292 THE VITAL FUNCTIONS. 



undergoing all the changes I have been describing, 

 present, during their whole lives, a great similitude 

 to the first stage of the tadpole. This is the case 

 with the Axolotl, the Proteus anguinns, the Siren 

 lacertina, and the Meuobranchus lateralis, which 

 permanently retain their external gills, while at the 

 same time they possess imperfectly developed lungs. 

 It would therefore appear that these animals, in 

 their adult state, are in a condition similar to that 

 of the tadpole ; with this difference, that in them 

 the developement of the organs stops there ; 

 whereas, in the latter, it proceeds to the metamor- 

 phosis of the animal into a quadruped breathing 

 only atmospheric air.^ 



In all warm blooded animals respiration becomes 

 a function of much greater importance, the con- 

 tinuance of life being essentially dependent on its 

 vigorous and unceasing exercise. The whole class 

 of Mammalia have lungs of an exceedingly de- 

 veloped structure, composed of an immense number 

 of minute cells, crowded together as closely as pos- 

 sible, and presenting a vast extent of internal sur- 

 ftice. The thorax, or cavity in which the lungs, 

 together with the heart and its great blood-vessels, 

 are inclosed, has somewhat the shape of a cone ; 

 and its sides are defended from compression by the 



* Geoffroy St. Hilaire thinks there is ground for believing that 

 Crocodiles and Turtles possess, in addition to the ordinary pulmonary 

 respiration, a partial aquatic abdominal respiration, effected by 

 means of the two channels of communication which have been found 

 to exist between the cavity of the abdomen and the external surface 

 of the body : and also that some analogy may be traced between this 

 aquatic respiration in reptiles, by these peritoneal canals, and the 

 supposed function of the swimming bladder of Hshes, in subserviency 

 to a species of aerial lespiratiou. 



