832 



ACCOMMODATION OF THE EYE. 



When the eye is at rest, it is accommodated for the greatest distance; 

 that is, sharp images are formed on the retina of objects at an infinite 

 distance (as, for example, the moon). In other words, parallel rays (ap- 

 proximately) that enter the eye are united on the retina of the normal- 

 sighted eye at rest; the principal focus is therefore in the retina. 

 Distant vision is thus accomplished without the aid of any muscular 

 action. 



That no muscular activity is actually required for distant vision is proved by 

 the following facts: (i) Normal-sighted persons see sharply and distinctly at a 

 distance without the slightest feeling of exertion. On opening the eyelids after 

 a considerable period of rest, distant objects are at once seen with sharp outlines. 



FIG. 281. Anterior Quadrant of a Horizontal Section of the Eyeball. Cornea and lens cut in the middle of their 

 vertical diameter: a, substantia propria of the cornea; b, Bowman's membrane; c, anterior corneal epithelium; 

 tf, Descemet s membrane; e, its epithelium; /, conjunctiva; g, sclera; h, iris; i, sphincter muscle of the iris; 

 7, pectinate ligament of the iris with the adjacent fenestrated tissue; k, canal of Schlemm; /. longitudinal, 

 m, circular fibers of the ciliary muscle; n, ciliary process; o, ciliary portion of the retina; q, canal of Petit, 

 with the zonule of Zmn (Z) in front of it, the posterior leaflet of the hyaloid membrane (p) behind it; r, 

 anterior, s, posterior, capsule of the lens; /, choroid; , perichoroidal space; T, pigment epithelium of the 

 ins; x, margin (equator) of the lens. 



(2) If the eye has lost its power of accommodation in consequence of paralysis of 

 the oculomotor nerve, sharp images of distant objects are still found on the retina. 

 Paralysis of the accommodation-apparatus is always associated with disturbance 

 f near vision, never of distant vision. Temporary paralysis, with the same 

 result, is produced by the instillation of atropin or duboisin (and by taking toxic 

 doses internally). 



When the eye is accommodated for near objects, the lens becomes 

 thicker and its anterior surface is more curved and protrudes further into 

 the anterior chamber. The mechanism producing this change is as 

 follows: While at rest the lens is kept flat against the vitreous by the 

 traction of the stretched zonule of Zinn (Fig. 281, Z), which is attached 

 to its margin. When the ciliary muscle (1, m) contracts to focus for 



