HEALTH AND DISEASE 



in the angle of this chamber at the line where the iris and cornea are 

 in contact. The channel is named the canal of Fontana, and communi- 

 cates with another canal named the canal of Schlemm. 



The Lens (fig. 240, j) is a perfectly transparent biconvex solid body, 

 about half an inch in diameter. The front surface is less convex than the 

 posterior. It lies behind the iris, and is lodged in a depression of the 

 vitreous humour. It is kept in position by a circular ligament which is 

 formed out of a continuation of the modified outer layer of the retina 

 and extends from the ciliary processes to the margin of the lens. 



The lens is enclosed in a capsule, which is thick in front and thin 

 behind. It is composed of long fibres with sawlike edges, which mutually 



interlock with those of the adjoining 

 fibres; and these are arranged in 

 many layers, the outermost being 

 soft, becoming denser as the central 

 part of the lens is reached. By the 

 lens, parallel or nearly parallel rays 

 of light are brought to a focus on 

 the retina, and a precise image is 

 there formed. Opacity of the lens 

 constitutes cataract. 



The Vitreous Humour (fig. 



240, h) is a semi-tiuid, uniform sub- 

 stance, like white-of-egg in consist- 

 ence, traversed by numerous delicate 

 fibres which appear to be the remains 

 of cells with greatly attenuated and 

 elongated processes. It is enclosed 

 in a transparent and very thin mem- 

 brane termed the hyaloid membrane. The vitreous humour presents a 

 canal running through its centre from behind, which originally contained 

 blood-vessels on their way to the posterior surface of the lens, but both 

 the canal and the blood-vessels disappear in infancy. In front the 

 vitreous humour is hollowed out, so as to give lodgment to the convex 

 surface of the lens in front of it. Near the margin of the lens' is a 

 structure named the Zonule of Tinn, composed of fibres which are super- 

 imposed upon the hyaloid membrane and forming the suspensory ligament 

 of the lens, whilst they are so arranged as to present alternate elevations 

 and depressions which correspond to the ciliary processes and the inter- 

 vening spaces. 



Fig. 246. —The Eye, showing the Lachrymal Gland 



A, Superior Oblique Muscle. B, Levator Oculi or 

 Superior Rectus. C, Abductor Oculi or External 

 Rectus. D, Depressor Oculi or Inferior Rectus. 

 E, Lachrymtil Gland. F, Optic Nerve. 



