THE RESPIRATORY EXCHANGE 487 



So far the discussion has been confined chiefly to the quality 

 of the expired air. The quantity of the expired air, if its com- 

 position be known, gives the measurement of the respiratory 

 exchange of the body ; a few samples, however, taken over short 

 periods, are not sufficient for exact determinations, for the depth 

 and frequency of respiration are easily altered, especially when a 

 mask is placed on the face. It is unnecessary to describe here 

 the methods used for the determination of the total respiratory 

 exchange in man and animals, but some account must be given 

 of the various factors which influence it. 



Relation between Functional Activity and Respiratory Exchange. 

 The investigation of the relation between functional activity and 

 oxidation in animals is beset with many difficulties and sources 

 of error. The animal, not the cell, is the physiological unit. 

 Surviving organs cannot be regarded as comparable to the organs 

 intact within the living body of a warm-blooded animal. Such 

 an animal on exposure to cold increases its activity and its pro- 

 cesses of oxidation, whereas its excised organs would react in 

 exactly the opposite manner. It is probable that much further 

 knowledge of respiration might be gained by more extensive 

 research in comparative physiology. Natural conditions vary 

 in different animals, one living process may be more marked than 

 another, or may show large variations in its range during different 

 seasons. Thus, hibernation is a physiological condition in which 

 the processes of combustion are reduced to the lowest ebb com- 

 patible with the life of mammals ; in such a condition the relation 

 between functional activity and oxidation can be conveniently 

 studied. It is well known that physiological activity is closely 

 associated with oxidation, but the study of the respiratory quotient, 

 the ratio of the intake of oxygen to the output of carbon dioxide, 

 shows the possibility of oxidation taking place in the body without 

 a coincident production of carbon dioxide, and on the other hand 

 the possibility of the production of carbon dioxide without a 

 coincident absorption of oxygen. 



Respiratory quotients as low as O3 have frequently been 

 observed in torpid animals ('). During some periods of its deep 

 winter sleep the animal may actually gain in weight, for, although 

 it loses water and carbon dioxide, it absorbs a greater weight of 

 oxygen. This oxygen does not reappear immediately combined 

 with carbon as carbon dioxide, but is used, it would seem, for the 

 partial combustion of the reserves of food material stored in the 



