176 THE BIOLOGY OF THE FROG chap. 



and reaches its minimum in the winter, the ratios of oxygen 

 absorption at a temperature of 20° C. being as follows : 

 spring, 134.5 ; summer, 82 ; winter, 54. The output of CO2 

 by the lungs varies in a similar manner (spring, 62 ; summer, 

 42; winter, 16.5). The cutaneous respiration is subject to 

 much less seasonal variation ; the absorption of oxygen is 

 practically constant ; the elimination of CO^ is considerably 

 increased during the breeding period, but for the rest of the 

 year it varies but little. While the amount of oxygen taken 

 in by the lungs during the spring and summer considerably 

 exceeds that absorbed by the skin, the cutaneous absorption 

 of oxygen becomes much greater than the pulmonary in the 

 winter. In winter, therefore, the skin becomes relatively 

 more important in respiration than during the rest of the 

 year. 



Whether the skin functions more efficiently as a respira- 

 tory organ in air or in water the few and contradictory 

 results of Bohr and Krogh do not enable one to determine. 

 Few experiments have been made upon the relation between 

 temperature and the rapidity of respiration, although it is 

 known that respiration takes place much more rapidly when , 

 the temperature is increased. At low temperatures respi- 

 ratory changes are slight. 



Moleschott and Fubini have shown that light has a marked 

 effect upon respiration of the skin, the amount of CO2 pro- 

 duced at a given temperature being much greater in the 

 light than in the dark. This was held to be due in part to 

 a direct action of light upon the skin, because the increase 

 occurs in frogs whose eyes have been removed, although to 

 a less extent than in normal specimens. The more refrangi- 

 ble rays have the greatest effect upon skin respiration, as 

 was shown by measuring the CO2 output in frogs exposed to 

 differently colored lights. The ratios of CO2 production under 



