DIOPTRIC MECHANISMS OF THE EYEBALL 543 



its centre, the pupil, and is provided with muscular fibres by means of which 

 the pupil may be constricted or dilated. The posterior surface of the iris, 

 like the inner surface of the choroid generally, is lined by a pigmented 

 epithelium. The posterior layer of the cornea is formed by a tough elastic 

 membrane (Descemet's membrane) which is covered posteriorly by a layer of 

 cubical epithelial cells. At the circumference of the cornea Descemet's 

 membrane, or the posterior elastic lamina, breaks up into a number of 

 fibres, which pass outwards and backwards to be inserted into the anterior 

 part of the choroid and root of the iris. These fibres are the ligamentum 

 pectinatum iridis. Between the fibres of the ligamentum pectinatum are a 

 number of spaces lined with endothelium continuous with the anterior 

 chamber and known as the spaces of Fontana, and outside these spaces, in the 

 corneo-sclerotic junction, is a circular sinus continuous with the venous 

 system, the canal of Schlemm or sinus venosus. The iris rests at its inner 

 margin on the lens, so dividing the cavity of the aqueous humour into two 

 parts. In front is the anterior chamber and behind the posterior chamber. 

 The latter is a small annular space, triangular in cross-section, and bounded 

 by the iris in front, the lens behind, and the ciliary processes externally. 



The crystalline lens, made up of radiating lens fibres, each of which 

 is produced by the modification of an epithelial cell, is biconvex, the posterior 

 surface being more convex than the anterior. It is surrounded by a tough 

 structureless membrane, the capsule of the lens, and rests in a cavity hollowed 

 out of the anterior surface of the vitreous humour. At its circumference it is 

 hung up and fastened to the ciliary processes by the suspensory ligament, or 

 zonule of Zinn (zonula ciliaris). (Fig. 271.) This ligament is formed in the 

 following way : 



The vitreous humour is bounded externally by the hyaloid membrane, 

 which separates it from the retina. In front the hyaloid membrane passes 

 behind the lens, but as it lies on the ciliary processes is closely adherent to 

 these structures and sends off fibres which pass radially from the ciliary pro- 

 cesses to the capsule of the lens and form the zonule, or suspensory ligament. 

 The greater part of the suspensory ligament, i.e. from the ora serrata pf the 

 retina to the edge of the ciliary processes, is closely attached to these pro- 

 cesses. From their edge a number of fibres pass and fuse at their inner 

 extremities with the lens capsule. These fibres are arranged in three groups : 



(1) The anterior group passing to the anterior capsule of the lens. 



(2) A middle group passing to the equator of the lens. 



(3) A posterior group lying close to the hyoid 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, may be measured by means which we shall 

 have to discuss later, and amounts to about 25 mm. Hg. As a result of 



