



122 RESPIRATION 



different animals, these qualities would be much more emphatically 

 noticed than in that of the figure made from Anolis. 



The respiratory movements, except so far as stopping them for a long 

 time is concerned, are very perfectly under the control of the animal's- 

 will. This we shall see more fully in studying speech. Thus, stetho- 

 grams have many more arbitrary interruptions in their course than 

 have, for example, sphygmograms, or tracings made from the pulse 

 of the heart, which is only rarely under voluntary control. Men by 

 practice can learn to stop their breathing for five minutes or so, as do 

 the pearl- and sponge-divers of the South Sea islands. No man can 

 commit suicide by this means, however, unless it be those rare indi- 

 viduals who have some voluntary control of their hearts and respiration 

 together. In this case death is caused by stopping for too long a period 

 the heart rather than the respiratory mechanism. 



The relation between these two rhythms, the cardiac and the respira- 

 tory, is close and tends to keep up the ratio of 4 to 1, whatever, within 

 limits, be the condition of either rhythm. As the breath-rate goes 

 high, as, for example, in pneumonia, the pulse-rate oftentimes fails to 

 keep up this its normal ratio. 



The Breath-rate. Few things in human function are more normally 

 variable within normal limits than is the number of respirations per 

 minute. The reason for this is that respiration is more closely related 

 to other functional conditions and more sensitive to mental influence 

 than almost any other of the basal functions. This may in turn be 

 due to the wide and, indeed, almost universal connections of the vagus y 

 the nerve which has so much to do with respiration. 



The breath-rate varies, for example, according to sex, age, season y 

 time of day, muscular and mental activity, temperature of the air, body- 

 temperature, recency of digestion, volition, atmospheric pressure,, 

 emotion, composition of inspired air, depth of breathing, pulse-rate, 

 sleep, and posture. These sixteen or seventeen conditions, at least, 

 besides the protoplasmic structure of the respiratory central neurones 

 and the respiratory state of the blood, determine the number of breaths 

 per minute in man. 



We need hardly do more than to point out the direction in which each 

 of these influences acts. Because the nervous system of the female is 

 more unstable than that of the male, women appear to breathe often- 

 times much more rapidly than do men. The average excess is really 

 small, and in childhood nearly nil; indeed, Milne Edwards supposed 

 that young men breathe somewhat more rapidly than young women. 

 On the average, it may be said that women breathe twice or thrice a 

 minute more than do men. The variation according to age is large, and 

 of much practical importance in medicine. From three hundred count- 

 ings, Quetelet derived the following numbers: 



