Chap, xvi.] PHYSIOLOGICAL OFFICE OF EESPIEATION. 251 



at the middle period of life. During the whole period of ovula- 

 tion, from the first appearance of the menses to their cessation, 

 the exhalation of carbonic acid is less in woman than in man ; 

 but the equality is sensibly re-established during pregnancy and 

 after the critical age. 



Many other causes vary the exhalation of carbonic acid. Thus, 

 in animals having a fixed temperature or hot blood, it augments 

 in proportion as the exterior temperature decreases. On the 

 contrary, in cold-blooded animals, respiratory combustion 

 diminishes in proportion as the temperature is lowered. This 

 is because the first resist refrigeration by a more active combus- 

 tion, whilst the second, being incapable of producing a sufficient 

 internal warmth, grow gradually colder till the moment when 

 they fall into a state of hibernal sleep or die. 



Every functional activity, whatever it may be, has as corollary, 

 or rather cause, a more active combustion within the organ or 

 functional apparatus. 



After having eaten copiously insects respire more energetically, 

 and they also die more promptly in confined air, or in an irre- 

 spirable gaseous medium. 



It has been observed that there is a proportion between the 

 digestive activity in frogs and the quantity of oxygen contained 

 in the ambient medium. 



According to Yierordt, Valentin, Scharling, and Horn, the 

 exhalation of carbonic acid augments rapidly in man after a 

 repast. Analogous facts have been stated by Spallanzani con- 

 cerning snails when fasting, by Story of various insects, and by 

 Boussingault of turtle-doves. 



In inanition, there is a lesser absorption of oxygen, and like- 

 wise a lesser exhalation of carbonic acid. The animal derives 

 from the atmosphere more oxygen than it restores to it. This 

 is simply because then it nourishes itself at the expense of its 

 own tissues ; it eats itself, and consequently respires as a car- 

 nivorous animal, but one badly nourished. In effect, Regnault 

 and Eeiset have seen that animals fed upon butcher's meat and 



