174 THE BIOLOGY OF THE FROG chap. 



practically the only respiratory organ. \ Frogs may be kept 

 alive when submerged in water at o° to 13° C. for several days. 

 At a higher temperature frogs tend to come to the surface 

 oftener for air, and if prevented from doing so, they may 

 die of asphyxiation. The skin functions as a respiratory 

 organ both in water and in air. If the nostrils of a frog be 

 plugged with wax, the animal may be kept alive in cool air 

 for several days. 



The experiments of several investigators have shown that 

 more carbon dioxide is given off through the skin than through 

 the lungs. Klug found that the ratio of CO2 given off by the 

 lungs to that given off through the skin varied in the different 

 specimens investigated from 1:2.5 to 1:4.46. The frogs 

 which Klug experimented upon were put in a chamber divided 

 by a partition which contained an aperture surrounded by rub- 

 ber. The frog was placed so that its head projected through 

 the partition, and was held tightly by the rubber so that one 

 chamber was completely shut off from the other. Air was 

 passed through both chambers, and the amount of carbon 

 dioxide given off into each measured and compared. The 

 one chamber received the output from the skin only, while 

 the other received that of the lungs together with the small 

 amount exhaled from the skin of the head. The method of 

 Klug was an improvement over those of his predecessors, 

 although not entirely free from objections, the principal one 

 being that the pressure of the rubber necessary to produce 

 an air-tight fit would impede the normal movements of res- 

 piration. Experiments of ligating or extirpating the lungs, 

 removing the skin, tying the cutaneous blood vessels, plung- 

 ing the frog in oil nearly up to its nostrils, etc., in order to 

 eliminate one or the other modes of respiration are all open to 

 the same criticism that they do not tell us anything of the 

 relation of skin and pulmonary respiration under ordinary 



