486 THE NERVOUS SYSTEM. 



the breathing becomes slower and slower, until it is at last so infre- 

 quent that it can no longer sustain life. 



Furthermore, the physical change in the pulmonary tissues is super- 

 added to their functional derangement. This alteration has no inflam- 

 matory character, but consists in a diminution of the air in the vesicles 

 of the lungs, and a passive accumulation of blood in the capillaries. 

 It combines with the causes already described, to interfere with the 

 aeration of the blood and to hasten the failure of the vital powers. 



Protection of the Glottis from Foreign Substances. The superior 

 laryngeal branch of the pneumogastric supplies to the mucous mem- 

 brane of the larynx a peculiar sensibility which is essential for the pro- 

 tection of the respiratory passages. It stands as a sort of sentinel, at 

 the entrance of the glottis, to prevent the intrusion of foreign sub- 

 stances. If a crumb of bread fall within the aryteno-epiglottidean 

 folds, or on the edges of the vocal chords, the sensibility of the parts 

 excites an expulsive cough, by which the foreign body is dislodged. 

 The impression conveyed inward by the superior laryngeal nerve is 

 reflected upon the expiratory muscles of the chest and abdomen, by 

 which the movement of coughing is accomplished. 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 

 nerve. 



Connection with the Voice. In addition to its function in respiration, 

 the larynx is an organ for the production of vocal sounds. The forma- 

 tion of the voice can be studied in animals after exposing the glottis 

 by the operation of pharyngotomy ; and in man by the use of the 

 laryngoscope. The first important fact demonstrated in this way is 

 that the voice is formed always in expiration^ never in inspiration. The 

 column of outgoing air is set in vibration by the glottis, and its res- 

 onance modified in the pharynx, mouth, and nasal passages. Secondly, 

 it requires tension and approximation of the vocal chords, by which the 

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

 When the vocal chords are relaxed during expiration, nothing can be 

 heard except a faint whisper of the air passing through the larynx. 

 In the production of a vocal sound the chords are made tense and 

 closely applied to each other ; and the air, driven by forcible expiration 

 through the narrowed chink of the glottis, between the vibrating vocal 

 chords, is itself thrown into sonorous vibration. The tone, pitch, and 

 intensity of the sound vary with the conformation of the larynx, the 

 tension and approximation of the vocal chords, and the force of expira- 

 tion. The narrower the opening and the greater the tension of the 

 chords, the more acute the sound ; while a wider opening and a lower 

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

 fied by the length of the column of air between the glottis and the 

 mouth, the tense or relaxed condition of the pharynx and fauces, and 

 the dryness or moisture of the mucous membrane. 



The production of a vocal sound takes place, therefore, in the larynx ; 



