CHANGES IN THE AIR DURING RESPIRATION. 



241 



Fig.75. 



In the respiratory motions of the glottis, as in those of the chest, 

 the movement of inspiration is an active one, and that of expira- 

 tion passive. In inspiration, the glottis 

 is opened by contraction of the posterior 

 crico-arytenoid muscles. (Fig. 75.) 

 These muscles originate from the pos- 

 terior surface of the cricoid cartilage, 

 near the median line ; and their fibres, 

 running upward and outward, are in- 

 serted into the external angle of the 

 arytenoid cartilages. By the contrac- 

 tion of these muscles, during the move- 

 ment of inspiration, the arytenoid car- 

 tilages are rotated upon their articula- 

 tions with the cricoid, so that their 

 anterior extremities are carried outward, 

 and the vocal chords stretched and sepa- 

 rate from each other. (Fig. 74.) In this 

 way, the size of the glottis may be in- glottis. ?. Arytenoid cartilages, a. 



-IP f\ ^ ~ ^ (\ ci*r i Cricoid cartilage, ee. Posterior crico- 



creased from O.lo to 0.27 square inch. aryt enoid mu.scies. /. Trachea. 



In expiration, the posterior crico- 

 arytenoid muscles are relaxed, and the elasticity of the vocal chords 

 brings them back to their former position. 



The motions of respiration consist, therefore, of two sets of move- 

 ments : viz., those of the chest and those of the glottis. These move- 

 ments, in the natural condition, correspond with each other both in 

 time and intensity. It is at the same time and by the same nervous 

 influence, that the chest expands to inhale the air, while the glottis 

 opens to admit it ; and in expiration, the muscles of both chest and 

 glottis are relaxed ; while the elasticity of the tissues, by a kind of 

 passive contraction, restores the parts to their original condition. 



CHANGES IN THE AIR DURING KESPIRATION. The atmospheric 

 air, as it is drawn into the cavity of the lungs, is a mixture of oxy- 

 gen and nitrogen, in the proportion of about 21 per cent., by volume, 

 of oxygen, to 79 per cent, of nitrogen. It also contains about one- 

 twentieth per cent, of carbonic acid, a varying quantity of watery 

 vapor, and some traces of ammonia. The last named ingredients, 

 however, are quite insignificant in comparison with the oxygen and 

 nitrogen, which form the principal parts of its mass. 



If collected and examined, after passing through the lungs, the 

 16 



