VOICE 553 



reeds and sounds are thus produced. These sounds may be 

 varied in loudness, pitch, and quality. 



The loudness, or amplitude of vibration, depends upon the 

 size of tlie larynx and of the resonating chambers — the pharynx, 

 naso-pharynx, and mouth — and upon the force of the blast of air 

 acting upon the cords. 



The pitch, or number of vibrations per second, depends upon 

 the length and tension of the vocal cords. The greater length 

 of the vocal cords in the male, as compared with the female, 

 makes the voice deeper. The tension of the cords is varied 

 by the action of the crico-thyreoid muscle. 



The power of varying the pitch of the voice differs greatly 

 in different animals. The average difference between the lowest 

 and the highest note which the ordinary human individual can 

 produce is about two octaves. 



The quality of the voice, upon which the characteristic sound 

 produced by each species of animal is largely due, depends upon 

 the overtones which are made prominent by resonance in the 

 pharynx, nose, and mouth. By varying the shape and size of 

 these cavities, and more especially of the mouth, the quality 

 of sound may be considerably altered. 



Roaring is the peculiar sound made by some horses when 

 exercised. It is due to paralysis of the posterior crico-arytenoid 

 muscle on the left side. This when in action abducts the vocal 

 cord of that side, but when not in action it allows it to be 

 drawn inward. This partially occludes the rima glottidis, and 

 thus not only causes the characteristic sound, but also 

 limits the entrance of air and oxygen to the lungs. The con- 

 dition appears to be due to disease of the recurrent laryngeal 

 nerve. It is more common in stallions than in mares, and 

 most frequent in thoroughbreds. By some it is considered to 

 be hereditary. Tracheotomy with the insertion of a tube into 

 the trachea relieves the evil effects of the condition. 



