244 



THE HUMAN MECHANISM 



compressible lens (that is, makes it less convex), and 

 the lens is always in this flattened condition in the resting 

 eye; for example, when one is asleep. The same condition 

 should obtain, as we shall learn, whenever we are looking 

 at distant objects. 



The pull of the tense choroid on the lens is, however, 

 overcome at times by the action of the sheetlike ciliary 



muscle. The fibers of this pe- 

 culiar muscle originate in the 

 sclerotic coat around and just 

 outside the cornea, and diverge 

 radially outward and backward 

 to end in the choroid beyond 

 the attachment of the suspen- 

 sory ligament of the lens. 

 ^Suspensory Fig. 94 shows how the con- 

 traction of this muscle, fixed 

 as it is near the cornea, must 

 draw the choroid forward and 

 so ease the pull of the latter 

 FIG. 93. Vertical section through on the ligament of the lens, 

 the ciliary region of the eye When this happens, the lens, 



Showing the structures concerned in owing to its OW11 elasticity, 

 accommodation (see sect. 7). This . . , , 



should be compared with the perspec- assumes its independent (more 



tive view into the hemisphere of the convex^) shape 



The curvature of the lens is 



Ligament 



eyeball, shown in Fig. 167 



thus variable, and is determined by the action of this muscle 

 of accommodation. When the ciliary muscle is relaxed, the 

 lens is kept flattened by the pull of the choroid on the liga- 

 ment; when the muscle contracts, this pull is eased off (or 

 slacked) and the lens becomes more convex. The entire 

 operation is known as accommodation, and we may now 

 inquire what part accommodation plays in vision. 



8. The formation of an image by a lens. The eye is a 

 camera, in that it forms on the retina an image of objects 



