MOVEMENT OF THE BLOOD IN THE VEINS. 191 



(4) The presence of numerous valves which permit the blood-stream to 

 move only in a centripetal direction (Fabricius ab Aquapendente). 

 They are absent from the smallest veins, and are most numerous in 

 those of middle size. 



Law of the Position Of Valves. The venous valves always have two 

 pouches, and are placed at definite intervals, which correspond to the 1, 2, 3, or 

 n th power of a certain "fundamental distance," which ig = 7 mm. for the lower 

 extremity and 5'5 mm. for the upper. Many of the original valves disappear. On 

 the proximal side of every valve a lateral branch opens into the vein, while on the 

 distal side of each branch lies a valve. The same is true for the lymphatics 

 (K. Bardeleben). 



Effect of Pressure. As soon as pressure is applied to the veins, the 

 next lowest valves close, and those immediately above the seat of 

 pressure open and allow the blood to move freely toward the heart. 

 The pressure may be exerted from without, as by anything placed 

 against the body; the thickened contracted muscles, especially the muscles 

 of the limbs, compress the veins. That the blood flows out of a 

 divided vein more rapidly when the muscles contract, is shown during 

 venesection. If the muscles are kept contracted, the venous blood 

 passing out of the muscles collects in the passive parts e.g., in the 

 cutaneous veins. The pulsatile pressure of the arteries accompanying 

 the veins favours the venous current (Ozanam). 



From a hydrostatic point of view, the valves are of considerable 

 importance, as they serve to divide the column of blood into segments 

 (e.g., in the crural vein in the erect attitude), so that the fine blood- 

 vessels in the foot are not subjected to the whole amount of the 

 hydrostatic pressure in the veins. 



The velocity of the venous blood has been measured directly (with the hama- 

 dromometer and the stromuhr 89). Volkmann found it to be 225 mm. per sec. 

 in the jugular vein. Reil observed that 24 times more blood flowed from an 

 arterial orifice than from a venous orifice of the same size. The velocity of the 

 venous current obviously depends upon the sectional area of the vessel. Borelli 

 estimated the capacity of the venous system to be 4 times greater than that of the 

 arterial ; while, according to Haller, the ratio is 9 to 4. 



As we proceed from the small veins towards the venee cavse, the 

 sectional area of the veins, taken as a whole, becomes less, so that the 

 velocity of the current increases in the same ratio. The velocity of the 

 current in the vente cavre may be about half of that in the aorta 

 (Haller). 



As the pulmonary veins are narrower than the pulmonary artery, 

 the blood moves more rapidly in the former. The velocity of the 

 blood-current in the veins is accelerated during inspiration compare 

 88 (De Jager). 



[Active pulsation occurs in the veins of the wing of the bat (Schiff).] 



