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HANDBOOK OF PHYSIOLOGY. 



of the body. As regards the arterial, system of the lungs (pulmonary 

 system) it begins at the right ventricle in the pulmonary artery, and is 

 distributed much as the arteries belonging to the general systemic circu- 

 lation 



Structure. The walls of the arteries are composed of three principal 

 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. 95 and 96 a), the 

 strongest and toughest part of the wall of the artery, is formed of areo- 

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

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

 arteries so distinct a layer as to be sometimes called the external elastic 

 coat (Fig. 99, e e). 



FIG. 97. 



FIG. 98. 



FIG. 97. Portion of a fenestrated membrane from the femoral artery, x 200. a, 6, c, Perfora- 

 tions. (Henle.) 



FIG. 98. Muscular fibre-cells from human arteries, magnified 350 diameters. (KoUiker.) a. 

 Nucleus. 6. A fibre-cell treated with acetic acid. 



(b) The middle coat (Fig. 95, m) is composed of both muscular and 

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

 arteries (Fig. 99) its thickness is comparatively as well as absolutely 

 much greater than in the small, constituting, as it does, the greater part 

 of the arterial wall. 



The muscular fibres, which are of the unstriped variety (Fig. 98), 

 are arranged for the most part transversely to the long axis of the artery 

 (Fig. 95, m)\ while the elastic element, taking also a transverse direc- 

 tion, is disposed in the form of closely interwoven and branching fibres, 

 which intersect in all parts the layers of muscular fibre. In arteries of 

 various size there is a difference in the proportion of the muscular and 

 elastic element, elastic tissue preponderating in the largest arteries, 

 while this condition is reversed in those of medium and small size. 



