THE SENSE OF SIGHT. 139 



muscular, fibres, about one-eighth of an inch long, running from before 

 backward; beneath the radiating fibres are small bands of circular fibres 

 running around the eye. It arises from the line of junction of the sclerotic, 

 cornea and iris ; passing backward it is attached to the outer surface of 

 the choroid ; it is the principal agent in accommodation, and innervated by 

 the 3d pair of nerves. 



The Retina forms the internal coat of the eye ; in the fresh state it is a 

 delicate, transparent membrane, but soon becomes opaque and of a pinkish 

 tint ; it extends forward almost to the ciliary processes, where it terminates 

 in the ora serrata. In the posterior portion of the retina, at a point cor- 

 responding to the axis of vision, is a rounded, elevated yellow spot, the 

 limbus luteus, having a central depression, ihefovea centralis ; about -^ of 

 an inch to the inner side is the point of entrance of the optic nerve, where 

 it spreads out to assist in the formation of the retina. The arteria centralis 

 retina pierces the optic nerve near the sclerotic, runs forward in its sub- 

 stance and is distributed in the retina as far forward as the ciliary processes. 



The Retina consists of ten distinct layers, from within outward, sup- 

 ported by connective tissue. I. Membrana limitans interna. 2. Fibres 

 of optic nerve. 3. Layers of ganglionic corpuscles. 4. Molecular layer. 

 5. Internal granular layer. 6. Molecular layer. 7. External granular 

 layer. 8. Membrana limitans externa. 9. Layer of rods and cones. 10. 

 The layer of pigment cells. 



The number of optic nerve fibres in the retina is estimated to be about 

 800,000, and for each fibre there are about seven cones, 100 rods, and 

 seven pigment cells. The points of the rods and cones are directed 

 toward the choroid, or away from the entering light, and dip into the 

 pigmentary layer. They, with the pigment layer, are the elements inter- 

 mediating the change of the ethereal vibrations into nervous force. Out of 

 these nerve messages the centre in the occipital lobe fashions the sensations 

 of light, color and form. 



In the Fovea centralis^ at the point of most distinct vision, all of the 

 layers disappear except the layer of rods and cones, which becomes some- 

 what longer and more slender. 



The Aqueous humor is a clear fluid, alkaline in reaction, occupying the 

 anterior chamber of the eye; this chamber is bounded in front by the 

 cornea, posteriorly by the iris. 



The Vitreous humor forms about four-fifths of the entire ball. It sup- 

 ports the retina, and is excavated anteriorly for the reception of the lens ; 

 it is transparent, of a jelly-like consistence, and surrounded by a structure- 

 less, transparent membrane, the hyaloid membrane. 



