502 HEARING, VOICE AND SPEECH 



Lagrange and Young regarded the difference tones as a kind of beats and 

 explained them on the assumption of subjective interference. If this were 

 shown to be true, it would constitute an absolute refutation of the resonance 

 theory, for these particular tones would then have no objective existence and so 

 could not, as the theory demands, excite resonators in the ear. The summation 

 tones would constitute still greater difficulty for the theory. 



Helmholtz, however, found an explanation for these tones by supposing that 

 either the tympanic membrane or the incus-malleus joint, or both, are not uni- 

 formly elastic, and that the combination really takes place therefore in the con- 

 ductors of the ear. Several authors do not find this explanation wholly satis- 

 factory and, because of this and other difficulties which cannot be entered into 

 here, have given up the resonance theory altogether and adopted other views. 

 When all has been said, however, it is the opinion of the author that the reso- 

 nance theory is better able to explain the essential features of the auditory 

 sensations than any of its rivals. It is, of course, not improbable that this 

 theory will need to be modified or extended in one way or another, as has been 

 done by Wundt and by Hermann, for example, but the ground principle the 

 analysis of sound by resonators in the internal ear will, it is the author's 

 belief, endure. 



SECOND SECTION 



PHYSIOLOGY OF VOICE AND SPEECH 



The physiology of voice and speech covers so wide a field that it will be 

 necessary for us here to limit ourselves to the most important facts. Let it 

 be expressly understood that what follows is to be regarded only as a brief 

 orienting survey. 



The uppermost part of the trachea, the larynx, is fashioned in a peculiar 

 way so as to serve for the production of the voice. The most essential parts 

 of the larynx are the vocal cords. These are thin, elastic bands stretched 

 across the lumen of the larynx, and like other structures of the kind, they 

 can be made to produce distinct tones by being set in vibration. In their 

 case the vibration is caused by a blast of air from the lungs forced through 

 the chink (glottis) between their free edges. The pitch and other qualities 

 of the tones thus produced are altered by varying the tension and mode of 

 vibration of the cords. This function it is the business of the laryngeal 

 muscles to discharge. 



1. ACTION OF THE LARYNGEAL MUSCLES 



The true vocal cords are attached at one end to the recurrent angle of the 

 thyroid cartilage and at the other to the vocal processes of the arytenoid 

 cartilages', consequently their tension and position can only be altered by 

 changing the distance from the thyroid cartilage to the arytenoids and the 

 distance from one arytenoid cartilage to the other. 



The arytenoids are fastened to the cricoid cartilage, so that every move- 

 ment of the latter produces a change in the position of the former ; hence, the 

 distance between the thyroid and the arytenoids can be altered by moving 

 the cricoid. 



The action of the separate muscles may be condensed somewhat as follows : 



