THE CONTROL OF THE RESPIRATION 371 



that it cannot be the HC0 3 -anion as some have supposed, that is the toxic 

 agent. The order of resistance of various protozoa to CO 2 was found to 

 bear no relation to that which they bear to other acids. It may be, however, 

 that the more toxic effect of the C0 2 is dependent upon the greater rate 

 at which it penetrates cell membranes than other acids. It would there- 

 fore enter the cells of the respiratory center and by dissociation cause 

 alteration of C H in the protoplasm. In the light of this possibility it is of 

 interest that whereas other acids cause cessation of the movements of flag- 

 ella in protozoa with no visible changes in the interior of the cells, C0 2 

 has little effect on the flagella but causes marked alterations in the intra- 

 cellular activities. Jacobs has also shown that an acid reaction may 

 develop within the petals of certain flowers when these are suspended 

 in a bicarbonate solution that is itself faintly alkaline. He has also 

 succeeded in arranging a laboratory experiment in which a similar 

 migration of undissociated carbonic acid through a layer of xylol caused 

 a neutral solution in contact with it to become acid. 78 



Relationship Between Alveolar CO, and Respiratory Activity. Vari- 

 ations in the respiratory hormone, whatever this may be, are associated 

 with changes in the C0 2 content of the alveolar air. Increase in the 

 alveolar C0 2 immediately stimulates respiration unless under certain 

 conditions which will be discussed later. Indeed the respiratory center 

 is so very sensitive towards this stimulus that whenever the percentage 

 of C0 2 in the inspired air tends to rise, pulmonary ventilation is excited to 

 a degree which is just sufficient to maintain the tension of* C0 2 in the 

 alveolar air at the normal level. There is therefore, no better method for 

 testing the excitability of the center than to measure the increase of 

 pulmonary ventilation produced by adding known percentages of CO., 

 to the inspired air. If the amount of C0 2 in the inspired air is sufficient 

 to raise the C0 2 in the alveoli, in spite of the greater breathing, hyperp- 

 nea becomes very marked ; thus, it has been found that if enough C0 2 

 is inspired to cause an increase of from 0.2-0.3 per cent in alveolar C0 2 

 in man the alveolar ventilation is doubled, or, more precisely stated, an 

 increase of ten liters in the air entering and leaving the alveoli per 

 minute results from raising the alveolar C0 2 tension by 2.2 to 3.1 mm. 

 Hg (Douglas 24 ). 



The relationship between breathing and alveolar C0 2 is by no means al- 

 ways so simple as in the instances just described. In these hyperpnea is sec- 

 ondary to an increase in alveolar C0 2 but there are many cases where the 

 reverse relationship obtains namely, where decreased alveolar C0 2 is sec- 

 ondary to hyperpnea caused by stimulation of the respiratory center by 

 some other agency than increase in C0 2 tension of the blood. These agen- 



*The tension is found by the equation given on page 374. 



