LECTURE V. ^19 



turns by vessels which, endhig in a single 

 trunk, form the great distributive vessel or 

 aorta of the animal, to which no heart is 

 connected; so that the blood receives no ad- 

 ditional impulse besides that derived from 

 the action of its containing vessels. There- 

 fore, these extremely agile, powerful, inde- 

 fatigable, and vivacious creatures are sup- 

 ported by a languid circulation and trivial 

 respiratory process : a fact which warrants 

 the conclusion, that the powers of life can 

 be exerted according to their allotted man- 

 ner, without that quick supply of materials, 

 or great degree of change produced in the 

 circulating fluids by respiration, both of 

 which many physiologists have been accus- 

 tomed to consider essential to the produc- 

 tion and maintenance of such high degrees 

 of vital action. 



In order to explain the varieties of hearts 

 met with in different animals, Mr. Hun- 

 ter seems to have sought for some generic 

 term, simply expressing the number of ca- 

 vities found in them, without any reference 

 to the connections or offices of those cavi- 



