382 PHYSIOLOGY AND HEALTH. 



aperture in the centre of the iris. The iris is expansible and 

 contractile: when it expands, it extends toward the centre, 

 and lessens the diameter of the pupil ; and when it contracts, 

 it draws back from the centre, and enlarges the pupil. By 

 this means, the amount of light received into the eye is regu- 

 lated. When we are in a dark place, the iris contracts, the 

 pupil is enlarged, and more rays are admitted. When the 

 light is increased, the iris expands, the pupil is contracted, 

 and fewer rays are admitted. When we first go from a bright 

 light, as from a well-lighted room to the darker air abroad in 

 the evening, we see with difficulty, because the pupil is so 

 small that few rays can enter the eye. But soon the pupil 

 enlarges, more rays enter, and we see with ease. On the 

 contrary, when we go suddenly from a dark to a very light 

 place, the pupil being large, much light enters, and the eye& 

 are dazzled ; but soon the iris expands, the pupil diminishes 

 and fewer rays enter, and we bear the light without incon 

 venience. 



899. The lids protect the eyes in front. They are corn 

 posed of cartilages adapted to the shape of the eye, the skin 

 without, and the lining membrane within. The lining of the 

 lids is continued over the front of the eye. It prepares and 

 throws out upon itself a thin mucous or glairy fluid, that oils 

 the surface and allows the lids to glide smoothly over the ball. 

 One circular muscle surrounds the open part of the eye, (Fig. 

 Lil. ft,) and closes the lids when it contracts. Another mus- 

 cle, attached to the upper eyelid by one end, and to the bone 

 of the socket by the other, opens the eye. 



891. The tears wash the eye and keep its surface dean. 

 The apparatus for this purpose consists of the lachrymal 

 glands, tubes, ducts, and canal. The lachrymal glands are 

 placed in the upper and outer corner of the socket, (Fig. 

 LXVII. a.) They prepare the tears, which then flow through 

 the ducts (Fig. LXVII. ft) under the upper lid into the eye. By 

 the motions of the lids the tears are spread over all the surface 

 of the eye, and wash away any particles of dust. Then they 

 fall into a little groove or trough in the upper edge of the 



