INTRODUCTION. XXV 



and after moistening the most minute fibres it is received 

 into the capillary extremities of the veins and brought 

 back to the heart, where it receives another impulse, and 

 performs again the round of the body, and so on in suc- 

 cession. This phenomenon is called the circulation. 

 When it exists in animals, blood is always to be found; 

 for the most part red, but in many species white or trans- 

 parent. The use of the blood in them is to receive from 

 the alimentary canal, from the skin and lungs, such 

 matter as has been assimilated, and to convey it to eve- 

 ry part of the body, for the purpose of repairing its 

 waste, or providing for its growth. It is at the very 

 extremities of the arteries that this deposite occurs, and 

 the blood getting into the veins loses its bright vermi- 

 lion colour, becomes of a modena or dark blue, and is 

 no longer fit for the purposes of life till some of the prin- 

 ciples which it has lost by this passage are restored to 

 it. This restoration takes place in the lungs> where a 

 sort of combustion is performed by^ the absorption of 

 oxygen, This process is called respiration, and it ex- 

 ists in all things that live, under various modifications 

 of the apparatus performing it. In man it is performed 

 in two cellular air bags, which have a heart indepen- 

 dent of the one just mentioned, for propelling the blood 

 through the ramifications of their vessels. In fish there 

 are gills, which have their surfaces exposed to the wa- 

 ter, and are aerated by the air contained in the water, 

 and the same heart which supplies the general circu- 

 lation, also fills a large artery, that is distributed very 

 minutely through the gills. But in insects, where 

 there are no blood vessels, and the nutritious fluid is 

 contained in cells, there are, distributed over their bo- 

 dies, air tubes, which transmit atmospheric influence. 



The blood vessels, iu addition to the function of car- 

 rying nutritious matter, perform an essential part of a 

 very different character. All the atoms of which the 



VoL.L 3 



