Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 SPECIAL SENSES 557 1. By making the refractive index of the lens higher at its centre than at its circumference. 2. By making the curvature of the lens less near its circumference than at the centre. 3. By ' stopping out' the peripheral rays of light by means of a diaphragm. The two latter methods are those used in most optical instruments. In the eye there is an attempt at all three, but the most important means is the third, the diaphragm being formed by the iris. This is a circular curtain wi
Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 SPECIAL SENSES 557 1. By making the refractive index of the lens higher at its centre than at its circumference. 2. By making the curvature of the lens less near its circumference than at the centre. 3. By ' stopping out' the peripheral rays of light by means of a diaphragm. The two latter methods are those used in most optical instruments. In the eye there is an attempt at all three, but the most important means is the third, the diaphragm being formed by the iris. This is a circular curtain with a hole in the middle, lying just on the anterior surface of the Fig. 249. Diagram to show couise of tlic impulses in the light rcfiex (marked by single arrows), and of tliose which, starting from the oculomotor nucleus, cause dilatation of the pupil (double aiuows). lens. Pigmented cells in it effectually stop out peripheral rays of light, and the size of the opening in it, the pupil, is controlled by the contraction or relaxation of a ring of unstriated muscular fibres situated near the margin of the pupil. The iris has a twofold nerve-supply from the third nerve through the short ciliary nerves, and from the cervical sym- pathetic through the Gasserian ganglion, ophthalmic branch of the fifth, and the long ciliary nerves (Fig. 249). Stimula- tion of the third nerve causes contraction of the pupil. The centre for this movement is in the anterior part of the floor
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