304 AVERS. [Vol. VI. 



easier from in front than from behind, from below than from 

 above ; and this condition being ever present, the co-ordinating 

 mechanism is adjusted to it, and thus our judgment of direction 

 is rendered easier and more frequently correct than it would 

 otherwise be. Sound is heard most distinctly when the auditory 

 canal is in the same straight line with the direction of the sound 

 waves, and for this reason we turn the head in various directions 

 in order to ascertain the position in which the sound seems the 

 loudest ; and the line passing through the centres of both ears 

 will lie in the plane in which the most intense sound waves are 

 propagated. This plane is of necessity at right angles to 

 antero-posterior or sagittal plane or plane of direct vision. All 

 judgments involving the use of the auditory apparatus for 

 determining direction and relations of external bodies must 

 necessarily start from this fundamental relation, which is so 

 early recognized as to be during adult life passed over unnoticed. 

 In the position of the ear above referred to one ear receives the 

 greatest possible, the opposite ear the least possible, amount of 

 sound, and the appreciation of this difference, i.e. the determi- 

 nation of the position of the head in which the greatest differ- 

 ence exists, is used directly in determining the direction of 

 sound, the judgment being based upon a comparison of the 

 intensities of stimuli. The necessity for making trial attempts 

 in order to establish the position of a sounding body by the ear 

 has been compared to the execution of movements in binocular 

 vision, by means of which we estimate distance, determine direc- 

 tion, etc. The assistance gained from the eyes in determining 

 direction in such a case is not slight, but is usually an uncon- 

 scious effort, mainly unnoticed because there is a continued 

 effort maintained by the nervous system for the proper orien- 

 tation of the body in space such as is necessary for its perfect 

 control. All sensations whatsoever going to the brain from the 

 surface of the body likewise aid materially in its orientation. 



Although the ordinary method of transmission is through the 

 air, liquid and solid substances prove even better conductors 

 when brought in contact with the hard parts of the head. When 

 the head is immersed in water, all sounds that are transmitted 

 to the water pass with increased facility and intensity to the 

 internal ear, in part through the bones of the head direct, but 

 also in part through the mediation of the tympanic membrane 



