The Special Senses 



253 



The ciliary muscle is thus known as the muscle of accom- 

 modation, because it has the power to accommodate the eye 

 to near and distant objects. In this respect it corresponds 

 in its use to the adjusting screw in the opera glass and the 

 ^microscope. 



396. The Refractive Media of the Eye. The eye is a 

 closed chamber, into which no light can pass except through 



FIG. 131. Diagram showing the Change in the Lens during 

 Accommodation. 



(On the right the lens is arranged for distant vision, the ciliary muscle is relaxed, 

 and the ligament D is tense, so flattening by its compression the front of 

 the lens C; on the left the muscle A is acting, and this relaxes the ligament 

 and allows the lens B to become more convex, and so fitted for the vision 

 of near objects.) * 



the cornea. All the rays that enter the eye must also 

 pass through the crystalline lens, which brings them to 

 a focus as would an ordinary lens. 



Now, if the media through which the light from an 

 object passes to reach the retina were all of the same 

 density as the air, and were also plane surfaces, an 

 impression would be produced, but the image would not 

 be distinct. 



The action of the crystalline lens is aided by several 

 refractive media in the eye. These media are the cornea, 

 the aqueous humor, and the vitreous, humor. By reason 

 of their shape and density these media refract the rays of 

 light, and bring them to a focus upon the retina, thus 



