54 RESPIRATORY MECHANISMS 



These conditions are almost always fulfilled, although in 

 some heavily chitinized insects the permeability may be 

 extremely low. 



There is reason to believe that in many pulmonate gastro- 

 pods a considerable proportion of the oxygen is normally 

 absorbed through the skin, because the lung can remain 

 closed for long periods, while some of the aquatic (Limnaa and 

 Helicosoma) can stay submerged for an indefinite time at low 

 temperature (Cheatum, 1934). 



The respiration through the general surface is of consider- 

 able importance also in the Amphibia, and the circulation is 

 modified so as to distribute highly venous blood both to the 

 lungs and to the skin. 



A comparative study of the cutaneous and pulmonary 

 respiration in the frog was made by Krogh (1904) and by 

 Dolk and Postma (1927). The functional difference between 

 the two organs comes out very clearly. By large variations 

 in total exchange (70-170 ml/kg/hour) the oxygen intake 

 through the skin, which is limited mainly by the conditions 

 for diffusion, remains practically constant at about 50, while 

 the variations are brought about by the ventilation of the 

 lungs and the blood-flow through them. C0 2 , which diffuses 

 so much more rapidly, is eliminated at all metabolic levels 

 mainly through the skin where the blood is exposed to an 

 atmosphere practically C0 2 free. The elimination through 

 the lungs is limited by the alveolar C0 2 tension and is there- 

 fore closely correlated with the ventilation. 



In a small number of vertebrates (tortoise, pigeon, man) 

 Krogh (1904.2) determined the cutaneous uptake of 2 . It 

 is too small to be of any respiratory significance. 



In blowfly larvae having a very thin cuticle, Fraenkel and 

 Herford (1938) find that in a normal atmosphere about 10% 

 of the total oxygen is absorbed through the integument. 



Air-breathing gills, formed by the internal branches of some 

 of the abdominal feet and protected by the external, are found 

 in some terrestrial isopods (Ligia, Oniscus) living in moist sur- 



