RESPIRATION IN REPTILES. 231 



longing to this class, which, instead of 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 Jlxolotl, the Proteus anguinus, 

 the Siren lacertina, and the Menobranchus lateralis, which 

 permanently retain their external gills, while at the same 

 time they possess imperfectly developed lungs. It would 

 therefore seem as if, in these animals, the progress of deve- 

 lopment had been arrested at an early stage, so that their 

 adult state corresponds to the larva condition of the frog.* 



In all warm-blooded animals respiration becomes a func- 

 tion of much greater importance, the continuance of life 

 being essentially dependent on its vigorous and unceasing 

 exercise. The whole class of Mammalia have lungs of an 

 exceedingly developed structure, composed of an immense 

 number of minute cells, crowded together as closely as pos- 

 sible, and presenting a vast extent of internal surface. The 

 thorax, or cavity in which the lungs, together with the heart 

 and its great blood vessels, are enclosed, has somewhat the 

 shape of a cone; and its sides are defended from compres- 

 sion by the arches of the ribs, which extend from the spine 

 to the sternum, or breast-bone, and produce mechanical sup- 

 port on the same principle that a cask is strengthened by 

 being girt with hoops, which, though composed of compara- 

 tively weak materials, are yet capable, from their circular 

 shape, of presenting great resistance to any compressive 

 force. 



While Nature has thus guarded the chest, with such pe- 

 culiar solicitude, against the efforts of any external force, 

 tending to diminish its capacity, she has made ample provi- 

 sion for enlarging or contracting its diameter in the act of 



* Geoffrey St. Hilaire thinks there is ground for believing that Crocodiles 

 and Turtles possess, in addition to the ordinary pulmonary respiration, a par- 

 tial 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 1 bladder of fishes, in sub- 

 serviency to a species of aerial respiration. 



