222 PHYSIOLOGY FOR DENTAL STUDENTS. 



asthma is caused by the constriction of the bronchioles by spasm 

 of the bronchial muscles. Atropin, a drug which pnraly/es cer- 

 tain nerves is of therapeutic use in this disease, since it paraly/es 

 the nerve endings in the bronchial muscles.. Adrenalin is also 

 sometimes of use. 



The Effect of Changes in the Respired Air on the Respiration. 

 A very slight increase in the percentage of carbon dioxide in the 

 alveolar air is accompanied by a very marked quickening of re- 

 spiration. On the other hand, the carbon dioxide content of the 

 atmosphere may be increased to about one per cent without em- 

 barrassing the respiratory function, except during muscular 

 work, and it is only at concentrations of carbon dioxide of three 

 or four per cent of an atmosphere that the respiratory function 

 is seriously excited. The reason for this is that the inspired 

 air becomes greatly diluted before it reaches the alveoli, so that 

 a slight increase up to one per cent of carbon dioxide in the 

 atmosphere only quickens and deepens the respiration sufficient- 

 ly to maintain the pressure of carbon dioxide at its normal level 

 in the alveoli. 



An increase in the oxygen pressure has no such effect. In fact 

 pure oxygen has scarcely any influence on the rate of breathing 

 in the normal man. In persons suffering with heart failure or 

 diseases in which the respiratory function of the lungs is im- 

 paired, however, the presence of a high concentration of oxygen 

 in the alveoli may make it possible for the oxygen-starved blood 

 to obtain enough of this gas to saturate it and thus improve the 

 general condition. The reason for these effects of oxygen is that 

 under normal conditions the pressure of oxygen in the atmos- 

 phere is more than sufficient to saturate the haemoglobin of the 

 blood, so that an increase in the oxygen pressure will add only a 

 small amount more of oxygen to that dissolved in the plasma al- 

 ready. On the other hand, the oxygen pressure in the atmos- 

 phere may be reduced to less than half that found at sea level 

 without destroying life. This brings up the interesting question 

 of mountain sickness. 



MOUNTAIN SICKNESS. At an altitude of 5,000 metres (about 

 16,000 feet) the air is reduced to a little over half an atmos- 



