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 transmission of the vital spirits,* and were therefore named 

 arteries (cb}g TT^SIV, 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 sys- 

 temic 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 aorta, from which they are given off 

 as branches, and divide and subdivide to their ultimate ramifications, 

 constituting the great arterial tree which pervad.es by its minute 

 subdivisions 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 common iliacs, common carotid into the external and inter- 

 nal, &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 axis, as the thyroid and cosliac axis. 



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

 the combined areas of the two branches are greater than that of the 

 single trunk ; and if the combined area? 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 

 numerous distributing branches, was flowing through a conical tube 

 of which the apex might be represented by the aorta, and the base 



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

 not air. 



