565 



different days, care must be taken that the inquiries are all made at 

 rest, at the same hour, in the same posture, and at the same period 

 after meals ; and even then the results will be liable to great variation. 

 No comparison can be made of the rates at different periods of the 

 year, nor indeed on any two consecutive days, if there have been any 

 considerable atmospheric changes or material variation of the daily 

 habits. With increase of temperature the rate of pulsation increases, 

 whilst that of respiration declines. The effect of moderate exertion 

 continues for several minutes, and of severe exertion for about half 

 an hour. An unusual rapidity of pulsation in one half of the day 

 will be attended by the opposite state in the other half, if the condi- 

 tions have been changed at those periods, as, for example, an increase 

 from exertion, followed by rest, or a decrease from fasting, followed 

 by food. Extreme differences of 30 pulsations per minute occur in 

 the 24 hours. The chief objection to the use of instruments to 

 register the rate is their liability to miss the record from the diffi- 

 culty of closely applying them to the wrist or chest, and the great 

 variations in the extent of pulsatory and respiratory movement in 

 sleep and wakefulness. 



The usual practice amongst medical men of ascertaining the rate 

 of pulsation, regardless of posture of the body, of meals, and of the 

 hour of the day, is evidently liable to the greatest error. In order to 

 obtain even approximately correct results, the visit should be made 

 at about the same hour daily, and the inquiry pursued always in one 

 posture. 



QUANTITY OF AIR INSPIRED *. 



The same attention to posture is required as that noted under 

 the previous heading. It is more satisfactory to measure the inspired 

 than the expired air, since no correction is required for expansion by 

 heat. Moreover, when the object is to ascertain the vital capacity of 

 the lungs, it is important to remember that the inspiratory force at 

 the end of a deep inspiration is greater than the expiratory force at 

 the end of a deep expiration. It is necessary to cover or close the 

 nose, since otherwise a portion of air is unconsciously inhaled through 

 it. If a tube be inserted into the mouth for the purpose of inspi- 

 ration, the lips must be closely pressed against it, and particularly 

 at the angles of the mouth ; but there are many persons who cannot 

 * See Phil. Trans. 1859. 



