RESPIRATORY MOVEMENTS 83 



the tidal air, by the most forcible expiration; it is about 

 1500 cc. 



These volumes combined form the vital capacity (4500 cc), 

 i.e. the greatest possible inhaled and exhaled volume of air. 



Residual air is the air which, after the deepest expiration, 

 remains in the lungs and may amount to about 1200 cc. 



Because of the radiation of heat and the evaporation of 

 water from the mucous membranes, the inspired air in its 

 passage to the lungs is warmed to the body temperature and 

 saturated with water vapor. 



Dust which has been brought to the air-passages by inspiration 

 is forced out by the movement of the cilia of the epithelial cells 

 of mucous membrane. 



Respiratory sounds. During breathing the movement of the 

 air produces sounds which may be heard by placing the ear upon 

 the chest-wall. Above the trachea and the bronchi there is heard 

 a blowing noise, like the sound of the German " ch " (bronchial 

 breathing), both during inspiration and expiration. Above the 

 tissue of the lungs we hear a sighing sound (vesicular murmur) 

 which is strong during inspiration, feeble during expiration. 



3. FREQUENCY AND RHYTHM OF RESPIRATORY 

 MOVEMENTS. INNERVATION OF THE MUSCLES 

 OF RESPIRATION 



Adults breathe about 18 times in one minute, children 

 oftener (during the first year, on the average, 44 times). 



Expiration follows immediately upon inspiration. The 

 proportion of the length of the inspiration to that of expira- 

 tion is about as 10 : 12. Between the end of expiration and 

 the beginning of the next inspiration there is, as a rule no 

 pause. 



The motor nerves for the muscles of respiration proceed 

 from the spinal cord at the anterior roots of the cervical and 

 dorsal region. They are the phrenic nerves for the dia- 

 phragm and the intercostal nerves for the intercostal muscles. 



The respiratory centre lies in the medulla oblongata, on 

 both sides of the posterior point of the fovea rhomboid. 



