98 CIKCULATION OF THE BLOOD. 



tion 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 oppo- 

 site direction. On slitting open a vein, we observe the shape, attachment, and extreme 

 delicacy of structure of the valves. When the vessel is empty, or when fluid moves 

 toward the heart, they 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. Fabricius noted the following 

 peculiarity in the arrangement of the valves : 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 have a 

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

 side. 



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 we come to the crural vein ; but occasionally there is a double 

 valve at the origin of the 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 are entirely deprived of valves, 

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



In addition to the double, or more rarely triple valves which have just been described, 

 there is another variety, found in certain parts, at the point where a tributary vein opens 

 into a main trunk. This consists of a single fold, which is attached to the smaller vessel 

 but projects into the larger. Its action is to prevent regurgitation, by the same mech- 

 anism as that by which the ileo-csecal valve prevents the passage of matters from the 

 large into the small intestine. These valves are much less numerous than the first variety. 



The veins form a system which is adapted to the return of blood to the heart in a 

 comparatively slow and unequal current. Distention of certain portions is provided for ; 

 and the vessels are so protected with valves, that whatever influences the current must 

 favor its flow in the direction of the heart. 



Course of the Blood: in the Veins. The experiments of Hales and Sharpey, showing 

 that defibrinated blood can be made to pass from the arteries into the capillaries and out 

 at the veins by a pressure less than that which exists in the arterial system, and the 

 observations of Magendie upon the circulation in the leg of a living dog, showing that 

 ligation of the artery arrests the flow in the vein, points which have already been fully 

 discussed in treating of the causes of the capillary circulation, have established, beyond 

 question, the fact that the force exerted by the left ventricle is sufficient to account for 

 the venous circulation. The heart must be regarded as the prime cause of all movement 

 in these vessels. Eegarding this as definitely ascertained, there remain to consider, in 

 the study of the course of the blood in the veins, the character of the current, the influence 

 of the vessels themselves, the question of the existence of forces which may assist 

 the vis a tergo from the heart, and circumstances which may interfere with the flow of 

 blood. 



As a rule, in the normal circulation, the flow of blood in the veins is continuous. The 



