186 THE VITAL FUNCTIONS. 



alone; the lateral communicating vessels just clescril)ecl; while 

 De jjlainvillc and Morrcn ascribe to it a course precisely the 

 reverse. Amidst these conflicting testimonies, it is extreme- 

 ly difficult to determine on which side the truth lies; and a 

 suspicion will naturally arise, that tlie course of the blood in 

 the vessels may not be at all times uniform, but may be lia- 

 ble to ])artial oscillations, or be even completely reversed, by 

 the operation of particular distiirbinii; causes. 



The larger Crustacea possess a circulatory apparatus still 

 more extensive and complete, accompanied by a correspond- 

 ing increase in the energy of the vital functions. As we 

 follow this system in the more higlily organized tribes of 

 this class, we find the powers of the dorsal vessel becoming 

 more and more concentrated in its anterior extremity; till, 

 in the Decapoda, a family which comj^rchends the Lobster 

 and the Crab, we find this part dilated into an oval or globu- 

 lar organ, with very muscular coats, capable of vigorous 

 contractions, propelling its contents with considerable force 

 into the vessels, and therefore clearly entitled to the appel- 

 lation of heart. The distinction between arteries and veins, 

 which can scarcely be made with any precision in the sys- 

 tems of the inferior tribes, is here perfectly determined by 

 the existence of this central organ of propulsion: for the ves- 

 sels 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 carajDace, or shell of the 

 dorsal region of the thorax, directly over the stomach; its 

 pulsations are very distinct, and are performed 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 continual and rapid renewal of nutrient fluid, and render 

 necessary the introduction of farther refinements into its 



