146 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON 



of Preyer and other responses to auditory tests. Both the guinea- 

 pig and rat react most evidently to sounds. The former animal 

 responds usually five to six hours after birth. In the rat, however, 

 the development of the function is, as already stated, relatively 

 retarded and usually first appears at about ten days of age. 



To test the presence of hearing there are several methods, 

 as for example, Preyer V Ohrmuschelreflex' and the reflex 

 closure of the eyelid, and these I used in making my observations. 

 As the source of the sound I selected the hand clap, a whistle 

 (Triller-pfeife, about c 4 ) and a low sound made by drawing in 

 the breadth with nearly closed lips (about c 1 ). 



Since it is my purpose merely to determine the first appear- 

 ance of a response to sound, it is not nescessary to carry out such 

 refined examinations as did Hunter ('14, '15, '18) Watson 

 ('07), and others on white rats, and Marx ('09) on the guinea- 

 pig, nor was it necessary to use the tuning-fork which by the 

 way is not so good for tests on animals as for those on man. 

 Since sometimes we may have a defect of hearing in animals, 

 as shown by many investigators, it was deemed necessary to 

 use several sources of sound and to take care not to produce 

 ah* currents striking the test animal or to touch it in any way. 

 For this purpose a large sheet of glass was placed between the 

 source of sound and the rats to be tested. While the rat was 

 resting I suddenly produced the sound and noted whether the 

 rat responded. When the animals already had their eyes open 

 the test was made from behind to avoid visual reflexes. 



Observations 



Rats at birth show no response to auditory stimuli. Most 

 of them respond at twelve days of age very clearly, sometimes 

 at ten to eleven days Under certain circumstances, the time 

 of the reflex can be rather accurately noted. For example, 

 while in the morning at ten days no reflex was noted it was 

 present in the evening of the same day. Fortunately, I obtained 

 five nine-day-old rats belonging to one litter and in nearly the 

 same condition of nourishment and developnent. One of these 



