230 A MANUAL OF PHYSIOLOGY 



monary branches of the vagus. That the action is direct is 

 shown by the fact that after section of the vagi, the sympathetic, 

 and the spinal cord below the origin of the phrenics, an increase 

 in the respiratory movements is still produced by tetanizing a 

 limb. 



It is generally acknowledged that the respiratory centre may 

 be excited both by blood that is rich in carbon dioxide and by 

 blood that is poor in oxygen, the actual stimulating substance 

 in the latter case being, perhaps, an easily oxidizable body 

 possibly lactic acid which rapidly disappears from properly 

 oxygenated blood. 



But it has been the subject of long - continued discussion 

 whether excess of carbon dioxide or deficiency of oxygen is the 

 more potent stimulus. The best evidence points to the conclusion 

 that comparatively small alterations in the amount of carbon 

 dioxide in the inspired air cause a relatively great increase in 

 the respiration, while in the case of the oxygen the departure 

 from the normal proportion must be much more decided to 

 bring about any notable effect. Nor is it at all out of harmony 

 with this that, when very large quantities of carbon dioxide 

 (30 per cent, and upwards in rabbits) are inhaled, a condition 

 of narcosis comes on without any previous respiratory distress. 

 For many substances act differently in large and in small doses. 

 Haldane has pointed out how exquisitely sensitive the respiratory 

 centre is to even small changes in the partial pressure of carbon 

 dioxide in the alveolar air, and therefore in the blood and the 

 centre itself, and has demonstrated that this is the way in which 

 the amount of the pulmonary ventilation (the volume of air 

 breathed per unit of time) is chiefly regulated in ordinary 

 breathing. 



For instance, an increase of as little as o - 2 per cent, of carbon 

 dioxide in the alveolar air .corresponding to an increase of 1-4 mm. 

 of mercury in the partial pressure (p. 248) of the gas, caused an 

 increase in the pulmonary ventilation of 100 per cent. The 

 alveolar oxygen pressure had to be diminished to 13 per cent. 

 of an atmosphere before any decided increase in the respiration 

 occurred. During moderate muscular work the percentage of 

 carbon dioxide in the alveolar air, and therefore in the blood, 

 increases slightly, causing an increase in the ventilation, and this 

 is one of the ways in which the hyperpncea associated with 

 muscular exercise is brought about. In severe work lack of 

 oxygen, with accumulation of lactic acid and other metabolic 

 products, which stimulate the respiratory centre or render it 

 excitable by smaller pressures of carbon dioxide, also plays a 

 part. 



To sum up, the regulation of normal breathing is twofold a 



