604 PHYSIOLOGY 



leaves the sclerotic and hangs as a curtain into the cavity of the 

 aqueous humour, between the cornea and the lens. The curtain, 

 which is known as the iris, presents a circular orifice at 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 endo- 

 thelium 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 oj 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, which 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. 269.) This ligament is formed in the 

 following way : 



The vitreous humour is bounded externally by the hyaloid mem- 

 brane, 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 processes to the capsule of the lens and form 

 the zonule, or suspensory ligament. The greater part of the sus- 

 pensory ligament, i.e. from the ora serrata of the retina to the edge 

 of the ciliary processes, is closely attached to these processes. From 

 their edge a number of fibres pass and fuse at their inner extremities with 

 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. 



