76 RELATIONS OF THE VESSELS. 



minutest portion of the body ! I am quite impatient to 

 know more of these curious organs. 



Dr. B. The blood vessels are strong membranous 

 tubes composed of three coats. The external is formed 

 of cellular substance ; the middle of muscular ; and the 

 internal is one of a peculiar nature, possessing considera- 

 ble elasticity and power of dilatation. In their course 

 they are constantly subdividing and giving off branches, 

 until from being larger than a man's thumb at their main 

 trunk, they become at their extremities of a size almost 

 imperceptible. They are distinguished into two kinds ; 

 one conveying the blood from the lungs to every part of 

 the body, called arteries ; the other, conveying the blood 

 from every part of the body to the lungs, called veins. 

 Each of these sets of vessels may be compared, in their 

 disposition, to two trees united by their trunks, the branch- 

 es of one originating in the lungs, the other in every part 

 of the body. In order that you may have a perfectly 

 distinct idea of the relative situation of these vessels, I 

 have prepared this diagram* in which the dotted lines 

 represent the arteries, and the others the veins. You 

 see the branches of the arteries originating in L the lungs> 

 running together till at last they form one larger trunk, 

 and again branching off to terminate in B, or every part 

 of the body. The veins originate where the arteries. 



terminate, run together and form one larger trunk, branch 

 off again, and finally extend their minute ramifications 

 to every part of the lungs. This now, constitutes all that 



*The reader must bear in mind the distinction between a plate and 

 a diagram. The former is a representation of the actual appearance of 

 an object ; the purpose of the latter is merely to illustrate some par- 

 ticular principle, or manner in which an object acts, without adhering 

 at all to the exact form or appearance of the object. 



