THE EAR AND THE SENSE OF HEARING 135 



prevent such accidents. They do not often do harm, and 

 may be removed by dropping warm water into the ear. 



DEMONSTRATIONS AND EXPERIMENTS 



67. Dissection of the Ear. The external and the middle ear can be 

 fery easily studied by making a dissection of the head of a cat or 

 other domestic animal. Remove the lower jaw, expose and open the 

 temporal bulla. The latter is in many animals a conspicuous rounded 

 protuberance near the articulation of the jaw. The tympanic cavity 

 with the contained ossicles, etc., can then be studied. The internal ear 

 cannot be easily examined because of its smallness and well-nigh in- 

 accessible situation in the rocky portion of the temporal bone. The 

 bone which contains it can be dissected out, and the general outlines 

 of the cochlear region discerned. Or, this portion of the skull may 

 be treated with weak solution of muriatic acid for some days, after 

 which the parts may be partly dissected out. 



68. Effect of Varying A ir Tension in the Tympanum. While listening 

 to a ticking watch, close both nose and mouth, and expel as much air 

 as possible from the lungs, thus forcing the air through the Eustachian 

 tube into the middle ear. The ticking sounds fainter. Or under like 

 conditions inhale as much as possible. The result is as before. 



69. Judgment of the Direction of Sounds. Let a pupil, seated, keep 

 his eyes closed. Clink together two coins at varying distances and 

 directions from his head, and require him to indicate the direction of 

 the sound. Observe that while he rarely fails to distinguish between 

 right and left, he often errs in respect to other directions. Have him 

 hold his hands vertically one in front of each ear, and see if his judg- 

 ment of direction is thereby altered. Close one of his ears with 

 absorbent cotton, and try the effect upon his location of sounds. 



70. A uditory Fatigue. Strike a tuning fork, press the stem down 

 upon the crown of the head, and hold it there until the sound dies 

 away. Then remove it, and after a short interval replace it. The 

 sound will be heard again, but very faintly. 



