THE CONTROL OF THE RESPIRATION 377 



ing the ordinate which is crossed by the thick continuous line. In 

 this observation the subject withstood oxygen deficiency without alarm- 

 ing symptoms until a percentage of nearly six was reached. These 

 symptoms are giddiness and then fainting along with various psy- 

 chological effects. The respiratory volume, the pulse rate and the 

 systolic and diastolic blood pressures are also shown (see legend of 

 figure for details) and it will be observed that the respiratory vol- 

 ume did not change greatly until the oxygen percentage had fallen 

 to below twelve. Usually, however, the volume is slightly increased 

 even with a slight reduction in the oxygen, the depth rather than the 

 rate of the breathing being responsible for the change. Ellis 57 has more 

 particularly investigated the precise percentage of oxygen at which the 

 respiratory volume becomes just perceptibly increased and found it to be 

 somewhat above eighteen. By the time 8 per cent 2 is reached the 

 majority of men show about a doubling in the amount of air breathed. 

 If they do not respond in this way, they are almost certain to break down 

 by fainting, because the increased respiration is the mechanism by which 

 the deficiency of oxygen is compensated for. There is usually little change 

 in the pulse rate until the oxygen has fallen below fifteen per cent. A 

 total acceleration of 15-40 beats per minute is normal when the 2 is 

 lowered to seven per cent. The systolic blood pressure usually remains 

 unchanged down to a partial pressure corresponding to 14 or even 9 per 

 cent of oxygen. Below these levels the systolic pressure rises 15-20 mm. 

 Hg. above normal. If it rises more than this it is considered unfavora- 

 ble, since it indicates that vasodilatation has not occurred as it ought 

 to as a result of the low oxygen. A sudden fall in systolic pressure 

 precedes fainting. The diastolic pressure remains practically unchanged 

 throughout the test, but it ought to show a slight decline as the systolic 

 pressure rises. The decline shows that vasodilatation is occurring. 



When the anoxemia is continued for a time, the pulse usually returns 

 towards the normal rate and the systolic and diastolic pressures, if 

 they were affected, also tend to return to the normal levels (Lutz and 

 Schneider 58 ). These authors found the same results whether the 2 

 was reduced by lowering of the barometric pressure or by reduction of 

 the percentage of the gas. 



Cyanosis is a normal reaction but it should be delayed in its onset. 

 In some cases a pale and death-like color develops in place of cyanosis. 

 This is an unfavorable sign. 



The actual time taken to reach the limit of endurance in the test de- 

 pends of course on the capacity of the spirometer. This is usually chosen 

 so that a test occupies from 20 to 30 minutes. 



The Symptoms During Lowering of the Barometric Pressure in a Pneu- 



