CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 655 



crico-thyroid muscle ; hence when this nerve is divided on one 

 side the corresponding vocal cord is relaxed and high notes become 

 impossible. It is worthy of notice that this, the chief tensor, and 

 therefore the most important, muscle of the larynx, has a separate 

 and distinct nervous supply. According to some authors the 

 arytenoideus posticus also receives its nervous supply from this 

 nerve ; but this is denied by others. 



The inferior laryngeal or recurrent branch supplies all the other 

 muscles. When this nerve is divided the voice is lost, since the 

 approximation and parallelism of the vocal cords can no longer be 

 effected. When in a living animal both recurrent nerves are 

 divided, the glottis is seen to become immobile and partially 

 dilated, the vocal cords assuming the position in which they are 

 found in the body after death, and which may be considered as the 

 condition of equilibrium between the dilating and constricting 

 muscles. During forcible inspiration the glottis passes from this 

 condition in the direction of more complete dilation; during 

 forcible expiration, the change is one of constriction. When the 

 peripheral portion of one recurrent nerve is stimulated, the vocal 

 cord of the same side is approximated to the middle line ; when 

 both nerves are stimulated, the vocal cords are brought together 

 and the glottis is narrowed. Though the nerve is distributed to 

 both dilating and constricting muscles, the latter overcome the 

 former when the nerve is artificially stimulated. In the complete 

 closure of the glottis, which is so important a part of the act of 

 coughing (p. 383), the group of muscles which we have spoken of 

 as constituting a sphincter is thrown into forcible contractions by 

 the recurrent laryngeal nerve. 



Though fundamentally a voluntary act, the utterance of a given 

 note is not effected by the direct passage of simple volitional im- 

 pulses down to the laryngeal muscles. So complex and coordinate 

 a movement as that of sounding even a simple and natural note, 

 requires a coordinating nervous mechanism in which, as in other 

 complex muscular actions, afferent impulses play an important part. 

 Auditory sensations, if not as important for an accurate manage- 

 ment of the voice as are visual sensations for the movements of the 

 eye, are yet of prime importance. This is recognized when we say 

 that such and such a one whose power over his laryngeal muscles is 

 imperfect, 'has no ear.' 



A person may speak or sing in two kinds of voice. In the 

 one the sounds are full and strong, and the resonance chamber which 

 is supplied by the trachea, bronchi and indeed by the whole chest, 

 is thrown into powerful and palpable vibrations ; hence this voice 

 is spoken of as the chest-voice. The other kind of voice, called the 

 falsetto, is thin and poor, deals chiefly with high notes, and is not 

 accompanied by the same conspicuous vibrations of the chest. 

 Much controversy has taken place as to the exact manner in which 

 these two voices are respectively produced. The prevailing opinion 



