

STRUCTURE OF THE LARGER BLOOD- VESSELS. Ill 



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 endothelium-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. 



The larger arteries and veins possess blood-vessels, vasa vaswum, and 

 lymphatics, both of which ramify chiefly in the external coat. Nerves 

 are distributed to the muscular tissue of the middle coat, after forming 

 a plexus in the outer coat. 



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, etc. ; 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 (although 

 occurring abundantly in some of the tributaries of the portal vein), in those 

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

 umbilical vein. 



