340. Effect of Alcohol upon the Eye. The earlier and slighter forms of injury done to the eye by the use of intoxicants are quite familiar: the watery condition of the eye and of the lids, and the red and bleared aspect of the organ. Both are the result of chronic inflammation, which crowds the blood into the vessels of the cornea, making them bloodshot and visible. The nerves controlling the circulation of the eye are partially paralyzed, and thus the relaxed vessels become distended. But more serious results ensue. Long use of intoxicants produces diseases of the retina, involving in many cases marked diminution of acuteness as well as quickness of vision, and at times distorted images upon the surface of the retina. In other instances, the congestion of the optic nerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worse and worse, till total blindness may ensue. It is beyond question that a wide comparison of cases by careful observers proves that a large fraction of those who indulge in strong drink suffer from some form of disease of the eye. 341. Effect of Tobacco upon Vision. Tobacco, in its distribution of evil effects, does not neglect the senses and especially the eye. A variety of vicious results is produced. The pungent smoke inflames the lids. The narcotic dilates the pupil, causing dimness and confusion of vision. A diseased condition occurs with severe pain in the eye followed by impaired vision. Oculists speak impressively of the ill effects of tobacco, and especially of cigarettes, upon the eyes of the young. They mention a well-known disease, tobacco blindness, usually beginning with color-blindness, and progressing occasionally with increasing dimness of vision to entire loss of sight.[47] 342. The Sense of Hearing. The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending. The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head; so deep that sounds cannot directly affect it. Some arrangement, therefore, is required for conducting the sounds inwards to this true organ. Illustration: Fig. 138.—The Pinna, or Auricle. In studying the structure of the ear, and how it is fitted to respond to sonorous vibrations, we may divide it into three parts: the sound-conducting part, known as the external ear, the middle ear, and the deeply placed nerve portion, the inner ear.