MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 233 



inspiration and the beginning of expiration. Nor, although the 

 chest collapses more gradually than it expands, is there any distinct 

 interval in ordinary breathing between the end of expiration and 

 the beginning of the succeeding inspiration. When, however, the 

 respiration 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 in- 

 fluences 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; 150 respira- 

 tions 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 

 less than a minute, in ordinary individuals, the 

 desire to breathe becomes imperative. 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 physi- 

 ologist. After a period of forced respiration 

 the breath can be held for a much longer time. 

 This is due to the ' washing out ' of the carbon 

 dioxide, the normal stimulus to the respiratory 

 centre (p. 276). After six minutes of forced 

 breathing the interval of voluntary inhibition can be extended be- 

 yond four minutes. A professional diver has remained under water 

 in a tank for about four and three-quarter minutes. When oxygen 

 is inhaled instead of air during the last few breaths of the forced 

 respiration, the interval during which the breath can be held may 

 be much increased (up to nine or ten minutes). In animals the rate 

 of respiration 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. 



a line of small spurs 

 which, after the can- 

 nula C has been 

 pushed without ad- 

 mission of air through 

 an intercostal space 

 into the pleural 

 cavity, stick in the 

 parietal pleura and 

 securely fasten the 

 cannula. Traction 

 being made on the 

 cannula, a ligature is 

 tied at L around the 

 protruding tissue for 

 greater security. S, 

 side-tube by which the 

 cannula is connected 

 with a manometer or 

 tambour. 



