THE SANGUIFEROUS SYSTEM. 209 



when the blood circulates, it must move from the heart, through 

 the aorta and its branches, to the different parts of the body, 

 and return from these parts through the venae cavae, to the 

 heart ; that, when deposited in the heart by the venae ca/vae, it 

 must proceed through the pulmonary artery to the lungs, and 

 return from the lungs through the pulmonary veins to the 

 heart, in order to pass again from that organ into the aorta. 



It is also certain, that the blood is forced from the heart into 

 the arteries, by the contraction of the muscular fibres of which 

 the heart is composed ; and that the blood-vessels likewise 

 perform a part in the circulation, they propelling the blood 

 which is thus thrown into them : but their action appears to 

 depend upon the causes of a complex nature. 



General Anatomy of the Arterial System. 



The arteries are so much concerned in the important function 

 of the circulation of the blood, that every circumstance con- 

 nected with them is. very interesting. 



They are composed of coats or tunics, which are very elastic 

 and strong, and which are also very thick. In consequence of 

 the firmness of their coats, they continue open, after their 

 contents are discharged, like hard tubes. They submit to great 

 dilatation, and elongation, when fluids are forced into them, and 

 return to their former dimensions when the distending cause is 

 withdrawn. This elasticity is particularly subservient to the 

 circulation of the blood. It admits the artery to distend readily, 

 and receive the blood which is thrown into it by the contraction 

 of the heart. It also produces the contraction of the artery ; 

 which takes place as soon as the action of the heart ceases ; and 

 this contraction of the heart necessarily forces the blood for- 

 ward, as the valves at its orifice prevent it from returning to 

 the heart. 



The motion of the artery, which is so easily perceived by 

 the touch, and in many instances also by the eye, is completely 

 explained by the discharge of blood into the artery from the 

 heart, and by the elasticity of the vessel, by which it reacts 

 upon the blood. In some cases it is not simply the diameter 



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