238 



FUNCTIONS OF NUTRITION. 



cavity. Thus it will need from eight to ten respirations entirely to 

 renovate the air in the lungs. 



Respiratory Movements of the Glottis. Beside the movements of 

 respiration belonging to the chest, there are similar changes of expan- 

 sion and collapse in the larynx. If the respiratory passages be exam- 

 ined after death, the opening of the glottis will be found smaller in 

 calibre than the cavity of the trachea. The air-passage at the level 

 of the glottis is a narrow chink; but it widens considerably in the 

 lower part of the larynx, while the trachea is a spacious cylindrical tube. 

 In man the space between the vocal chords has an area, on the average, 

 of only one square centimetre ; while the calibre of the trachea in the 

 middle of its length is 2.81 square centimetres. But this disproportion 

 does not exist during life. In respiration there is a regular movement 

 of the vocal chords, synchronous with that of the chest, by which the 

 size of the glottis is alternately enlarged and diminished. At inspira- 



FIG. 55. 



FIG. 56. 



HUMAN LARYNX, viewed from above in its 

 ordinary post-mortem condition. a. Vocal 

 chords, b. Thyroid cartilage, c, c. Aryte- 

 noid cartilages, o. Opening of the glottis. 



The same, with the glottis opened by separa- 

 tion of the vocal chords. a. Vocal chords. 

 b. Thyroid cartilage, c, c. Arytenoid carti- 

 lages, o. Opening of the glottis. 



tion the glottis opens, admitting the air freely into the trachea; at 

 expiration it collapses, as the air is expelled from below. These move- 

 ments are the "respiratory movements of the glottis." They corre- 

 spond in every respect with those of the chest, and are excited or 

 retarded by similar causes. When the general movements of respira- 

 tion are hurried, those of the glottis are also accelerated ; and when 

 the movements of the chest are slower or fainter than usual, those of 

 the glottis are diminished in the same proportion. 



In the glottis, as in the chest, the movement of inspiration is an 

 active one, and that of expiration passive. In inspiration, the glottis 

 is opened by contraction of the posterior crico-arytenoid muscles; 

 which originate from the posterior surface of the cricoid cartilage, and, 

 running thence upward and outward, are inserted into the external 

 angles of the arytenoid cartilages. By these muscles, the arytenoid 

 cartilages are rotated upon their articulations, so that the vocal chords, 



