214 



THE CIRCULATORY SYSTEM. 



[CH. XVI3I. 



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 walls of the arteries are composed of three coats, 

 termed (a) the external or tunica adventitia, (b) the middle or 



tunica media, and (c) the internal or 

 tunica intima. 



(a) The external coat or tunica 

 adventitia (figs. 2 14. and 2 15, a), the 

 strongest and toughest part of the 

 wall of the artery, is formed of areolar 

 tissue, with which is mingled through- 

 out 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. 215, e). 



(6) The middle coat (fig. 215, m) 

 is composed of both muscular and 

 elastic fibres, with a certain propor- 

 tion of areolar tissue. In the larger 

 arteries (fig. 217) 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. 216), and are arranged for the most part transversely 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 here 

 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. 

 215, e), which make it smooth, so that the blood may flow with 

 the smallest possible amount of resistance from friction. 



Immediately external to the endothelial lining of the artery 

 is fine connective tissue (sub-endothelial layer), with branched 

 corpuscles. Thus the internal coat consists of three parts, (a) an 

 endothelial lining, (b) the sub-endothelial layer, and (c) elastic layer. 



Fig. 216. Muscular fibre-cells from 

 human arteries, magnified 350 

 diameters. (Kolliker.) a. Nu- 

 cleus, b. A fibre- cell treated 

 with acetic acid. 



