THE CIRCULATION OF THE BLOOD. 



181 



(a) The external coat or tunica adventitia (figs. 152 and 153, ), the 

 strongest and toughest 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 

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

 elastic coat (fig. 153, e). 



(b) The middle coat (fig. 153, m) is composed of both muscular and 

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

 arteries (fig. 153) its thickness is comparatively as well as absolutely 

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



Fig. 152. 



Fig. 153. 



Fig. 154. 



Fig. 152. Minute artery viewed in longitudinal section, e, Nucleated endothelial membrane, 

 with faint nuclei in lumen, looked at from above ; t, thin elastic tunica intima ; m, muscular coat 

 or tunica media ; a, tunica adventitia. (Klein and Noble Smith.) X 250. 



Fig. 153. Transverse section through a large branch of the inferior mesenteric artery of a pig. 

 . Endothelial membrane ; i, tunica elastica interna, no subendothelial layer is seen ; m, muscular 

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

 media from the connective tissue adventitia a. (Klein and Noble Smith.) X 850. 



Fig. 154. Muscular fibre-cells from human arteries, magnified 350 diameters. (Ko'lliker.) a, 

 Nucleus. 6, a fibre-cell treated with acetic acid. 



of the arterial wall. The muscular fibres are unstriped (fig. 154), and 

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

 (fig. 155, 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, and 

 unstriped muscle in those of medium and small size. 



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

 i\\Q femstrated coat of Henle. It is peculiar in its tendency to curl up, 

 when peeled off from the artery, and in the perforated and streaked ap- 

 pearance which it presents under the microscope. Its inner surface is 

 lined with a delicate layer of elongated endothelial cells (fig. 153, e), 



