ON ZOOLOGY. 125 



effectually, as to answer the desired ends; and 

 that the propelling vessels or arteries should be 

 strong and muscular, to give an additional impulse 

 to the circulation after the contraction of the heart 

 has been completed. We have here then, a very 

 wonderful apparatus to accomplish the most im- 

 portant operation in the animal economy; and as 

 it must appear from this explanation, that the liv- 

 ing principle is entirely dependent upon the agen- 

 cy of atmospherical air, and as we know it is 

 instantly suspended upon the least interruption to 

 its admission ; it is most obvious that the great- 

 est precaution was necessary against accidents, 

 which, even only of momentary operation, might 

 be instantly destructive to animal life. 



We accordingly find, that while vegetables are 

 made up of soft and fragile materials suitable to 

 their functions, animals are composed of those 

 which are strong and resisting ; their most im- 

 portant organs, especially the heart and lungs, 

 to which we now more particularly allude, being 

 so effectually secured within a bony exterior, 

 that no injury, short of extraordinary force, or 

 some unusual impulse, could possibly reach them. 



If we look more nicely into the structure and 

 relative situation of those organs, we find the 

 heart, whose cavities are liable to great disten- 

 sion, formed of a thick, muscular substance of 

 sufficient flexibility, to admit of their being dila- 



