VISION 



373 



of cutting out divergent rays, which would not be clearly 

 focussed when objects near at hand are looked at. The 

 posterior surface of the iris and the inner surfaces of the ciliary 

 processes are covered with dense black pigment. It is this 

 pigment, showing through the uncoloured connective tissue 

 and plain muscle-fibres of which the iris is composed, that 

 gives their colour to grey and blue eyes. In many eyes the 

 iris contains a brown pigment in its substance. 



OpHt Nerve 



Central Artery, 



of Retuta. 



FIG. 27. HORIZONTAL SECTION THROUGH THE RIGHT EYE. 



The slight depression in the retina in the axis of the globe is the fovea centralis, or yellow spot 

 the optic nerve pierces the ball to its inner or nasal side. The lens, with its suspensory 

 ligament, separates the aqueous from the vitreous humour. On the front of the lens rests 

 the iris, covered on its posterior surface wtih black pigment. On either side of the lens is 

 seen a ciliary process, with the circular fibres of the ciliary muscle cut transversely, and its 

 radiating fibres disposed as a fan. 



The back portion of the globe of the eye is covered with a 

 curtain, the retina, formed by the spreading out of the fibres 

 of the optic nerve in front of various layers of nerve-cells and 

 the sensory cells of the organ of vision, rods and cones. The 

 retina lies between the hyaloid membrane, which encloses the 

 vitreous humour, and a layer of pigment which " backs " it, 

 as a photographer backs a plate when he proposes to use it 

 towards a source of light to take a photograph of a window 

 from within a room. The serrated margin of the retina is 

 somewhat anterior to the equator of the eyeball. The pig- 

 ment which backs the retina is contained in a sheet of cells 



