86 PHYSIOLOGY AND TEMPERANCE. 



valves, similar to those guarding the pulmonary artery, but 

 much stronger, are placed at the opening of the aorta. 



8. The Arteries. The strong elastic tubes which carry 

 the blood from the heart to all parts of the body are called 

 arteries. We have mentioned the aorta. This is the largest 

 artery in the body. It receives the blood from the left ven- 

 tricle, carries it along, and distributes it to its numerous 

 branches. After leaving the heart, the aorta arches back- 

 ward, like the curve on a walking-stick, and passes down the 

 back part of the thorax into the abdomen, where it divides 

 to supply the lower extremities. Along this course it gives 

 off such important branches as those to the arms and head, 

 and further on, it sends branches to supply the walls of the 

 chest and the organs in the abdomen. These are the main 

 branches from the aorta, like the limbs of a tree from the 

 parent trunk. Follow them on, and we find, as in a tree, 

 the branches divide and subdivide, growing smaller and 

 smaller as we near their termination. Ultimately they he- 

 come so small and so numerous as to form a close network. 

 This network, at its finest parts, constitutes what are called 

 the capillaries, or hair-like tubes. 



The web of the frog's foot affords a good example for 

 seeing, under the microscope, the flow of blood through the 

 capillaries. We can see the corpuscles wending their way 

 in single file along these narrow passages, and occasionally 

 swaying to and fro, stopping for a moment, but soon to be 

 rushed on again with the ever-constant stream. 



9. The Veins. Gradually the blood in the capillaries, 

 now dark and impure, is gathered into small tubes, called 

 veins. As they proceed toward the heart, the veins join 

 each other, becoming fewer in number, but- larger in size, 

 until all those from the legs and abdomen are joined into one 

 great vein, which opens into the right auricle at its lower 

 part ; and those from the head and arms form another large 



