Part 59 (1/2)

910. What are found on the internal surface of the cartilage of the eyelids? Where do they open, and what is their use? 911. With what are the edges of the eyelids furnished? What are their uses? 912. Of what does the lachrymal apparatus consist? 913. Describe the lachrymal gland. How many ducts pa.s.s from this gland, and what do they convey to the eye? Why is the eye constantly moist? 914. Where do the lachrymal ca.n.a.ls commence?

915. The NASAL DUCT is a short ca.n.a.l, about three quarters of an inch in length, directed downward and backward to the inferior channel of the nose, where it terminates by an expanded orifice.

[Ill.u.s.tration: Fig. 141. 1, The lachrymal gland. 2, Ducts leading from the lachrymal gland to the upper eyelid. 3, 3, The puncta lachrymalia. 4, The nasal sac. 5, The termination of the nasal duct.]

916. The fluid (tears) secreted by the lachrymal gland, is conveyed to the eye by the small ducts before described. It is then imbibed by the puncta lachrymalia, and carried by the lachrymal ca.n.a.ls into the lachrymal sac, from which it is pa.s.sed to the nasal cavities by the nasal ducts.

What are they called? With what do they communicate? 915. Describe the nasal duct. 916. How are the tears conveyed from the lachrymal gland to the nose?

CHAPTER XLV.

PHYSIOLOGY OF THE ORGANS OF VISION.

917. To comprehend the theory of vision, it is not sufficient to know the structure of the eye. We must be familiar with some of the properties of a subtile fluid, which is constantly emanating from all luminous bodies, called _light_.

918. It is the province of natural philosophy, rather than physiology, to enter minutely upon the properties of light. It may be observed, however, that, when light pa.s.ses through any medium of the same density, the rays are in straight lines; but, when it pa.s.ses from one medium into another of different density, it is refracted, or turned from a straight course, unless it strikes the medium in a perpendicular direction--then light pa.s.ses through without a change of direction.

919. When a ray of light meets with a body, it either pa.s.ses through it, or is reflected by it, or it may be absorbed. Again, in proportion as the rays of light become distant from the body from which they emanate, they diverge one from the other. In accordance with the laws of optics, the rays of light, in pa.s.sing through an optical instrument like the eye, must cross each other, and thus produce an inverted image of the object from which the rays proceed. With the general view of the structure of the eye, we will now examine the use of each part in the function of vision.

917-933. _Give the physiology of the organs of vision._ 917. What is necessary in order to understand the theory of vision? 918. When light pa.s.ses through a medium of the same density, in what direction will be its rays? Of a different density? What exception? 919. When light meets with a body, what takes place? What is said in reference to rays of light in pa.s.sing through the eye?

920. The sclerotic coat not only gives form to the body of the eye, but protection to the interior and more delicate parts. The choroid coat seems to be chiefly composed of a tissue of nerves and minute blood-vessels; the latter give nourishment to the different parts of the eye. One of the uses of this coat is, to absorb the rays of light immediately after they have pa.s.sed through the retina. This is effected by the black pigment that lines its inner surface. Were it not for this provision, light would be too intense, and vision indistinct.

_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 quant.i.ty of light admitted through the pupil. If the iris is thin, and the rays of light pa.s.s 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 const.i.tutes vision.