250 



ESSENTIALS OF PHYSIOLOGY. 



produced by the passage of air through the larger respiratory passages, 

 the sound being modified in its conduction through the substance of the 

 lung. On listening over the trachea and large bronchi, the sound is 

 louder and is audible both during inspiration and expiration. The 

 character of the sound is altered in disease of the lungs or pleura. 



SECTION II. 

 THE CHEMISTRY OF RESPIRATION. 



The composition of expired air differs considerably from that of the 

 atmosphere, the average difference being as follows : 



The increased percentage of carbonic acid in expired, as compared 

 with inspired, air is 4 '06, the difference between the percentage of 

 oxygen in inspired and expired air being 4-46, so that the total volume 

 of the air expired is less than that inspired. It is for this reason that 

 the relative amount of nitrogen in expired air is slightly increased. 

 The ratio of the amount of carbonic acid leaving the body to the amount 

 of oxygen taken into the body and not reappearing in expired air, is 



known as the respiratory quotient, and is usually expressed as 



CO 



_ ? 



-' 



In the table it is = 0-90. Its significance will be discussed 

 4'4o 



subsequently (p. 343). 



In addition to containing less oxygen and more carbonic acid, the 

 expired air is fully saturated with water, and is at the body temperature. 

 It has been stated that small quantities of poisonous substances are also 

 present in expired air, and that the accumulation of these substances in 

 crowded and ill-ventilated rooms is the cause of the headache and other 

 uncomfortable symptoms experienced in these circumstances. Careful 

 experiments have shown that this is not the case, and that the 

 symptoms are due partly to the accumulation of carbonic acid in the 

 air, and make their appearance when this reaches O'l per cent, or more, 

 and partly, as Leonard Hill suggests, to a high temperature and lack of 



