THE PNEUMOGASTRIC. 565 



Protection of the Glottis from the Intrusion of Foreign Substances. 

 The influence of the pneumogastric nerve in the larynx is not confined 

 to its motor action upon the muscles ; it also supplies, by its superior 

 laryngeal branch, a peculiar sensibility to the mucous membrane of these 

 parts, which is essential for the protection of the respiratory passages. 

 In the first place, it stands as a sort of guard, or sentinel, at the entrance 

 of the larynx, to prevent the intrusion of foreign substances. If a crumb 

 of bread fall within the aryteno-epiglottidean folds, or on the edges of 

 the vocal chords, or upon the posterior surface of the epiglottis, the 

 sensibility of the parts excites an expulsive cough, by which the foreign 

 body is dislodged. The impression received and conveyed inward by 

 the sensitive fibres of the superior laryngeal nerve, is reflected upon the 

 expiratory muscles of the chest and abdomen, by which the movements 

 of coughing are accomplished. Touching the above parts with the point 

 of a needle, or pinching them with the blades of a forceps, will produce 

 the same effect. This reaction is dependent on the sensibility of the 

 laryngeal mucous membrane ; and it can no longer be produced after 

 section of the superior laryngeal branch of the pneumogastric nerve. 



Connection with the Formation of the Voice. In addition to its func- 

 tion in the mechanism of respiration, the larynx is also an organ for 

 the production of vocal sounds. The formation of the voice can be 

 studied in the lower animals by exposing the larynx and glottis. in the 

 manner described above, and in man by the use of the laryngoscope ; 

 that is, a small mirror held at a suitable angle at the back of the pharynx 

 in such a way as to reflect a more or less complete view of the laryngeal 

 orifice. The first important fact to be observed in this respect is that 

 the voice is formed always in expiration, never in inspiration. It is 

 the column of outgoing air which is set in vibration to produce a vocal 

 sound, and which continues and modifies its resonance while passing 

 through the pharynx, mouth, and nasal passages. Secondly, it requires 

 an active tension and close approximation of the vocal chords, so that 

 the orifice of the glottis is narrowed to a comparatively minute crevice. 

 So long as the vocal chords preserve their relaxed condition during 

 expiration, nothing is heard except the faint whisper of the air passing 

 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, thus diminishing considerably the diameter of the orifice : 

 and the air, driven by forcible expiration through the glottis, in passing 

 between the vibrating vocal chords, is itself thrown into vibrations which 

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

 vary with the conformation of the larynx, the degree of tension and 

 approximation of the vocal chords, and the force of expiration. The 

 narrower the opening of the glottis, and the greater the tension of the 

 chords, 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 



