REGULATION OF BREATHING 13 



depth, and vice versa. It is an entire mistake to judge 

 of the amount of air breathed by the mere frequency 

 of the breathing. With very rapid and shallow 

 breathing only a little of the pure inspired air clears 

 the air-passages and enters the lungs. The very rapid 

 and shallow breathing of a dog in hot weather does 

 not over- ventilate its lungs, and is only designed to 

 promote evaporation from its tongue, and consequent 

 cooling, since a dog sweats with its tongue, and not 

 with its skin. 



If the breathing is obstructed, so that considerable 

 effort is needed to draw in and expel air, as in breath- 

 ing through a partially closed tap, there is still no 

 appreciable rise in the alveolar CO z percentage. The 

 breathing is less frequent ; biH it is also deeper, and the 

 fundamental regulation is practically undisturbed. 



It was shown by Paul Bert that the physiological 

 actions of C0 2 and various other gases depend upon 

 the pressure which they exercise. This pressure de- 

 pends on the number of molecules of the gas present 

 in a given volume. For instance, 5 per cent of C0 2 

 present in dry air at the normal sea-level pressure of 

 760 millimetres of mercury has a pressure of 760 X 

 %oo = 38 mm., and exercises the same pressure as 10 

 per cent of C0 2 in air at 380 mm. barometric pressure. 

 It also contains the same number of molecules in a 

 cubic centimetre. The air in the lung alveoli is satu- 

 rated with aqueous vapour at the body temperature, 

 and this vapour has a pressure of 47 mm., which must 

 be allowed for in calculating from an analysis the 

 pressure of C0 2 in alveolar air. As already seen the 



