258 THE VITAL FUNCTIONS. 



in its anterior extremity ; till in the Decapoda, 

 a family which comprehends the Lobster and 

 the Crab, we find this part dilated into an oval 

 or globular organ, with very muscular coats, 

 capable of vigorous contractions, propelling its 

 contents with considerable force into the vessels, 

 and therefore clearly entitled to the appellation 

 of heart. The distinction between arteries and 

 veins, which can scarcely be made with any 

 precision in the systems of the inferior tribes, is 

 here perfectly determined by the existence of 

 this central organ of propulsion : for the vessels 

 into which the blood is sent by its contractions, 

 and which, ramifying extensively, distribute 

 it to distant parts, are indisputably arteries; 

 and conversely, the vessels, which collect the 

 blood from all these parts, and bring it back 

 to the heart, are as decidedly veins. The heart 

 of the lobster is situated immediately under the 

 carapace, or shell of the dorsal region of the 

 thorax, in a plane posterior to the stomach, 

 where it is not liable to be pressed against the 

 resisting shell, when the stomach is distended. 

 Its pulsations are very distinct, and are per- 

 formed with great regularity. 



The importance of the heart, as the prime 

 agent in the circulation, increases as we advance 

 to the higher classes of animals, whose more 

 active and energetic functions require a con- 

 tinual and rapid renewal of nutrient fluid, and 

 render necessary the introduction of further re- 



