106 THE HUMAN BODY. 



terein, to contain). For the same reason, and with more 

 accuracy, they named the tube which conveys the air to the 

 lungs the trachea-artery. 



Galen discovered the presence of blood in the arteries, 

 but he retained the name, and others have continued it, 

 although it does not accord with their functions. 



The walls of the arteries are composed of three superposed 

 coats. The outer one is nbro-cellular, vascular, and very 

 resistant; the middle one, the membrane proper or elastic, 

 is less resistant, and changes its texture under the influence 

 of age or other causes. The inner is extremely thin, and is 

 analogous in texture to the endocardium. When a ligature 

 is applied to an artery, the internal and middle coats are 

 broken through by the pressure, but the external one 

 resists it. 



The arteries communicate with each other in their course 

 through the body, and especially toward the extremities by 

 numerous anastomoses that, is, they join each other either 

 by means of branches, or by forming a net-work, the meshes 

 of which, rounded and in arches, are closer in proportion 

 as the twigs are smaller. They terminate in innumerable 

 microscopic ramifications, called the capillary vessels, which 

 are intermediate between the ends of the arteries and the 

 veins. 



The walls of the arteries are nourished, like all other parts 

 of the body, by the vasa vasorum, or vessels of the vessels. 

 And lastly, the arteries are enveloped in their course by 

 numerous nervous filaments from the great sympathetic 

 nerve, and by lymphatic vessels. 



The arteries which penetrate the substance of muscles, 

 like those of the thigh and leg, are protected by an aponeu- 

 rotic sheath, and by fibrous rings which prevent them from 

 being pulled out of place or compressed during the contrac- 

 tion of the muscles which surround them. 



Principal arteries. The aorta, which is the main trunk of 

 the arterial system which carries the red blood, is the largest 

 artery in the system. It commences at the upper portion of 

 the left ventricle, not far from the ventriculo-aortic orifice; 

 it has three valves, called the sigmoid or semi-lunar valves, 



