276 RESPIRATION 



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 0-2 per cent, of carbon 

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

 of mercury in the partial pressure (p. 246) 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 atmo- 

 sphere 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 hyperpnoea 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 chemical 

 regulation (through the carbon dioxide] of the amount of air moved into 

 and out of the lungs per unit of time ; and a nervous regulation (chiefly 

 through the vagi) of the rate and depth of the movements necessary to 

 effect the given amount of ventilation. 



When the vagi have been divided, an increase in the carbon 

 dioxide pressure within certain limits is responded to by an increase 

 in the total ventilation, just as in the normal animal, but the form 

 of the response is different. Whereas in the normal animal both 

 the rate and the depth of respiration are increased, in the vagoto- 

 mized animal there is a marked increase in depth, with little or no 

 increase in rate (Scott). 



When the gaseous exchange in the lungs from any cause becomes 

 insufficient, the respiratory movements are exaggerated, and ulti- 

 mately every muscle which can directly or indirectly act upon the 

 chest-wall is called into play in the struggle to pass more air into 

 and out of the lungs. To a lesser and greater degree of this exag- 

 geration of breathing the terms Hyperpnaa and Dyspnceah&ve been 

 respectively applied. If the gaseous interchange remains insuffi- 

 cient, or is altogether prevented, asphyxia sets in. Sometimes in 

 man impending asphyxia from loss of function by a part of the lungs 

 (with crippling of the lesser circulation), as in pneumonia, may be 

 warded off by inhalations of oxygen. Increase in the temperature 



