142 THE CIRCULATION. 



four are sometimes placed together, or near one another, in the 

 largest veins, such as the subclavian, and at their junction with 

 the jugular veins. The valves are semiluuar; the unattached 

 edge being in some examples concave, in others straight. They 

 are composed of inextensile fibrous tissue, and are covered 

 with epithelium like that lining the veins. During the period 

 of their inaction, when the venous blood is flowing in its proper 

 direction, they lie by the sides of the veins ; but when in ac- 

 tion, they close together like the valves of the arteries, and 

 offer a complete barrier to any backward movement of the 

 blood (Figs. 54 and 55). 



Valves are not equally numerous in all veins, and in many 

 they are absent altogether. They are most numerous in the 

 veins of the extremities, and more so in those of the leg than 

 the arm. They are commonly absent in veins of less than a 

 line in diameter, and, as a general rule, there are few or none 

 in those which are not subject to muscular pressure. Among 

 those veins which have no valves may be mentioned the supe- 

 rior and inferior vena cava, the trunk and branches of the portal 

 vein, the hepatic and renal veins, and the pulmonary veins ; 

 those in the interior of the cranium and vertebral column, those 

 of the bones, and the trunk and branches of the umbilical vein 

 are also destitute of valves. 



The principal obstacle to the circulation is already over- 

 come when the blood has traversed the capillaries; and the 

 force of the heart which is not yet consumed, is sufficient to 

 complete its passage through the veins, in which the obstruc- 

 tions to its movement are very slight. For the formidable 

 obstacle supposed to be presented by the gravitation of the 

 blood, has no real existence, since the pressure exercised by 

 the column of blood in the arteries, will be always sufficient to 

 support a column of venous blood of the same height as itself: 

 the two columns mutually balancing each other. Indeed, so 

 long as both arteries and veins contain continuous columns of 

 blood, the force of gravitation, whatever be the position of the 

 body, can have no power to move or resist the motion of any 

 part of the blood in any direction. The lowest bloodvessels 

 have, of course, to bear the greatest amount of pressure; the 

 pressure on each part being directly proportionate to the height 

 of the column of blood above it : hence their liability to disten- 

 sion. But this pressure bears equally on both arteries and 

 veins, and cannot either move, or resist the motion of, the fluid 

 they contain, so long as the columns of fluid are of equal 

 height in both, and continuous. Their condition may, in this 

 respect, be compared with that of a double bent tube, full of 

 fluid, held vertically ; whatever be the height and gravitation 



