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, percentage. The 
breathing is less frequent ; but it is also deeper, and the 
fundamental regulation is practically undisturbed. 
It was shown by Paul Bert that the physiological 
actions of CO, 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 CO, 
present in dry air at the normal sea-level pressure of 
%60 millimetres of mercury has a pressure of 760 X 
5400 = 38 mm., and exercises the same pressure as 10 
per cent of CO, 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 CO, in alveolar air. As already seen the 
