198 PHYSIOLOGY CHAP. 



greatest when the watch is held laterally to the head in the axis 

 of the meatus. Grelle demonstrated the function of the pinna in 

 appreciating the direction of sounds by a very simple experiment. 

 If the pinnae are eliminated by inserting the two ends of a 

 rubber tube 50 cm. long into the auditory meati, so as to close them 

 completely, and a watch is then placed at the centre of the loop 

 after blindfolding the subject's eyes, the same sensations of sound 

 are received in both ears, and they do not alter if the rubber loop 

 is placed above or behind the head. 



Another older experiment of Weber confirms this. If the two 



lobes are bandaged closely to the skull, with closed eyes we are no 



longer able to distinguish the forward or backward direction of a 



sound, because the conditions of the conduction of sound in both 



directions have become approximately equal. 



/ Weber further noticed that if the hands are placed in front of 



' Ahe ears, like horns directed backward, then with the eyes shut 



/the sound of a tuning-fork placed in front of the head seems to 



I come from behind. This is due to the diminution of the tone by 



\ the shield formed of the hands, just as, under normal conditions, 



the shields are represented by the projecting posterior surface of 



the lobe. 



~^- The canal or external auditory meatus which extends from 

 the concha to the tympanic membrane conveys the vibrations of 

 sound to the middle chamber of the ear. It is 23-32 mm. in 

 length. The calibre of the passage is smaUest in the osseous 

 part of the canal, and at a few millimetres from the tympanum it 

 becomes wider again. It has a somewhat tortuous course, so that 

 to look through the meatus at the tympanum in adults the lobe 

 must be pulled a little upward and backwards. The skin that 

 covers it is provided in the outer portion with hairs and sebaceous 

 glands that secrete cerumen. The thick subdermal tissue is rich 

 in convoluted tubular glands similar in structure to the sweat 

 glands, which do not seem concerned in the secretion of cerumen, 

 as is often supposed. 



The external meatus may be regarded as an organ for the 

 protection of the middle and inner ear. Its tortuous course, the 

 sensitive hairs at its entrance, and the cerumen with which its 

 surface is smeared, make it difficult for insects to penetrate into 

 the canal. The wax further serves to keep the tympanic 

 membrane from drying up, and makes it supple. The meatus is 

 also a protection against variations of temperature. 



Lastly, the external auditory meatus, like all hollow spaces, 

 can function as a resonator, as well as a conductor, of sound- 

 waves. According to Helmholtz and Hensen the proper tone of 

 the auditory passage is too high (between c and a of the fourth 

 octave) for it to have any perceptible effect on hearing. 



III. The membrana tympani is attached obliquely at the end 



