en. xviir.] 



THE ARTERIES 



215 



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, for the supply of the lower ex- 

 tremities. The arterial branches wherever given off divide and sub- 

 divide, until the calibre of each subdivision becomes very minute, and 

 these minute vessels lead 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 inos- 

 culate) with other arteries. The 

 branches are usually given off at an 

 acute angle, and the sum of the sec- 

 tional areas 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 (aprrjpia, the 

 windpipe) was given them, as the 

 ancients believed that they conveyed 

 air to the various parts of the body. 

 As regards the arterial system of the 

 lungs, the pulmonary artery is dis- 

 tributed much as the arteries belong- 

 ing to the general systemic circulation. 

 Structure. The wall of an artery 

 is composed of the following three 

 ooats : 



(a) The external coat or tunica adventitia (figs. 223 and 224, a), 

 the strongest part of the wall of the artery, is formed of areolar 

 tissue, with which is mingled throughout a network of elastic fibres. 

 At the inner part of this outer coat the elastic network forms, in 

 some arteries, so distinct a layer as to be sometimes called the 

 external elastic coat (fig. 224, e). 



(b) The middle coat (fig. 224, ra) is composed of both muscular 

 and elastic fibres, with a certain proportion of areolar tissue. In the 

 larger arteries (fig. 226) its thickness is comparatively as well as 

 absolutely much greater than in the small ones ; it constitutes the 

 greater part of the arterial wall. The muscular fibres are unstriped 

 (fig. 225), and are arranged for the most part transversely to the 



FIG. 224. Transverse section through a 

 large branch of the inferior mesenteric 

 artery of a pig. e, endothelial mem- 

 brane ;. i, tunica elastica intema, no 

 subendothelial layer is seen ; m, mus- 

 cular 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. 



