THE CIRCULATION. 115 



icately smooth lining membrane. They are also elastic, 

 and thus re-enforce the action of the heart : they always 

 remain open when cut across, and after death are always 

 found empty. 



31. The early anatomists observed this phenomenon, 

 and supposing that the same condition existed during life, 

 came to the conclusion that these tubes were designed to 

 act as air-vessels, hence the name artery, from a Greek 

 word which signifies containing air. This circumstance 

 aifords us an illustration of the confused notions of the 

 ancients in reference to the internal operations of the 

 body. Cicero speaks of the arteries as "conveying the 

 breath to all parts of the body," 



32. The arterial system springs from the heart by a 

 single trunk, like a minute and hollow tree, with number- 

 less branches. As these branches leave the heart they 

 divide and subdivide, continually growing smaller and 

 smaller, until they can no longer be traced with the 

 naked eye. If, then, we continue the examination by the 

 aid of a microscope, we see these small branches sending- 

 off still smaller ones, until all the organs of the body are 

 penetrated by arteries. 



33.. The Pulse. With each contraction of the left 

 heart, the impulse causes a wave-like motion to traverse 

 the entire arterial system. If the arteries were exposed to 

 view, we might see successive undulations speeding from 

 the heart to the smallest of the branches, in about one- 

 sixth part of a second. The general course of the arteries 

 is as far as possible from the surface. This arrangement 

 is certainly wise, as it renders them less liable to injury, 

 the wounding of an artery being especially dangerous. It 

 also protects the arteries from external and unequal pres- 

 sure, by which the force of the heart would be counter* 



3 1 . Early anatomist* ? The service of the illustration ? 



32. The arterial system ? The branches and sub-branches of the arteries ? 



33. Successive undulations from the heart ? Course of the arteries * Protec 

 tion of the arteries ? General location of the arteries ? 



