92 CIRCULATION OF THE BLOOD IN THE VESSELS. 



ery. In the great majority of instances, the valves exist in pairs, but they 

 are occasionally, although very rarely in the human subject, found in groups 

 of three. They are seldom if ever found in veins of a less diameter than one 

 line (2-1 mm.). The valves are formed in part of the lining membrane of 

 the veins, with fine fibres of connective tissue, elastic fibres and non-striated 

 muscular fibres. There exists, also, a fibrous ring following the line of 

 attachment of the valvular curtains to the vein, which renders the vessel 

 much stronger and less dilatable here than in the intervals between the 

 valves. The valves are most abundant in the veins of the lower extremities. 

 They are generally situated 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 observed in any of the superficial veins. If the flow of blood be 

 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 be 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 portion of the vessel by pressing the blood in 

 the opposite direction (Harvey). On slitting open a vein, it is easy to ob- 

 serve the shape, attachment and extreme delicacy of structure of the valves. 

 When the vessel is empty or when fluid moves toward the heart, the valves 

 are closely applied to the walls ; but if liquid or air be forced in the opposite 

 direction, they project into its caliber, and by the application of their free 

 edges to each other, effectually prevent any backward current. When closed 

 the application of their free edges form 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 have a direction from before back- 

 ward, in the sets above and below, the lines run from side to side (Fabricius). 

 There are certain 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 the horse. They do not 

 exist, however, in this situation in the human subject. Generally, in following 

 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 origin of tl)e 

 external iliac. In some of the inferior animals, there 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. This is 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 orifice of the coronary 

 vein. There are no valves at the openings 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 very 

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



