230 STUDIES IN PHYSIOLOGY 



are developing on the outside of the body, lungs are forming 

 within, and by the time the tail has disappeared and the 

 legs have become full-grown, the animal is provided with a 

 good pair of lungs ready for air breathing. These lungs are 

 very simple affairs, however. The whole interior is a hollow 

 cavity connected with a short windpipe, through which the 

 animal swallows air taken into the mouth cavity through 

 the nostrils. In the thin walls which inclose the lungs, run 

 the pulmonary blood vessels. The skin of the frog is 

 always moist, and a considerable amount of respiration is 

 carried on through this outer surface also, much as respira- 

 tion is carried on by the earthworm. 



All reptiles, birds, and mammals breathe throughout life by 

 lungs, and little, if any, respiration is carried on through the 

 more or less thickened skin. Respiration is most complete 

 in the birds, since air sacs, connected with the lungs, are 

 found in the neck, wings, abdomen, and legs, and even run 

 out, as we have already learned, into the cavities of the 

 bones. Hence, if the windpipe were closed and an opening 

 were made into one of these air sacs, respiration could still 

 be carried on. 



Comparison of the Organs of Respiration Studied. In single- 

 celled animals the whole body may be said to function in 

 respiration, since each bit of protoplasm takes from the sur- 

 rounding water the oxygen it needs and gives off to the 

 water its carbon dioxid. 



The respiratory region in worms is somewhat more lim- 

 ited and specialized. The whole outer skin functions as a 

 lung, but a circulatory system is rendered necessary by the 

 size of the animal in order to carry oxygen to the internal 

 organs and to remove the wastes from them. 



In all vertebrates specialization is carried still farther, 

 well-developed gills or lungs being provided to carry on the 

 function of respiration. In most of the vertebrate groups, 

 too, there is a special pulmonary blood system to carry the 

 blood to, through, and from the lungs. 



