RESPIRATION. 423 



The same observers have also shown that when an individual is caused 

 to inhale air the percentage of the oxygen of which had been reduced from 

 20 to 13 and therefore to about 8 per cent, in the alveolar air instead of about 

 15 per cent, no particular change in either the frequency or the depth of 

 the inspiratory movements was noticed, but when the percentage of the 

 oxygen was lowered below this amount the inspiratory center became more 

 irritable as shown by an increase in the rate and depth of the inspiratory 

 movement. As a rule the oxygen percentage in the alveolar air must be 

 reduced fully one-half and thereby the percentage and pressure of the oxygen 

 in the arterial blood fully one-third before the respiratory center is stimulated 

 to increased activity. A reason assigned for this result is the presence in 

 the blood of some non-oxidized metabolic product, probably lactic acid, that 

 is acting as the stimulus. All recent experimental work confirms the view 

 that the specific stimulus to the inspiratory center is the normal pressure of 

 the CO 2 in the blood and so responsive is it to this agent that an increase 

 in even 0.2 per cent, in the alveolar air is sufficient to almost double the 

 respiratory ventilation. 



MODIFICATIONS OF THE RESPIRATORY RHYTHM. 



The character of the respiratory movements is materially modified by a 

 change in the quantitative and qualitative composition of the air and 

 blood as well as by changes of a pathologic nature of the respiratory appa- 

 ratus itself. 



Eupnea. So long as the air retains its normal composition and the 

 respiratory mechanism its structural integrity, so long do the respiratory 

 movements exhibit a normal rhythm and frequency. To the condition of 

 easy tranquil breathing the term eupnea is given. In this condition the 

 percentages of oxygen and carbon dioxid in the blood are such as to favor at 

 least the rhythmic discharge of nerve impulses to the respiratory mus- 

 cles, of sufficient energy and frequency for the maintenance of normal 

 respiration. 



Hyperpnea. The normal rate of the respiratory movements is increased 

 by a rise in body-temperature, by active exercise, and by emotional states. 

 Whatever the cause, the increase in rate and probably in depth is termed 

 hyperpnea. 



Febrile states characterized by a rise in the temperature of the blood 

 increase considerably the respiratory activity. This is due in all probability 

 to a warming of the respiratory center, in consequence of which its excitabil- 

 ity is heightened; for surrounding the carotid arteries with warm tubes and 

 heating the blood on its way to the medulla has the same effect. It is also 

 possible, however, that the high temperature of febrile conditions may inter- 

 fere with the absorbing power of hemoglobin, and thus by diminishing the 

 quantity of oxygen absorbed lead to more frequent respirations. To the 

 hyperpnea induced by heat the term thermo-polypnea is frequently given. 



Muscle activity, especially if it is violent and indulged in by those unac- 

 customed to exercise, is generally followed by increased rate and depth of 

 breathing, and not infrequently it is attended with such extreme difficulty 

 that the condition approximates that of dyspnea. This condition is attrib- 



