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condition of the venous bbod causes this increased activity, whether it 

 is due to deficiency of oxygen or excess of carbonic acid in the blood. 

 This has been answered by the experiments, which show on the one hand 

 that dyspnoea occurs when there is no obstruction to the exit of carbonic 

 acid, as when an animal is placed in an atmosphere of nitrogen, and 

 therefore cannot be due to the accumulation of carbonic acid, and sec- 

 ondly, that if plenty of oxygen be supplied, dyspnoea proper does not. 

 occur, although the carbonic acid of the blood is in excess. The respir- 

 atory centre is evidently stimulated to action by the absence of sufficient 

 oxygen in the blood circulating in it. 



The method by which the vagus is stimulated to conduct afferent im- 

 pulses, influencing the action of the respiratory centre, appears to be by 

 the venous blood circulating in the lungs, or as some say by the condition 

 of the air in the pulmonary alveoli. And if either of these be the stimuli 

 it will be evident that as the condition of venous blood stimulates the 

 peripheral endings of the vagus in the lungs, the vagus action which tends 

 to help oil the discharge of inspiratory impulses from the centre, must 

 tend also to increase the activity of the centre, when the blood in the 

 lungs becomes more and more venous. No doubt the venous condition 

 of the blood will affect all the sensory nerves in a similar manner, but it 

 has been shown that the circulation of too little blood through the 

 centre is quite sufficient by itself for the purpose; as when its blood sup- 

 ply is cut off increased inspiratory actions ensue. 



Effects of Vitiated Air. Ventilation. We have seen that the 

 air expired from the lungs contains a large proportion of carbonic acid 

 and a minute amount of organic putrescible matter. 



Hence it is obvious that if the same air be breathed again and again, 

 the proportion of carbonic acid and organic matter will constantly increase 

 till fatal results are produced; but long before this point is reached, 

 uneasy sensations occur, such as headache, languor, and a sense of oppres- 

 sion. It is a remarkable fact that the organism after a time adapts itself 

 to such a vitiated atmosphere, and that a person soon comes to breathe, 

 without sensible inconvenience, an atmosphere which, when he first 

 entered it, felt intolerable. Such an adaptation, however, can only take 

 place at the expense of a depression of all the vital functions, which must 

 be injurious if long continued or often repeated. 



This power of adaptation is well illustrated by the experiments of 

 Claude Bernard. A sparrow is placed under a bell-glass of such a size 

 that it will live for three hours. If now at the end of the second hour 

 (when it could have survived another hour) it be taken out and a fresh 

 healthy sparrow introduced, the latter will perish instantly. 



The adaptation above spoken of is a gradual and continuous one: thus 

 a bird which will live one hour in a pint of air will live three hours in 

 two pints; and if two birds of the same species, age, and size, be placed 



