1 6 RESPIRATION 



rebreathed air at the end of the experiment; but even this made 

 no difference to the breathing. When, on the other hand, a mixture 

 containing a greatly reduced oxygen percentage, without any 

 addition of CO 2 , was breathed, the breathing was increased sensi- 

 bly, as shown by graphic records, when the oxygen fell to about 

 12 per cent, and was greatly increased by lower percentages. 

 With extremely low percentages, such as 2 per cent, consciousness 

 was lost quite suddenly after about 50 seconds, before there was 

 time to notice any increase in the breathing. 



It was evident from these experiments that when the same air 

 is rebreathed, or an insufficient proportion of fresh air is supplied, 

 the increased breathing produced is due simply to excess of CO 2 , 

 until, at least, the oxygen percentage becomes extremely low. It 

 appeared, therefore, that the variations in ordinary breathing in 

 response to variations in the respiratory exchange must be due 

 to the increased CO 2 produced, and not to the increased consump- 

 tion of oxygen. This conclusion was the same as that of Miescher, 

 and supported his views as to the regulation of respiration. 



When more than about 10 per cent of CO 2 was breathed the 

 effect of the mixture was to produce stupefaction, which was very 

 marked with higher percentages. This effect was already well 

 known in animals, and CO 2 was one of the gases tried as an an- 

 aesthetic by Sir James Simpson before he adopted chloroform. 

 The effect of excess of CO 2 in producing ataxia, stupefaction, and 

 loss of consciousness has become very familiar to me in connection 

 with experiments with mine-rescue apparatus and diving appa- 

 ratus. These effects are readily produced in the presence of a large 

 excess of oxygen, and are therefore quite independent of the 

 effects of want of oxygen. The narcotic effect of a large excess of 

 CO 2 quiets down the respiration, and this effect in animals led 

 many previous observers to overlook almost entirely the ordinary 

 effects of CO 2 in stimulating the breathing. 



During the next few years after our first experiments I was 

 engaged in the investigation of other problems connected with 

 general metabolism, respiration, and blood gases, but in 1903 

 returned to the regulation of breathing in a long series of experi- 

 ments carried out in conjunction with Dr. J. G. Priestley, who 

 was then a student at Oxford. 



It seemed pretty evident that in order to reach clear ideas on 

 the regulation of breathing it was necessary to study very care- 

 fully the composition of the alveolar air which is in contact, 

 through the alveolar epithelium, with the blood passing through 



