208 



THE VITAL FUNCTIONS. 



53 2 



and ultimately collect the blood into those large 

 internal tubes, or reservoirs, which, pursuing the 

 analogy, correspond to the stem or trunk of its 

 respective tree : while, on the other hand, the tubes 

 termed arteries, which, proceeding from the trunks, 

 divide and subdivide into smaller and smaller 

 vessels, and carry the blood to every part of the 

 body, are represented by the branches and rami- 

 fications of this tree. The finer vessels composing 

 the ultimate ramifications are termed, from their 

 minuteness, the Capillaries. 



We may next conceive these two trees placed 

 with their trunks parallel to each other, but each 

 in an inverted position with respect to the other : 

 so that the extremities of the branches of the one 

 are continuous with the extremities of the roots of 

 the other. The diagram, Fig. 332, 

 may assist us in this analysis of the 

 organs of circulation. Theoxygen- 

 ising tree, with its roots, or veins, r, 

 its stem o, and its branches, or 

 arteries, a, being represented as 

 situated on the left side of the dia- 

 gram ; and the nutrient tree, with 

 its roots, or veins, v, its stem, n, and 

 its branches, or arteries, a, on the 

 right, but in an inverted position 

 with respect to the former. As the 

 branches of the one tree are con- 

 tinuous with the roots of the other, a complete 

 circle is established : and the blood may move 

 along this circle in the same constant direction, 

 indicated in the Figure by the arrows; being, in the 

 oxygenising system, towards r, the organ of respi- 



