140 MOVEMENT OF THE BLOOD IN THE VEINS. 



in different parts of its course. Amongst these are : (1) The relative lateness, 

 great (listensHAlity, and the ready compressibility of the walls, even of the thickest 

 veins. (2) The incomplete filling of the veins, which does not amount to any con- 

 siderable distension of their walls. (3) The numerous and free anastomoses between 

 adjoining veins, not only between veins lying injthe same plane, but also between 

 superficial and deep veins. Hence, if the course of the blood be obstructed in one 

 direction, it readily finds another outlet. (4) The presence of numerous valves 

 which permit the blood-stream to move only in a centripetal direction. They are 

 absent from the smallest veins, and are most numerous in those of middle size. 



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 is = 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. Bardcleben). 



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. From a hydro- 

 static 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 hsemadromonieter and 

 the rheometer 89). Volkmann found it to be 225 mm. per sec. in the jugular vein. Reil 

 observed that 2i 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. 



Large Veins. As we proceed from the small veins towards the venae cava?, 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 venae cavae 

 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. 



97. SOUNDS WITHIN ARTERIES. The sounds produced within arteries are, speaking 

 strictly from a physical point of view, only noises or bruits. Still, following Skoda's lead, they 

 are spoken of by physicians as "tones." Clinically, there is no sharp distinction between 

 "tones," sounds, noises, or bruits. In four-fifths of all healthy men two sounds correspond- 

 ing in duration and other characters to the two heart-sounds are heard in the carotid (Conrad, 

 Weil). Sometimes only the second heart-sound is distinguishable, as its place of origin is near 

 to the carotid. They are not true arterial sounds, but are simply "propagated heart-sounds." 

 Sometimes the sound of the pulmonary artery can be heard in this way ( Weil, Bettelheim). 

 These murmurs, sounds, or bruits occur either spontaneously, or are produced by the application 

 of exUrvpal prestwrt, whereby the lumen of the vessel is diminished. Hence one distinguishes : 

 (1) Spontaneous Murmurs, and (2) Pressure Murmurs. 



Arterial Sounds or murmurs are readily produced by pressing upon a strong 

 artery, e.g., the crural in the inguinal region, so as to leave only a narrow passage 

 for the blood (" stenosal murmur "). A fine blood-stream passes with great 

 rapidity and force through this narrow part, into a wider portion of the artery 

 lying behind the point of compression. Thus arises the " pressure-stream " 

 (P. Niemeyer), or the " fluid vein " (" veine flnide " of Chauveau). The particles 



