THE CIRCULATION 161 



made of tough, fibrous materials, so that they sustain the mighty 

 impulse of the heart, and are not ruptured. In common with 

 the heart, the arteries have a delicately smooth lining mem- 

 brane. They are also elastic, and thus re-enforce the action 

 of the heart ; they always remain open when cut across, and 

 after death are usually found empty. 



31. The early anatomists observed this condition, and sup- 

 posing that it existed during life, came to the conclusion that 

 these tubes were designed to act as air-vessels hence the 

 name artery, from Greek words which signify " containing air." 

 This circumstance affords us an illustration of the mistaken 

 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 numberless branches. 

 As these branches leave the heart, they divide and subdivide, 

 continually growing smaller and smaller, until they can no 

 longer be traced by 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 side of 

 the 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 waves speeding from the heart 

 to the smallest of the branches. The general course of the ar- 

 teries 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 pressure, by 

 which the force of the heart would be counteracted and wasted. 

 Accordingly, we generally find these vessels close to the bones, 



81. Early anatomists ? The service of the illustration ? 

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

 38. Successive undulations from the heart ? Course of the arteries ? Protection of the 

 arteries ? General location of the arteries ? 



