FREQ UENC Y OF RESPIRA TION IN MAN. 



747 



lungs, and other factors which bear upon the composition of the air in 

 the lungs. 



The frequency of respiration in man. Under normal conditions 

 this could be readily and exactly determined, were it not liable to 

 variations as soon as the attention of the subject is directed to the 

 breathing. Apart from this, the most important causes of variations 

 in the frequency of respiration are age, exercise, and temperature. 



Age. The frequency of breathing decreases from birth to old age, 

 as shown by the following table, the result of three hundred observations 

 made by Quetelet l upon human subjects of the male sex. 



In healthy infants the respiration is very irregular in frequency, 

 and often of the Cheyne- Stokes type. 2 



The average frequency of respiration in 1897 adult males was found 

 by Hutchinson 3 to be 20 per minute, one-third of the cases breathed 

 at that rate, and 1731 between 16-24 per minute. 



Exercise increases not only the frequency but also the depth of 

 breathing. This hyperpnoea is not due to a deficiency of oxygen or an 

 accumulation of carbon dioxide in the blood, but probably to some pro- 

 duct which is derived from the metabolism in the muscles, and stimulates 

 the respiratory centre. 4 



The physiological explanation of the condition, well known to athletes as 

 " second wind," appears to be unknown ; during violent exercise, such, as 

 running or rowing, there is, after a time, considerable dyspnoea, but if the 

 exercise be continued this discomfort disappears, sometimes quite suddenly; 

 the man has now got his " second wind," and can continue the exertion in com- 

 parative comfort. The dyspnoea in these cases appears to be partly cardiac, 

 for the pulse-rate may be more than doubled, but when the " second wind " is 

 obtained, there appears to be a marked decrease in the frequency of the heart's 

 contraction. 5 The causes of this accommodation are unknown. 



1 " Sur 1'homme et le deVeloppement de ses faculte"s," Paris, 1835. 



2 See Preyer, "Specielle Physiologic des Embryo," Leipzig, 1885, S. 179; Eckerlein, 

 Ztachr.f. Geburtsh. u. GynciL, Stuttgart, 1890, Bd. xix. S. 120. 



3 Med.-Chir. Trans., London, vol. xxix. p. 137 ; art. "Thorax," Todd's " Cyclopaedia of 

 Anat. and Physiol.," vol. iv. p. 1085. 



4 Geppert and Zuntz, Arch. f. d. ges. Physiol., Bonn, 1888, Bd. xlii. S. 189. 



5 Result of a few observations by Pembrey and Reynolds. 



