RESPIRATION. 



537 



The quantities of O and of CO 2 exchanged, although in a general way 

 closely related, are in a measure independent of each other, but, as a rule, an 

 increase or a decrease in one is accompanied by a rise or a fall in the other. 

 The most important conditions affecting the quantities of O absorbed and CO 2 

 given off are species, body-weight and body-surface, age, sex, constitution, rate 

 and depth of the respirations, the period of the day, digestion, food, internal 

 and external temperature, activity, atmospheric pressure, the composition of 

 the inspired air, and the condition of the nervous system. 



Most of the studies have been made solely by determinations of the quan- 

 tities of CO 2 given off, the results being taken as standards for the relative 

 volumes of O absorbed ; but such deductions are of very uncertain value and 

 may be entirely misleading. (See Respiratory Quotient, p. 544.) 



Respiratory activity in different species in proportion to body-weight is less 

 in cold-blooded than in warm-blooded animals, the difference being due chiefly 

 to the larger supply of O demanded by the more active heat-producing pro- 

 cesses in the latter, and in part to the more active character generally of the 

 bodily operations. If we take as a standard for cold-blooded animals the 

 respiratory activity in the frog (which is 0.07 gram of O per kilogram of body- 

 weight per hour), and compare this with the standards for warm-blooded ani- 

 mals, in the latter it will be from 6 to 18 times greater, according to the species. 

 Respiratory activity is higher in proportion to body-weight in birds than in 

 mammals. The following tabular statement of the intensity of the respiratory 

 interchange per kilogram of body-weight per hour, compiled chiefly from the 

 researches of Regnault and Reiset, Zuntz and Lehman n, Bossignault, Herzog, 

 and Grouven, illustrates these differences : 



As a rule, the smaller the species the greater (relatively, but not absolutely) 

 is the intensity of respiratory activity ; for instance, -the consumption of O 

 for each kilogram of body-weight is for the horse, 0.437 ; ass, 0.566 ; sheep, 

 0.499; rabbit, 0.92; and for birds, as high as 12.58. For different species 

 of the same class the same variations are observed ; thus, Richet records, as 

 the result of investigations on birds, the following figures as the number of 



