278 RESPIRATION OP REPTILES AND BIRDS. 



of Frogs is of great importance in their respiration in fact, 

 of almost as much consequence as their lungs. The necessity 

 for more energetic respiration increases in these animals with 

 the temperature, every rise in which excites them to greater 

 activity. During the winter, which they pass beneath the 

 water in a state of torpidity, the action of the water upon 

 their skin is sufficient to aerate their blood. When the re- 

 turning warmth of spring arouses them from their inaction, 

 they need a larger amount of respiration, and come occa- 

 sionally to the surface to take-in air by their lungs. And 

 when summer comes on, the greater heat increases their need 

 of respiration ; and they quit their ditches and ponds, so as 

 to allow the atmosphere to act upon their skin as well as 

 upon their lungs. If they are prevented from doing so, they 

 will die ; and the same result follows if the skin be smeared 

 with grease, so that the air cannot permeate it. Moreover, if 

 the lungs be removed, and the animal be made to breathe by 

 its skin alone, it may live for some time, if the temperature 

 be not high. These facts show the great importance of the 

 skin as a respiratory organ in Frogs. 



326. The respiration of BIRDS is more active than that of 



Trachea 



Pulmonary vessels 



___jBronchial tube 



Orifice of bronchial) 

 tube) 



("Bronchial tube 

 (.opened. 



Fig. 161. AIR-TUBES AND LUNGS OF BIRDS. 



any other Yertebrata ; that is, they consume more oxygen, 

 and form more carbonic acid, in proportion to their size. 



