534 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



7 P.M., 11.1. Guy gives the mean rate in the morning as 17 and in the 

 evening as 18. 



The rate increases with an increase in muscular activity (p. 413). 



Changes in ^external (surrounding) temperature have very little influence. 

 Vierordt noted'a rate of 12.16 at 8.47 C. and one of 11.57 at 19.4 C., and 

 that an increase of each degree C. decreases the period of each respiration 

 0.054. Alterations of internal temperature are associated with marked 

 changes, as is well illustrated in the increase in the rate observed in fevers, 

 which increase, in turn, is closely related to the rise in the pulse-rate and 

 the bodily temperature. 



Season is not without its influence. In the spring the rate, according to E. 

 Smith, is 32 per cent, greater than at the end of summer. 



Ordinary changes in atmospheric pressure exert no influence, but under con- 

 siderable variations the rate rises and falls inversely with the pressure. 



The frequency of the respirations may be profoundly affected by our emo- 

 tions and by our mil. Mental excitement may increase or decrease the rate, 

 and, as is well known, we may greatly modify not only the rate but the depth 

 of the movements by volitional effort. 



If the composition of the inspired air becomes so altered that O falls below 

 13 volumes per cent., the respirations are increased in frequency and in depth. 

 In the same way, if the blood becomes deficient in O or overcharged with CO 2 , 

 movements of respiration are increased. 



Excitation and depression of the respiratory centres and nerves .through the 

 agency of operations, disease, poisons, etc. effect changes in the respiratory rate. 



The rate and the depth of the respirations bear generally an inverse relation 

 to each other : the greater the rate the less the depth, and vice versa; but the 

 quantity of air respired during a given period does not necessarily bear any 

 direct relation to either the rate or the depth alone, but rather to both. 



A general relationship exists between the frequency of the respirations and 

 the pulse-rate. Comparisons of a large number of observations by different 

 investigators give a ratio at twenty-five to thirty-five years, 1 : 4-4.5 ; at 

 fifteen to twenty years, 1 : 3.5 ; at six weeks, 1 : 2.5. 



D. THE VOLUMES OF Am, O, AND OO 2 RESPIRED. 

 During quiet respiration there occurs an inflow and outflow of air, desig- 

 nated tidal air, equal to about 500 cubic centimeters, or 30 cubic inches. The 

 volume of expired air is a little in excess of inspired air, owing to the expan- 

 sion caused by the increase of temperature, although the actual volume is less 

 (p. 519). The volume of air respired during each respiration bears generally 

 an inverse relation to the respiration-rate, and is affected by the position of the 

 body; thus, if in the lying posture the volume be 1, when sitting it will be 

 1.11, and when standing 1.13 (Hutchinson). Besides the term tidal air, 

 others are used to express definite volumes associated with the capacity 

 of the lungs under certain circumstances. Thus, Hutchinson distinguishes 



