ACCOMMODATION 33 



(Fig. 5, mm) are thus at right angles to those of the radial (Briicke's) 

 muscle, the two muscle masses are in no way antagonistic in their action 

 as are the sphincter and dilatator pupillae. The contraction of Miiller's 

 muscle heaves the ciliary processes inward toward the axis of the eyeball 

 and thus substantially supplements the action of Briicke's muscle in 

 letting up the tension in the zonule fibers. In fact, the muscle of Miiller 

 is much the more efficient of the two, since no component of its direction 

 of contraction is wasted in uselessly pulling any part of the ciliary body 

 forward in the eye. It is only the inward component of the action of the 

 diagonally-placed Briicke's muscle which is very useful. It is significant 

 that in far-sighted (hypermetropic) eyes, which must constantly make 

 extra accommodatory effort (Fig. 12), it is Miiller's muscle — not 

 Brucke's — which becomes hypertrophied if spectacles are not worn. 



To understand what happens to the lens when the zonule is relaxed, 

 we must recall the nature of the lens capsule and consider its structure 

 in a little more detail. The capsule is a firm, elastic membrane. If a 

 cut is made in it, the edges of the cut will tend to roll outward — thus 

 it is clear that the capsule is normally exerting pressure on the lens 

 fibers. If the capsule were equally thick throughout and the lens fibers 

 were plastic enough, the elasticity of the capsule would tend to mold 

 the lens into a ball if the flattening effect of the tensed zonule fibers 

 were to be eliminated by cutting them. 



Actually, however, the capsule varies greatly in thickness in different 

 parts and consequently varies locally in the force which its elasticity can 

 exert upon the lens capsule (Fig. 14). Fincham, who has revised and 

 modernized the F^elmholtz theory of human accommodation, has care- 

 fully studied the properties of the capsule and of the decapsulated lens. 

 Without its capsule, the body of the lens slowly takes on the flattened 

 form characteristic of the intact lens in situ in the resting eye. Hence 

 the bulged form of the lens in accommodation is brought about by the 

 capsule's assertion, upon it, of a molding force more than strong enough 

 to overcome the tendency of the lens body to flatten. Cutting the zonule 

 fibers allows the capsule to mold the lens into the same shape it has in 

 accommodation. The relaxation of the ciliary muscle allows the tensed 

 zonule fibers to effect a 'physiological decapsulation' of the lens, by pulling 

 so hard upon the equator of the capsule that the latter 's elasticity is ren- 

 dered ineffectual, and the lens body assumes the same flattened form 

 which it takes when removed from its capsule. The contraction of the 

 ciliary muscle, on the other hand, eliminates the pull of the zonule fibers 



