126 ON ZOOLOGY. 



ted without the risk of rupture, and protected 

 by a strong, resisting bag, named the pericar- 

 dium ; while the whole is nearly enveloped by 

 the lungs, which are also made of a very pliable 

 nature, that they may not be injured by the con- 

 stant action to which they are exposed. Thus 

 the circulating and respiratory systems so im- 

 portant to animal existence, display a wonderful 

 arrangement, in which security and effect are 

 equally conspicuous; and which, by evincing 

 the most benevolent attention to the welfare and 

 preservation of the individuals upon whom it has 

 been bestowed, is well deserving of your minuter 

 investigation. 



As the mechanism, upon which 1 have just 

 treated, has a particular reference to the admis- 

 sion of air into the blood vessels ; it is obvious 

 that in those animals which can subsist upon a 

 diminished supply of that fluid, aless complicated 

 machinery might answer the required purpose. 

 Nature, therefore, has made the proper distinc- 

 tion, by confining the double circulation and 

 a warmer blood to those animals which are not 

 only exposed to the common atmosphere and its 

 peculiar temperature, but also to a very active 

 use of their locomotive powers; as illustrated in 

 our own species, land quadrupeds, birds, and in 

 whales, the last of which are obliged to be 

 frequently on the surface of the water for a fresh 

 supply of air, and to expel the superfluous fluid 



