AUDITORY SENSATIONS 569 



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, con- 

 sisting 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 com- 

 pound tone changes slightly in quality as its position in relation to the ear 

 is altered. This is partly due to the fact that the auricle may reflect a funda- 

 mental tone more strongly than the partial or the converse. According to 

 Rayleigh this difference in quality is determined chiefly by the fact that diffrac- 

 tion of the sound waves occurs as they pass round the head to the ear remote 

 from the source of the sound, so that the partial tones reach the two ears in 

 different degrees of intensity and determine a difference in quality of the sound 

 as heard by the two ears. 



The external auditory meatus in man is about one inch long and 

 directed forwards, inwards, and slightly upwards. Its general func- 

 tion, other than as a mere conductor of the sound waves, is to protect 

 the delicate vibrating membrana tympani which closes its inner end. 

 Opening on the skin of the meatus are special sebaceous glands which 

 secrete a yellow wax (cerumen) with bitter taste and peculiar odour. 

 The wax not only protects the cuticle of the ear and the membrana 

 tympani from drying, but, together with the hairs at the orifice of the 

 meatus, serves to repel insects and prevent their entering. By the 

 length of the meatus moreover the drum is protected from draughts 

 and its temperature is maintained constant. 



The sound waves which pass down the external meatus impinge 

 on the drum of the ear and set this into vibration. The vibrations 



