Observation. In albinos, where there is an absence of the black pigment, the rays of light traverse the iris, and even the choroid coat, and so overwhelm the eye with light, that their vision is quite imperfect, except in the dimness of evening, or at night. In the manufacture of optical instruments, care is taken to color their interior black, for the same object, namely, the absorption of scattered rays.
921. The iris, by means of its powers of expansion and contraction, regulates the quantity of light admitted through the pupil. If the iris is thin, and the rays of light pass through its substance, they are immediately absorbed by the uvea, and, if that layer be insufficient, they are taken up by the black pigment of the choroid coat.
Observation. When we look toward the bottom of the eye, the pupil appears like a black spot, instead of an opening. This is caused by seeing the black pigment through the retina and humors of the eye.
920. What is the use of the sclerotic coat? Of what is the choroid coat chiefly composed? What is the use of this coat? How is it effected? What is said of albinos? What care is taken in the manufacture of optical instruments? 921. What is the use of the iris? When we look toward the bottom of the eye, why does the pupil look like a black spot, instead of an opening?
922. The cornea, and the aqueous, crystalline, and vitreous humors, are transparent; so that rays of light traverse these parts of the eye, and fall upon the retina. The office of these humors and the cornea is to refract the rays of light in such proportion as to direct the image in the most favorable manner upon the retina.
923. The office of the retina is to receive the impression of the rays of light which leave an object at which we look, and it is upon it that a small but very clear image of that object is formed. The impression thus produced by the reflected light is transmitted by the optic nerve to the brain, which receives the sensation. This constitutes vision.
924. The optic nerve has but one function, that of sight. Sensibility is conferred on this organ by a large branch from the fifth pair of nerves, which ramifies upon the different parts of the eye and its appendages. These parts, however, receive some nervous filaments from the seventh pair.
Observations. 1st. The large number of sensitive nervous filaments renders the visual organ very impressible to bodies that cause irritation, as dust, or intense light. This compels us to use due care to shield the eye from the influence of agents that would impair or destroy vision.
2d. Although particles of dust, when in contact with the delicate parts of the eye, induce severe pain, yet these parts may be cut in surgical operations, and the patient’s sufferings are not as great as when an incision is made in the skin to remove a small tumor.