180 THE EYE. 



hyaloid membrane round the circumference of the crystalline 

 lens, which hyaloid membrane is continuous with the fibro- 

 cellular elements of the retina, which ends at the ora serrata 

 (about one-third back from front). The anterior ends of 

 the processes project into the posterior chamber of the aqueous 

 humour, touching the /m, and bounding peripherally that 

 chamber. 



The Iris, which receives its name from the various colours it 

 assumes in different individuals, is the circular screen, or curtain, 

 attached at its outer border to the ciliary ligament, and interposed 

 between the cornea and lens. Its aperture is commonly spoken of 

 as the pupil. Its anterior surface is the seat of that variety of 

 colour to which, in common parlance, the colour of the eye 

 itself is attributed. At birth the anterior surface of the iris is 

 invariably blue, at which time this colour does not depend on 

 pigment, but is due to interference phenomenon in the same 

 manner as the blue of the sky. Pigment begins to be deposited 

 a few weeks after birth. The posterior surface is covered with 

 pigmented cells. Albinoes are an exception, and have no pigment 

 anywhere, and so, in their pink eyes, the colour of the blood is 

 shown up ; the light reflected back from the interior of the eye, 

 being absorbed in others by the pigment. The substance of the 

 iris contains involuntary muscular fibres near its free edge, forming 

 a circular band, and closing by its contraction the pupil ; it also 

 contains fibres acting as dilators, radiating outwards from the 

 central band. 



The Retina, Fig. 3 {c). — On entering the orbit, the optic 

 nerve, protected by an additional sheath (derived from the dura 

 mater)y after a slightly curved course, enters the globe slightly to 

 the nasal side of the posterior pole. The outer sheath blends with 

 the sclerotic, and the inner one ends at the lamina cribrosa^ as 

 the sieve -like opening in the sclerotic is called, so that the nerve 

 fibrils only enter quite transparent, and accompanied by the 

 artery of the retina and its veins. The uncovered nerves bend 

 round, and end in the layer of ganglion cells of the retina as far 

 as the ora serrata. The artery sub-divides and breaks up in the 

 granular layers into a very fine set of capillaries. 



