THE CIRCULATION OF THE BLOOD. 



105 



Structure of the Valves. The valves of the heart are formed essen- 

 tially of thick layers of closely woven connective and elastic tissue, over 

 which, on every part, is reflected the endocardium. 



II. The Arteries. 



Distribution. 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 



FIG. 95. 



FIG. 96. 



FIG. 95 Minute artery viewed in longitudinal section, e. Nucleated endothelial membrane, 

 with faint nuclei in lumen, looked at from above, i. Thin elastic tunica intirna. m. Muscular coat 

 or tunica media, a. Tunica adventitia. (Klein and Noble Smith.) x 250. 



FIG. 96. Transverse section through a large branch of the interior mesenteric artery of a pig. 

 e. endothelial membrane ; i, tunica elastica interna, no subendothelial layer is sesn ; m, muscular 

 tunica media, containing only a few wavy elastic fibres ; e, e, tunica elastica externa, dividing the 

 media from the connective-tissue adventitia, a. (Klein, and Noble Smith.) X 350. 



various organs and tissues it passes on its way. In the abdomen it di- 

 vides into two chief branches, for the supply of the lower extremities. 

 The arterial branches wherever given off divide and subdivide, until the 

 calibre of each subdivision becomes very minute, and these minute ves- 

 sels pass into capillaries. Arteries are, as a rule, placed in situations 

 protected from pressure and other dangers, and are, with few exceptions, 

 straight in their course, and frequently communicate (anastomose or in- 

 osculate) with other arteries. The branches are usually given off at an 

 acute angle, and the area of the branches of an artery generally exceeds 

 that of the parent trunk; and as the distance from the origin is increased, 

 the area of the combined branches is increased also. 



After death, arteries are usually found dilated (not collapsed as the 

 veins are) and empty, and it was to this fact that their name was given 

 them, as the ancients believed that they conveyed air to the various parts 



