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CHAPTER V. 

 ON THE ARTERIES. 



THE arteries are the cylindrical tubes which convey the blood from 

 the ventricles of the heart to every part of the body. They are dense 

 in structure, and preserve for the most part the cylindrical form when 

 emptied of their blood, which is their condition after death : hence 

 they were considered by the ancients as the vessels for the transmis- 

 sion of the vital spirits,* and were therefore named arteries (uvg r^iTv, 

 to contain air). 



The artery proceeding from the left ventricle of the heart contains 

 the pure or arterial blood, which is distributed throughout the entire 

 system, and constitutes with its returning veins the greater or 

 systemic circulation. That which emanates from the right ventricle, 

 conveys the impure blood to the lungs ; and with its corresponding 

 veins establishes the lesser or pulmonary circulation. 



The whole of the arteries of the systemic circulation proceed from 

 a single trunk, named the aorfo, from which they are given off as 

 branches, and divide and subdivide to their ultimate ramifications, 

 constituting the great arterial tree which pervades by its minute sub- 

 divisions every part of the animal frame. The mode in which the 

 division into branches takes place is deserving of remark. From the 

 aorta the branches, for the most part, pass off at right angles, as if for 

 the purpose of checking the impetus with which the blood would 

 otherwise rush along their cylinders from the main trunk ; but in the 

 limbs a very different arrangement is adopted ; the branches are given 

 off from the principal artery at an acute angle, so that no impediment 

 may be offered to the free circulation of the vital fluid. The division 

 of arteries is usually dichotomous, as of the aorta into the two com- 

 mon iliacs, common carotid into the external and internal, &c. ; but in 

 some few instances a short trunk divides suddenly into several branches 

 which proceed in different directions ; this mode of division is termed 

 an owis, as the thyroid and coeliac axis. 



In the division of an artery into two branches, it is observed that 

 the combined areae of the two branches are somewhat greater than 

 that of the single trunk ; and if the combined areas of all the branches 

 at the periphery of the body were compared with that of the aorta, it 

 would be seen that the blood, in passing from the aorta into the nu- 

 merous distributing branches, was flowing through a conical tube, of 



* To Galen is due the honour of having discovered that arteries contained 

 blood, and not air. 



