REGULATION OF THE RESPIRATORY MOVEMENTS 1131 



cent. A rise of carbon dioxide to 3 per cent, increases the total ventilation 

 of the alveoli by 126 per cent. An amount of carbon dioxide equivalent 

 to 6 per cent, increases the depth of each respiration by 272 per cent., and 

 the total alveolar ventilation by 757 per cent. 



FIG. 518. Effect of C0 2 on respiratory movements of rabbit. (SeoTT.) 



Upper line, tracing of diaphragm slip (Head's method). Lower tracing, carotid 

 pressure. During the first period indicated on the signal line the animal breathed 

 9-6 per cent. C0 2 in air, and during the second period 10 per cent. C0 2 with 33 per 

 cent, oxygen. Time tracing = 2 sees. Scale = mm. Hg. blood pressure. 



If we examine the last column of figures in this Table, representing the 

 percentage of C0 2 in the alveolar air, it will be seen that, in spite of the very 

 large .variations in the air breathed, the alveolar content in C0 2 remained 

 practically constant until the C0 2 in the atmosphere was increased to such 

 an extent that the processes of compensation were no longer efficient. We 

 must conclude therefore that the respiratory centre is so arranged as to 

 react to the slightest increase of C0 2 tension in the blood, any increase in 

 this gas giving at once a compensatory increase in depth and frequency of 

 respiration, so that the alveolar C0 2 content may be maintained almost 

 constant. 



That it is the tension of C0 2 in the alveolar air and therefore in the blood 

 bathing the centres, and not the percentage amount of this gas which is the 

 determining factor, is shown by a comparison of the composition of the 



