THE ARTERIES 



149 



The Arteries. The arterial system begins at the left ventricle in 

 a single large trunk, the aorta, which, almost immediately after its origin, 

 gives off in the thorax three large branches for the supply of the head, neck, 

 and upper extremities; it then traverses the thorax and abdomen, giving 

 off branches, some large and some small, for the supply of the various organs 

 and tissues it passes on its way. In the abdomen it divides into two chief 

 branches. The arterial branches, wherever given off, divide and subdivide 

 until the caliber of each subdivision becomes very minute. These smallest 

 arteries are called arterioles. These arterioles are continuous with the capil- 

 laries. Arteries frequently communicate or anastomose with other arteries. 

 The arterial branches are usually given off at an acute angle, and the areas 

 of the branches of an artery generally exceed that of the parent trunk, and, 



, , m 



FIG. 144. 



FIG. 145. 



FIG. 146. 



FIG. 144. Minute Artery Viewed in Longitudinal Section, e, Nucleated endothelial mem- 

 brane, with faint nuclei in lumen, looked at from above; *', thin elastic tunica intima; ra, muscular 

 coat or tunica media; a, tunica adventitia. (Klein and Noble Smith.) 



FIG. 145. Transverse Section through a Large Branch of the Inferior Mesenteric Artery of a 

 Pig. e, End9thelial membrane; *", tunica elastica interna, no subendothelial layer is seen; m, 

 muscular tunica media, containing only a few wavy elastic fibers; e, c, tunica elastica externa, di- 

 viding the media from the connective-tissue adventitia, a. (Klein and Noble Smith.) Magnifica- 

 tion 350 diameters. 



FIG. 146. Muscular Fiber Cells from Human Arteries. Magnified 350 diameters. (Kolliker.) 

 a, Nucleus; B, a fiber cell treated with acetic acid. 



as the distance from the origin is increased, the area of the combined branches 

 is increased also. As regards the arterial system of the lungs, the pulmonary 

 artery and its subdivisions, they are distributed in much the same manner 

 as the arteries belonging to the general systemic circulation. 



The walls of the arteries are composed of three principal coats, the ex- 

 ternal or tunica adventitia, the middle or tunica media, and the internal or 

 tunica intima. The external coat, figures 144 and 145, a, the strongest and 

 toughest part of the wall of the artery, is formed of areolar tissue, with which 

 is mingled throughout a network of elastic fibers. The middle coat, figure 



