xi\ THF-: EYK AND THE SENSE OF SKJIIT 215 



Shade the eye of a person with your hand and notice the 

 size of the pupil, then let the light from a candle fall on the 

 eye : you will notice the pupil quickly becomes smaller. Ask 

 the person to look at some distant object, and notice the size 

 of his pupil, then ask him to look at the point of a pencil put 

 up about ten inches in front of him ; as he docs so you will 

 notice his pupil becomes smaller. The pupil is larger 

 \vlu-n the light is dim, and also when we look at distant 

 objects; it is smaller when the light is strong and when we 

 look at near objects. How does the iris act so as to cover 

 under some circumstances more, and under other circumstances 

 less, of the crystalline lens ? 



The loose fibrous connective tissue of which the iris con- 

 sists contains, in addition to pigment and other cells, un- 

 striated muscle tissue. The muscular fibres are placed near 

 the margin of the iris, around which they run in a circular 

 manner. They form what is called a sphincter muscle. 

 When this muscle contracts, the pupil becomes smaller, the 

 iris covering more of the lens. When this muscle relaxes, the 

 pupil becomes again larger. There is in addition at the back 

 of the iris a thin layer of muscle fibres, or at least something 

 like them, stretching outwards all round between the margin 

 of the pupil and the outer edge of the iris. This layer can, 

 by its contraction, draw back the iris, and so make the pupil 

 larger. It is. however, chiefly the sphincter muscle which 

 causes, by its contraction or its relaxation, the ordinary varia- 

 tions in the size of the pupil. 



We have, so far, spoken of the mechanisms by which 

 images of objects, whether near or far, or bright or dim, arc 

 formed on the retina ; we must now turn to the retina itself, 

 and see through what means the images on the retina excite 

 the endings of the optic nerve, so that proper impulses are 

 carried by it to the brain. 



Structure of the Retina. Just as there are in the 

 organs of the sense of smell and of taste delicate, rod-shaped 

 cells, or cells with delicate processes to receive the impressions 

 and to transmit them to the sensory nerves concerned, so also 

 in the retina the light excites rod-like processes of cells, and im- 

 pressions pass from these to the optic nerves. These cells lie in 

 the retina in a definite layer, called the layer of rods and cones. 



