RESPIRATION 219 



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 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 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 

 physiologist. 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 respira- 

 tory centre (p. 231). After six minutes of forced breathing the 

 interval of voluntary inhibition can be extended beyond 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. 



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

 of respiration is affected by the same circumstances as the fre- 

 quency of the heart (p. 98), and in the same direction. And, 

 indeed, in health, these two physiological quantities, amid all 

 their absolute variations, maintain to each other a fairly con- 

 stant 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. 



The total quantity of air expired, or, what comes to the 

 same thing, the alteration in the capacity of the chest during 

 expiration, can be measured by means of a gas-meter or of a 

 spirometer (Fig. 101), which consists of an inverted graduated glass 

 cylinder dipping by its open mouth into water and balanced by 

 weights. The vessel is sunk till it is full of water, the air being 

 allowed to escape by a cock. The expired air is now permitted 

 to enter it through a tube, and displaces some of the water. 

 The spirometer is adjusted so that the level of the water inside 

 and outside is the same, and then the volume of air contained 



