240 THE INTF.RNAL ENVIRONMENT OF Till BODY Part III 



also belonging to the vagus nerve is aflected by the collapse of the lungs and 

 starts stimuli and stimulate a new inspiration. 



Volume of Air in the Lungs 



The amount of air which can be taken into or forced out of the lungs is 

 easily measured by a spirometer. A person at rest, breathing about 16 times 

 per minute, regularly inspires and expires about one pint of air. By great effort, 

 three pints more can be expelled in addition to the pint of the regular inspira- 

 tion. Even after such a forced expiration there is still about a pint left in the 

 lungs. There is. therefore, a reserve supply of over five pints of air with which 

 the fresh pint in the regular inspiration is mixed. It must not be concluded that 

 ail the oxygen of inspired air is extracted with each breath; expired air con- 

 tains about three-fourths of its previous content of oxygen. Air is breathed 

 over and over again. Note the possibilities in the next crowded bus! 



Voice 



Voice is due to the expulsion of air across the vocal cords, folds of the 

 lining of the larynx which contain bands of dense elastic tissue and muscle. 

 Although called vocal cords, they are not cords and do not resemble them. 

 The upper folds are called the false vocal cords and the lower ones the true 

 vocal cords. The larynx, characteristic of higher vertebrates, is located just 

 below the glottis or opening of the trachea (Fig. 13.14). 



Electronic devices have proven that the deep sea is far from noiseless; fishes 

 have no larynx but they make grunts and kindred sounds with their swim 

 bladders; whales and their kin, being all good air-breathers, have an equipment 

 for voice. The really vociferous vertebrates are the birds and mammals. In 



Hydroid 

 cartilage 



Thyroid 



cgrtiloge 

 ('Adorns 



opple") 



^sr^^^^^^p 



igloftis 



Tracheal 



ring of 



cartilage 



Epiglottis 



Opening 

 nto trachea 



B 



Vocal 

 cord 



Fig. 13.14. The human larynx. A, Front view from above, looking into the 

 throat; B, with vocal cords swung away from one another when at rest; C, cords 

 swung near together during speech. Loudness depends on the pressure with which 

 air is exhaled between them. Pitch depends partly upon the tightness of the con- 

 traction of the cords. Quality of voice depends upon many factors. (Redrawn 

 after Brash, ed.: Cunningham's Textbook of Anatomy, ed. 9. New York, Oxford 

 University Press, 1951.) 



