. 168 THE CIRCULATION 



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. 



33. 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.' 7 



34. 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. 



35. Histology of the Arteries. The arteries have' three prin- 

 cipal coats. An inner, or epithelial coat; a middle coat con- 

 taining elastic fibrous tissue, and unstriped muscular fibres 

 arranged transversely around the vessel ; and an external coat 

 of condensed connective tissue. The principal difference be- 

 tween the -larger and smaller arteries is in the structure of 

 their middle coat. In the smaller arteries this is composed 

 exclusively of muscular fibres. In arteries of medium size it 

 contains both muscular and elastic tissue, whilst in those of 

 the largest calibre it consists of elastic tissue only ; therefore, 

 the large arteries possess much elasticity and but little con- 

 tractility, while in the smaller these conditions are reversed. 



33. Early anatomists ? The service of the illustration ? 



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



85. What are the three principal coats of the arteries ? The principal difference between 

 the larger and smaller arteries ? 



