502 



ANGIOLOGY 





fibers, and is covered on both surfaces with endothelium, the arrangement of which differs on 

 the two surfaces. On the surface of the valve next the wall of the vein the cells are arranged 

 transversely; while on the other surface, over which the current of blood flows, the cells are 

 arranged longitudinally in the direction of the current. Most commonly two such valves are 

 found placed opposite one another, more especially in the smaller veins or in the larger trunks 

 at the point where they are joined by smaller branches; occasionally there are three and some- 

 times only one. The valves are semilunar. They are attached by their convex edges to the 

 wall of the vein; the concave margins are free, directed in the course of the venous current, and 

 lie in close apposition with the wall of the vein as long as the current of blood takes its natural 

 course; if, however, any regurgitation takes place, the valves become distended, their opposed 

 edges are brought into contact, and the current is interrupted. The wall of the vein on the 

 cardiac side of the point of attachment of each valve is expanded into a pouch or sinus, which 

 gives to the vessel, when injected or distended with blood, a knotted appearance. The valves 

 are very numerous in the veins of the extremities, especially of the lower extremities, these vessels 



Endothelium - - ^._ 



Elastic layer . 



Middle coat 



Outer coat , 



FIG. 452. Section of a medium-sized vein. 



having to conduct the blood against the force of gravity. They are absent in the very small 

 veins, i. e., those less than 2 mm. in diameter, also in the venae cavse, hepatic, renal, uterine, and 

 ovarian veins. A few valves are found in each spermatic vein, and one also at its point of junc- 

 tion with the renal vein or inferior vena cava respectively. The cerebral and spinal veins, the 

 veins of the cancellated tissue of bone, the pulmonary veins, and the umbilical vein and its 

 branches, are also destitute of valves. A few valves are occasionally found in the azygos and 

 intercostal veins. Rudimentary valves are found in the tributaries of the portal venous system. 

 The veins, like the arteries, are supplied with nutrient vessels, vasa vasorum. Nerves also 

 are distributed to them in the same manner as to the arteries, but in much less abundance. 



THE BLOOD. 



The blood is an opaque, rather viscid fluid, of a bright red or scarlet color 

 when it flows from the arteries, of a dark red or purple color when it flows from 

 the veins. It is salt to the taste, and has a peculiar faint odor and an alkaline 

 reaction. Its specific gravity is about 1.06, and its temperature is generally about 

 37 C., though varying slightly in different parts of the body. 



