CH. XIX.] 



THE ARTERIES 



217 



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 inosculate) with other arteries. The 

 branches are usually given off at an acute angle, and the sum of 

 the sectional 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 distributed 

 much as the arteries belonging to the general systemic circulation. 



Structure. The arterial wall is composed of the following 

 coats : 



(a) The external coat or tunica adventitia (figs. 196 and 197, 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. 196, e). 



(b) The middle coat (fig. 196, m) is com- 

 posed of both muscular and elastic fibres, 

 with a certain proportion of areolar tissue. 

 In the larger arteries (fig. 196) its thickness 

 is comparatively as well as absolutely much 

 greater than in the small ones; it consti- 

 tutes the greater part of the arterial wall. 

 The muscular fibres are unstriped (fig. 198), 

 and are arranged for the most part trans- 

 versely to the long axis of the artery; 

 while the elastic element, taking also a 

 transverse direction, is disposed in the 

 form of closely interwoven and branching 

 fibres, which intersect in all parts the layers 

 of muscular fibres. In arteries of various 



sizes there is a difference in the proportion of the muscular and 

 elastic element, elastic tissue preponderating in the largest arteries, 

 and unstriped muscle in those of medium and small size. 



(c) The internal coat is formed by a layer of elastic tissue, called 

 the fenestrated membrane of Henle. Its inner surface is lined with a 

 delicate layer of elongated endothelial cells (fig. 196, e), which make 

 it smooth, so that the blood may flow with the smallest possible 



FIG. 198. Muscular fibre-cells 

 from human arteries, magni- 

 fied 350 diameters. (Kolliker.) 

 a. Nucleus, b. A fibre-cell 

 treated with acetic acid. 



