STRUCTURE OF THE LARGER BLOOD-VESSELS 91 



from the external coat, which is relatively better developed in the veins 

 than in the arteries, so that, although thinner, their walls are often 

 stronger. 



Many of the veins are provided with valves, which are semilunar 

 folds of the internal coat strengthened by a little fibrous tissue : a few 

 muscular fibres may be found in the valve near its attachment. The 

 layer of the inner coat is rather thicker, and the epithelium-cells are 

 more elongated on the side which is subject to friction from the current 

 of blood than on that which is turned towards the wall of the vessel. 



Variations in different veins. The veins vary in structure more than 

 do the arteries. In many veins longitudinal muscular fibres are found 

 in the inner part of the middle coat, as in the iliac, femoral, umbilical, 

 &c. ; in others they occur external to the circularly disposed fibres, and 

 are described as belonging to the outer coat. This is the case in the 

 inferior vena cava and also in the hepatic veins and in the portal vein 

 and its tributaries. In the superior and in the upper part of the inferior 

 vena cava the circular fibres of the middle coat are almost entirely 

 absent. The veins of the following parts have no muscular tissue, 

 viz. pia mater, brain and spinal cord, retina, bones, and the venous 

 sinuses of the dura mater and placenta. 



It is only the larger veins and especially those of the limbs that 

 possess valves. They are wanting in most of the veins of the viscera, 

 in those within the cranium and vertebral canal, in the veins of the 

 bones, and in the umbilical vein. 



