SIGHT. 501 



the pupillary opening. The action of the circular fibres, at the 

 same time, is much the most marked and important of the two. 

 For when the whole muscular apparatus of the eye is paralyzed 

 by the action of belladonna, or by the division of the third pair of 

 nerves, or in the general relaxation of the muscular system at the 

 moment of death, the pupil is invariably dilated, probably by the 

 passive elasticity of its tissues. 



During life, however, these different conditions of the pupil cor- 

 respond with the different degrees of light to which the eye is ex- 

 posed. In a strong light, the pupil contracts and shuts out the 

 superfluous rays; in a feeble light, it dilates, in order to collect 

 into the eye all the light which can be received from the object. 

 This contractile and expansive movement of the pupil is a reflex 

 action. It is not produced by the direct impression of the light 

 upon the iris itself, but upon the retina; since, if the retina be 

 affected with complete amaurosis, or if the light be entirely shut out 

 from it by an opacity of the lens, no such effect is produced, though 

 the iris itself be exposed to the direct glare of day. From the 

 retina the impression is transmitted, through the optic nerve, to the 

 optic tubercles and the brain, thence reflected outward by the oculo- 

 motorius nerve to the ophthalmic ganglion, and so through the 

 ciliary nerves to the iris. 



The pupil is subject, however, to various other nervous influences 

 beside the impressions of light received by the retina. Thus in 

 poisoning by opium, it is contracted ; in coma from compression of 

 the brain, it is dilated ; in natural sleep it is contracted, and the eye- 

 ball rolled upward and inward. In various mental conditions, the 

 pupil is also enlarged or diminished, and thus modifies the expres- 

 sion of the eye; and in viewing remote objects, it is generally 

 enlarged, while, in looking at near objects, it is comparatively con- 

 tracted. But still, the most constant and important function be- 

 longing to the iris is the admission or exclusion of the rays, accord- 

 ing to the intensity of the light. 



Our impressions of distance and solidity, in viewing external 

 objects, are produced mainly by the combined action of the two eyes. 

 For, as the eyes are seated a certain distance apart from each other 

 in the head, when they are both directed toward the same object, 

 their axes meet at the point of sight, and form a certain angle with 

 each other ; and this angle varies with the distance of the object. 

 Thus, when the object is within a short distance, the axes of the 

 two eyes will necessarily be very convergent, and the angle which 



