CHAPTER XIII 



VISION 



THE eye is enclosed in a globe of fibrous tissue, of which the 

 front part, or cornea, being transparent, admits light. The 

 epithelial layer which covers the cornea, conjunctiva, is also 

 transparent. No bloodvessels enter these colourless tissues, 

 unless as the result of inflammation due to infection or to 

 exposure to sunshine or dust. For nutrition they are depen- 

 dent upon the plasma which, exuding from, and returning to, 

 the vessels which surround them, circulates in their tissue- 

 spaces. In advancing years, when the circulation is less brisk, 

 a ring of opaque tissue, arcus senilis, encroaches on the cornea. 

 In the interior of the globe, just behind the cornea, is a pro- 

 jecting shelf, formed of a ring of tissue supported by buttresses, 

 ciliary processes. It is continued inwards as the iris, a mus- 

 cular curtain. The " hyaloid membrane " lines the back por- 

 tion of the globe. Continued on the inner side of the ciliary 

 processes, it splits into several layers, which pass, one in front 

 of the lens, others to its edge, to which they are attached, and 

 still another, very thin, behind it. Since it holds the lens in 

 place, the anterior portion of the hyaloid membrane is known 

 as its " suspensory ligament." Thus the eyeball is divided into 

 three chambers. The anterior is filled with watery lymph, 

 aqueous humour. In it, resting on the anterior surface of the 

 suspensory ligament of the lens, is the iris. The middle 

 chamber contains the lens. The posterior chamber is filled 

 with a liquid jelly, vitreous humour. 



By the contraction of the circular fibres of the iris, the 

 aperture of the pupil is diminished, limiting the light which 

 enters the globe. This adjustment occurs when the illumina- 

 tion is bright. It is also brought into action for the purpose 



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