66 



CIRCULATION" OF THE BLOOD. 



connective tissue running in a circular, longitudinal, and oblique direction. The longi- 

 tudinal and oblique fibres exist chiefly in the outer coat. The middle coat of the largest 

 arteries gives them their yellowish hue and the elasticity for which they are so remark- 

 able. 



The internal coat of the largest arteries does not differ materially from the lining 

 membrane of the rest of the arterial system. It is identical in structure with the endo- 

 cardium, the membrane lining the cavities of the heart, and is continued through the 

 entire vascular system. It is a thin, homogeneous, elastic membrane, covered with a 

 layer of elongated epithelial scales, with oval nuclei, their long diameter following the 

 direction of the vessel. 



The arteries of medium size possess considerable strength, some elasticity, and very 

 great contractility. In the outer and inner coats we do not distinguish any great differ- 

 ence between these and the largest arteries, even in thickness. The essential difference 



FIG. 20. Small artery from the mesentery of the frog, showing epithelium and circular muscular fibres : 

 magnified 500 diameters. (From a photograph taken at the United States Army Medical Museum.) 



in the anatomy of these vessels is found in the middle coat. Here we have a continua 

 tion of the elastic elements found in the largest vessels, but relatively diminished in 

 thickness and mingled with the fusiform, involuntary muscular fibres arranged, for the 

 most part, at right angles to the course of the vessel. These fibres are found chiefly in 

 the inner layers of the middle coat, and only in arteries smaller than the carotids and 

 primitive iliacs. In arteries of medium size, like the femoral, profunda femoris, radial, 

 or ulnar, they exist in several layers. There is no distinct division, as regards the middle 

 coat, between the largest arteries and those of medium size. As we recede from the 

 heart, muscular fibres gradually make their appearance between the elastic layers, pro- 

 gressively increasing in quantity, while the elastic elements are diminished. 



