112 RESPIRATION OF REPTILES. 



heat would boil them alive. I presume they are cautious 

 how they take exercise in such a terrifying medium. 



Dr. B. The respiration and circulation of birds are 

 upon the same plan as those of man and the large quad- 

 rupeds. These functions possess a higher degree of ac- 

 tivity than in the last animals, and in fact may be consid- 

 ered as at their maximum in the Bird. Birds have no 

 diaphragm; the lungs adhere to the ribs and extend far 

 down into the abdomen ; and the chest does not change 

 its capacity in respiration. The air is admitted into and 

 expelled from the lungs solely by their own force. Their 

 animal heat is also greater than that of any other ani- 

 mals, and may be stated as varying from 104 to 107 

 of Far. 



From a few experiments that have been made, it ap- 

 pears that birds are less capable of resisting the extremes 

 of heat and cold, than other animals. The migrations 

 of birds are evidently for no other purpose than to avoid 

 those extremes of temperature incident to many regions. 

 The swallows are unable to bear the cold of the New 

 England September, and a little premature cold weather, 

 hastens their departure several days, if not weeks. Of 

 all the birds that enliven our fields in summer, not one 

 remains to share with us the rigors of the cold and drea- 

 ry season. Even the little snow-bird which forms such 

 a lively feature in our winter scenery, has come from 

 higher and colder regions, to enjoy a more genial climate 

 in ours. 



Emily. I used frequently to see them in the country, 

 and though they seemed gay and active, as they flew 

 along from one field to another, yet I pitied them, for I 

 thought such tender things must suffer from the cold. 



Dr. B. The respiratory organs of reptiles, though 

 formed for breathing atmospheric air, are less perfe'ct in 

 their organization than the lungs of other animals. In- 

 stead of consisting of solid fleshy organs, penetrated 

 throughout with innumerable air-tubes, they are com- 

 posed of numerous delicate membranous bags, communi- 



