AUDITORY SENSATIONS 511 



(1) ' difference tones,' in which the frequency is the difference of the frequen- 

 cies of the generating tones ; (2) ' summation tones,' which have a pitch 

 corresponding to the sum of the vibrations of the tone of which they are 

 composed. By means of appropriate resonators these tones can be rein- 

 forced, showing that they have an objective existence and are not produced 

 in the ear itself. 



Not only can the ear appreciate differences between different musical 

 instruments, dependent on the varying overtones present in the sound pro- 

 duced by each instrument, but, when a number of these instruments are 

 sounded simultaneously, the ear can pick out from the compound sound the 

 notes due to the individual instrument, and a person with a trained ear can 

 with ease name notes composing any chord struck on an instrument such as 

 the piano. This power of analysis, which is possessed by the ear, or at any 

 rate by the auditory apparatus, may be stated in the form of the law, known 

 as Ohm's law, which is as follows : 



" Every motion of the air which corresponds to a composite mass of 

 musical tones is capable of being analysed into a sum of simple pendular vibra- 

 tions, and to each single vibration corresponds a simple tone, sensible to the 

 ear and having a pitch determined by the periodic time of the corresponding 

 motion of the air." 



THE PHYSIOLOGY OF HEARING 



The organ of the ear may be considered as consisting of an accessory part 

 and an essential part. The latter is formed by the terminal expansion of the 

 auditory nerve. The accessory part is constructed so as to bring the waves 

 of sound to act on the end organs. The ear is divided anatomically into three 

 parts the external ear, consisting of the pinna with the external auditory 

 meatus ; the middle ear, consisting of the tympanic cavity ; and the internal 

 ear, consisting of the osseous and membranous labyrinths with the terminal 

 branches of the auditory nerve. 



The external ear in the lower animals is fashioned so as to collect sound 

 waves from different directions. To this end it is provided with muscles and 

 in many cases is very movable. In such animals the immediate response to 

 a slight sound is a pricking up of the ears and a direction of their orifices 

 towards the source of sound, a reflex direction of attention which in man is 

 replaced by a conjugate deviation of the two eyes towards the side from 

 which the sound comes. The collecting function of the pinna in man is 

 rudimentary ; in fact a man can hear almost as well with his ear cut off 

 as normally. 



The form of the pinna in man may have a slight influence in the judgment of the 

 direction from which sounds proceed. It has been noticed that a compound tone 

 changes slightly in quality as its position in relation to the ear is altered. This ii 

 partly due to the fact that the auricle may reflect a fundamental tone more strongly 

 than the partial or the converse. According to Bayleigh this difference in quality is 

 determined chiefly by the fact that diffraction of the sound waves occurs as they pas 

 round the head to the ear remote from the source of the sound, so that the par 

 tones reach the two ears in different degrees of intensity and determine a d: 

 in quality of the sound as heard by the two ears. 



