206 A MANUAL OF PHYSIOLOGY 



breathing between the end of expiration and the beginning 

 of the succeeding inspiration. When, however, the respira- 

 tion is unusually slow, an actual pause (expiratory pause) may 

 occur at this point. Expiration takes somewhat longer 

 time than inspiration, the ratio varying from 7:6 to 3:2, 

 according to age, sex, and other circumstances. 



The frequency of respiration is by no means constant even 

 in health. All kinds of influences affect it. It is difficult 

 even to direct the attention to the respiratory act without 

 bringing about a modification in its rhythm. In the adult 

 15 to 20 respirations per minute may be taken as about the 

 normal. In young children the frequency may be twice as 

 great (new-born child, 50 to 70 ; child from I to 5 years old, 

 20 to 30 per minute). It is greater in a female than in a 

 male of the same age. A rise of temperature increases it, 

 and this is probably one of the causes of the increased rate 

 of respiration in fever; 150 respirations per minute have 

 been seen in a dog with a high temperature. Sudden 

 cooling of the skin, exercise, and various emotional states, 

 increase the rate, and sleep diminishes it. The will can 

 alter the frequency and depth of respiration for a time, and 

 even stop it altogether, but in about a minute, in ordinary 

 individuals, the- desire to breathe becomes imperative, nor 

 can any training extend this interval of voluntary inhibition 

 beyond three minutes. Cato's assertion that he could kill 

 himself at any time ' merely by holding his breath ' is only 

 a proof that he was a better philosopher than physiologist. 

 In animals the rate can be greatly affected by drugs and by 

 the section and stimulation of certain nerves ; but to this 

 we shall return when we come to consider the nervous 

 mechanism of respiration. 



It cannot fail to be observed that to a great extent the 

 rate of respiration is affected by the same circumstances as 

 the frequency of the heart (p. 95), and in the same direc- 

 tion. And, indeed, in health, these two physiological 

 quantities, amid all their absolute variations, maintain to 

 each other a fairly constant ratio (i to 4 or I to 5 in man). 

 Even in many diseases this proportion remains tolerably 

 stable, although in others it is disturbed. 



