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 pass from this gland, and what do they convey to the eye? Why is the eye constantly moist? 914. Where do the lachrymal canals commence? -=-=-=-=-=-=-=-=-=-=-=-= 915. The NASAL DUCT is a short canal, 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. [Illustration: 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 canals into the lachrymal sac, from which it is passed 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 passes through any medium of the same density, the rays are in straight lines; but, when it passes 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 passes through without a change of direction. 919. When a ray of light meets with a body, it either passes 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 passing 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. -=-=-=-=-=-=-=-=-=-=-=-=