 533. 

 Anterior quadrant of a horizontal section of the eyeball, cornea, and lens. 

 a, substantia propria of the cornea; b, Bowman's elastic membrane; c, 

 anterior corneal epithelium ; d, Descement's membrane ; e, its epithe- 

 lium ; /, conjunctiva ; g, sclerotic ; h, iris ; i, sphincter iridis ; /, liga- 

 mentura pectinatum iridis, with the adjoining vacuolated tissue ; k, canal 

 of Schlemm ; /, longitudinal, m, circular muscular fibres of the ciliary 

 muscle ; n, ciliary process ; o, ciliary part of the retina ; q, canal of Petit, 

 with Z, zonule of Zinn in front of it ; and p, the posterior layer of the hyaloid membrane ; 

 r, anterior, s, posterior part of the capsule of the lens ; t, choroid ; u, perichoroidal space ; 

 T, pigment epithelium of the iris ; x, margin of the lens. 



tinctly visible in the field of vision. (2) If, in consequence of paralysis of the mechanism of 

 accommodation {e.g., through paralysis of the oculomotor nerve 345, 7), the eye is unable to 

 focus images of objects placed at different distances, still distinct images are obtained of distant 

 objects. Thus, paralysis of the mechanism of accommodation is always accompanied by inability 

 to focus a near object, never a distant object. A temporary paralysis occurs with the same 

 results when a solution of atropin or duboisin is dropped into the eye, and also in poisoning 

 with these drugs ( 392). 



When the eye is accommodated for a near object, [positive accommodation], 

 the lens is thicker, its anterior surface is more curved (convex), and projects 

 farther into the anterior chamber of the eye {Cramer, 1851, v. Hehnholtz, 1853). 

 The mechanism producing this result is the following : During rest, the lens is kept 

 somewhat flattened against the vitreous humour lying behind it, by the tension of the 

 stretched zonule of Zinn, which is attached round the margin of the lens (fig. 533, Z). 

 When the muscle of accommodation, the ciliary muscle (I, m), contracts, it pulls 

 forward the margin of the choroid, so that the zonule of Zinn in intimate relation 

 with it is relaxed. [When we accommodate for a near object, the ciliary muscle 

 contracts, pulls forward the choroid, relaxes the zonule of Zinn, and this in turn 

 diminishes the tension of the anterior part of the capsule of the lens.] The lens 

 assumes a more curved form, in virtue of its elasticity, so that it becomes more 

 convex as soon as the tension of the zonule of Zinn, which keeps it flattened, is 

 diminished (tig. 534). As the posterior surface of the lens lies in the saucer-shaped 

 unyielding depression of the vitreous humour, the anterior surface of the lens in 

 becoming more convex must necessarily protrude more forwards. 



Nerves. According to Hensen and Vblckers, the origin of the nerves of accom- 



