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.

343.  The External Ear.  The external ear consists of an expanded portion known as the pinna or auricle, and of a passage, the auditory canal or meatus, leading inwards from it.  The surface of the auricle is convoluted to collect and transmit the vibrations of air by which sound is produced the auditory canal conducts these vibrations to the tympanic membrane.  Many animals move the auricle in the direction of the sound.  Thus the horse pricks up its ears when it hears a noise, the better to judge of the direction of sounds.[48]

The external auditory meatus, the passage to the middle ear, is curved and is about an inch and a quarter long.  Near its outer portion are a number of fine hairs slanting outwards to prevent the entrance of insects.  Embedded in the deeper parts of the canal are glands which secrete the cerumen, or ear-wax, which keeps the canal moist, and helps to protect it against foreign bodies and insects.  As the result of a cold, this wax may collect in sufficient quantities to block the passage, and to diminish to a considerable extent the power of hearing.