526 PHYSIOLOGY 



forward dining accommodation in both man and animals ; further, Hess has shown that, 

 when full aecommodation has been performed, the lens is only loosely supported, so that 

 gravity can act on it and cause it to sink slightly in relationship to the other eye struc- 

 tures. These effects appear to be definitely in favour of Helmholtz' theory, and against 

 that of Tscherning. The way in which the contraction of the ciliary muscles OMtti 

 slackening of the x.oiiula may therefore be given with some degree of confidence on 

 Helmholtz' theory. 



THE MECHANISM OF ACCOMMODATION. The cilhuv muscle con- 

 sists of two separate sets of unstriated muscle fibres, the more superficial 

 set of radial or longitudinal fibres, the deeper of bundles of radial fibres. 

 The former take their origin from the sclero- cornea! junction, and are attached 

 to the anterior part of the choroid coat behind the ciliary processes. When 

 these fibres contract, they draw the choroid forward and inward, the 

 ciliary processes tending to occupy a smaller circle. The circular 

 fibres lie in the substance of the base of the ciliary processes so that, 

 when they contract, they cause the apices of the processes to come 

 together. In addition to these two sets of fibres a third set has been 

 described as meridional. These are however part of the radio-longitudinal 

 set from which there does not appear to be much object in differentiating 

 them. The ciliary muscles therefore have a common action, causing the 

 ciliary processes to form a smaller circle. The zonula of Zinn, or sus- 

 pensory ligament, is formed of a large number of very fine fibres which 

 run from the ciliary processes to the capsule of the lens. Further those 

 which arise posteriorly are attached anteriorly and vice versa. When the 

 ciliary muscle is in a state of rest, the tension in the choroid set up 

 by the intraocular pressure causes the ciliary processes to be pulled 

 in an outward and backward direction and therefore puts tension on the lens 

 capsule through the zonula. The lens therefore tends to be flattened and 

 accommodated for distance. On stimulating the ciliary muscle the tension 

 in the choroid is opposed and the ciliary processes approximated. The 

 zonula thus becomes slack, and the tension of the lens capsule decreases, 

 allowing the lens to take up its more natural spherical shape, and thus to 

 focus the rays from nearer objects. 



FH; . LMii. Accommodation in the cat's eye. R, distance; \. for near vision. 



(After BlBB.) 



Two needles have been jiard tlm.u<:li (ht- edge of cornea into the eilinrv 

 bodies, to show forward movement f the latter durag accommodation. 



