VALVES OF THE VEINS 73 



most abundant in the veins of the lower extremities. They usually are sit- 

 uated just below the point where a small vein empties into one of larger 

 size, so that the blood as it enters finds an immediate obstacle to passage 

 in the wrong direction. The situation of the valves may be readily ob- 

 served in any of the superficial veins. If the flow of blood is obstructed, 

 little knots will be formed in the congested vessels, which indicate the 

 position and action of the valves. When the vein is thus congested and 

 knotted, if the finger is pressed along the vessel in the direction of the 

 blood-current, a portion situated between two valves may be emptied of 

 blood ; but it is impossible to empty any part of the vessel by pressing 

 the blood in the opposite direction (Harvey). On slitting open a vein, 

 it is easy to observe the shape, attachment and delicacy of structure of 

 the valves. When the vessel is empty or when the blood moves toward 

 the heart, the valves are closely applied to the walls ; but if liquid or 

 air is forced in the opposite direction, they project into its calibre, 

 and by the application of their free edges to each other, effectually 

 prevent any backward current. When closed, the application of 

 their free edges forms a line which runs across the vessel. It is 

 found that in successive sets of valves these lines are at right angles 

 to each other, so that if in one set this line has a direction from before 

 backward, in the sets above and below, the lines run from side to side 

 (Fabricius). 



There are exceptions to the general proposition that the veins of the 

 great cavities are not provided with valves. Valves are found in the 

 portal system of some of the inferior animals, as in the horse. They do not 

 exist, however, in this situation in the human subject. Usually, in fol- 

 lowing out the branches of the inferior vena cava, no valves are found 

 until the crural vein is reached ; but occasionally there is a double valve 

 at the opening of the external iliac. In some of the inferior animals, 

 the're exists constantly a single valvular fold in the vena cava at the 

 openings of the hepatic, and one at the opening of the renal vein. These 

 are not constant in the human subject. Valves are found in the spermatic, 

 but not in the ovarian veins. A single valvular fold has been described 

 at the opening of the right spermatic into the vena cava. There are 

 two valves in the azygos vein near its opening into the superior vena 

 cava. There is a single valve at the opening of the coronary vein. 

 There are no valves at the opening of the brachio-cephalic into the 

 superior vena cava ; but there is a strong double valve at the point 

 where the internal jugular opens into the brachio-cephalic. Between 

 this point and the capillaries of the brain, the vessels have no valves, 

 except in rare instances, when one or two are found in the course of the 

 jugular. 



