100 ELEMENTARY PHYSIOLOGY. 



as "the house in which we live." We may carry the 

 comparison a little farther and consider the different 

 tissues as the material used in the body-building, 

 where they serve a purpose much like that of sand, 

 stone, lime, glass, bricks and so on, in the construc- 

 tion of a building. 



MICROSCOPIC STRUCTURE. 



Here (1) we see the three coats of the 

 (')". arteries. The outer layer is made 

 up of a fibrous matter, more or less 

 elastic. Next to it lies the middle coat, which 

 consists of alternating layers of elastic tissue and 

 muscular fibers, and the inner lining consists prin- 

 cipally of a net- work of elastic tissue. 



As in the arteries, the veins under the 

 Vei^sf (*) microscope show three distinct coats, 



but the entire vein wall is much thin- 

 ner than that of the artery. Here at (2) the structure 

 of the vein is shown. We see also the valves, men- 

 tioned under the circulation, which consists of pouch- 

 like folds of the inner coat. Here we see three 

 valves; but sometimes there are but two, and even 

 one. From their shape' and position you may see 

 that they will allow blood to pass through them in 

 one direction, but not in the other. Thus, blood can 

 not flow backward, which, you will perceive, is a very 

 wise provision. The shape of these valves as shown 

 here will suggest the appropriateness of the name, 

 semilunar valves. 



