PNEUMOGASTRIC NERVE. 465 



except the ordinary faint whisper of the air passing gently through 

 the cavity of the larynx. When a vocal sound, however, is to be 

 produced, the chords are suddenly made tense and applied closely 

 to each other, so as to diminish very considerably the size of the 

 orifice; and the air, driven by an unusually forcible expiration 

 through the narrow opening of the glottis, in passing between the 

 vibrating vocal chords, is itself thrown into vibrations which pro- 

 duce the sound required. The tone, pitch, and intensity of this 

 sound, vary with the conformation of the larynx, the degree of ten- 

 sion and approximation of the vocal chords, and the force of the 

 expiratory effort. The narrower the opening of the glottis, and the 

 greater the tension of the chords, under ordinary circumstances, the 

 more acute the sound ; while a wider opening and a less degree of 

 tension produce a graver note. The quality of the sound is also 

 modified by the length of the column of air included between the 

 glottis and the mouth, the tense or relaxed condition of the walls 

 of the pharynx and fauces, and the state of dryness or moisture of 

 the mucous membrane lining the aerial passages. 



Articulation, on the other hand, or the division of the vocal sound 

 into vowels and consonants, is accomplished entirely by the lips, 

 tongue, teeth, and fauces. These organs, however, are under the 

 control of other nerves, and the mechanism of their action need not 

 occupy us here. 



Since the production of a vocal sound, therefore, depends upon 

 the tension and position of the vocal chords, as determined by the 

 action of the laryngeal muscles, it is not surprising that division of 

 the inferior laryngeal nerves, by paralyzing these muscles, should 

 produce a loss of voice. It has been sometimes found that in very 

 young animals the crico-thyroid muscles, which are the only ones 

 not affected by division of the inferior laryngeal nerves, are still 

 sufficient to give some degree of tension to the vocal chords, and 

 to produce in this way an imperfect sound ; but usually the voice 

 is entirely lost after such an operation. 



It is a very remarkable fact, however, in this connection, that all 

 the motor filaments of the pneumogastric, which are concerned in 

 the formation of the voice, are derived from a single source. It 

 will be remembered that the pneumogastric, itself originally a 

 sensitive nerve, receives motor filaments, on leaving the cranial 

 cavity, from no less than five different nerves. Of these filaments, 

 however, those coming from the spinal accessory are the only ones 

 necessary to the production of vocal sounds. For it has been found 

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