DIOPTRIC MECHANISMS OF THE EYEBALL 605 



(3) A posterior group lying close to the hyaloid membrane and 

 passing to the posterior lens capsule. 



The suspensory ligament is always in a condition of tension. 

 If the finger be pressed on the outside of the eyeball, it will be felt 

 that this organ presents a resistance to deformation which cannot be 

 ascribed simply to the firmness of the sclerotic coat, but must be 

 determined by the existence of a positive pressure in the fluid filling 

 the eyeball. This pressure, which is known as the intra-ocular pressure, 



Cornea 



Sinus venosus 



Conjunctiva 



Retina 



FIG. 269. Section through anterior part of eyeball to show mode of 

 suspension of lens. (After MEBKEL and KALLITJS.) 



may be measured by means which we shall have to discuss later* 

 and is found to amount to about 25 mm. Hg. As a result of this 

 pressure the membranes which confine the fluids of the eyeball are 

 distended, i.e. pressed outwards, and this pressure keeps the bases 

 of the ciliary processes pressed against the choroid coat and thus 

 enables them to withstand the pull exerted by the tense suspensory 

 ligament. 



The pull exerted by the suspensory ligament affects mainly the 

 tough anterior surface of the lens capsule and so has a constant 

 flattening effect on the anterior surface of the lens. This may be proved 

 by measuring the curvature of the lens in a recently excised eye, and 

 then removing the lens altogether from the eyeball and measuring 



