244 CIRCULATION. 



pends, is composed chiefly of the yellow elastic tissue. This 

 tissue is disposed in numerous layers. First we have a thin 

 layer of ramifying elastic fibres, and then a number of layers 

 of elastic membrane, with numerous oval longitudinal open- 

 ings, which has given it the name of the " fenestrated mem- 

 brane." According to Kolliker, between the layers of this 

 membrane are found a few unstriped or involuntary muscu- 

 lar fibres, but Robin states that muscular fibres are only found 

 in arteries of medium size. 1 Muscular fibres, if they exist at 

 all in the largest arteries, are very few, and of little physio- 

 logical importance. The middle coat of the largest arteries 

 gives them their yellowish hue, and the elasticity for which 

 they are so remarkable. 



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 membrane, covered with a layer of elongated 

 epithelial scales, with oval nuclei, their long diameter fol- 

 lowing 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 difference 

 between them and the largest arteries, even in thickness. 

 The essential 'difference in the anatomy of these vessels is 

 found in the middle coat. Here we have a continuation of 

 the elastic elements found in the largest vessels, but rela- 

 tively diminished in thickness, and mingled with the fusiform 

 involuntary muscular fibres, arranged at right angles to the 

 course of the vessel. These fibres are found in the inner 

 layers of the middle coat, and, according to Robin, 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 numerous layers. There is no 



1 ROBIN, in Nysten's Dictlonnaire de Mcdccine, 1858. Artere. 



