176 BLOOD-PRESSURE IN THE VEINS. 



undulations are occasionally observable in the jugular vein of a healthy 

 person ( 99). 



[Braune showed that the femoral vein under Poupart's ligament collapsed when 

 the lower limb was rotated outwards! and backwards, but filled again when the 

 limb was restored to its former position. All the veins which open into the 

 femoral vein have valves, which permit blood to pass into the femoral vein, but 

 prevent its reflux. This mechanism acts to a slight degree as a kind of suction 

 and pressure apparatus when a person walks, and thus favours the onward move- 

 ment of the blood.] 



(5.) Changes in the position of the limbs or of the body, for hydro- 

 static reasons, greatly alter the venous pressure. The veins of the 

 lower extremity bear the greatest pressure, while at the same time 

 they contain most muscle (K. Bardeleben, 65). Hence, when 

 these muscles from any cause become insufficient, dilatations occur in 

 the veins, giving rise to the production of varicose veins. 



[(6.) Movements of the Voluntary Muscles. Veins which lie between 

 muscles are compressed when these muscles contract, and as valves 

 exist in the veins the flow of the blood is accelerated towards the 

 heart; if the outflow of the blood be obstructed in any way, then 

 the venous pressure on the distal side of the obstruction may be 

 greatly increased. When a fillet is tied on the upper-arm, and the 

 person moves the muscles of the fore-arm, the course of the superficial 

 veins can be distinctly traced on the surface of the limb.] 



[(7.) Gravity exercises a greater effect upon the blood-stream in the 

 extensile veins than upon the stream in the arteries. It acts on the dis- 

 tribution of the blood, and thus indirectly on the motion of the blood- 

 stream. It favours the emptying of descending veins, and retards the 

 emptying of ascending veins, so that the pressure becomes less in the 

 former and greater in the latter. If the position of the limb be 

 changed, the conditions of pressure are also altered (Paschutin). If a 

 person be suspended with the head hanging downwards, the face soon 

 becomes turgid, the position of the body favouring the inflow of blood 

 through the arteries, and retarding the outflow through the veins. 

 If the hand hangs down it contains more blood in the veins than 

 if it is held for a short time over the head, when it becomes pale 

 and bloodless. As Lister has shown, the condition of the vessels in 

 the limb are influenced not only by the position of the limb, but also 

 by the fact that a nervous mechanism is called into play.] 



[Ligature Of the Portal Vein. The pressure and other conditions vary in 

 particular veins. Thus, if the portal vein be ligatured, there is congestion of the 

 capillaries and rootlets of the portal vein, and dilatation of all the blood-vessels in 

 the abdomen, and gradually nearly all the blood of the animal accumulates within 

 its belly, so that, paradoxical as it may seem, an animal may be bled into its own 

 belly. As a consequence of sudden and complete ligature of this vein, the arterial 



