164 THE BLOOD AND ITS CIRCULATION. 



organs : the veins of the dura mater offer the best example 

 of this arrangement. 



The veins are generally furnished with valves, arranged in 

 such a manner that when any abnormal pressure takes place, 

 they straighten under the influence of the current of blood, 

 which has a tendency to flow back, obliterate the lumen of 

 the vessel, and prevent the blood from returning to the 

 capillaries. These valves thus serve to neutralize, an 1 even 

 to utilize in regard to the circulation, the action uf the 

 shock, and of accidental pressure (for instance, on the part 

 of the neighboring muscles, when contracted) ; they also 

 serve to support, by their division, the long blood columns, 

 as, for example, the venous column of the lower limbs. The 

 veins supporting long columns of this kind have remarkably 

 thick coats ; thus the coats of the saphenous veins resemble 

 in appearance those of the arteries, and remain open after 

 incision, in the same manner as the large arterial vessels. 

 Where local pressure is rare, no valves are found in the 

 veins, as in the venous apparatus of the brain and lungs. 



As the phenomena of the flow outwards and backwards of 

 the blood through the cardiac orifices gives rise to particular 

 sounds (sounds of the heart, page 140), so the peripheral 

 circulation occasions sonorous phenomena, which may be 

 better observed in pathological cases (anemia) than in the 

 normal condition, and are heard especially about the neck, no 

 doubt because the aponeuroses of this region, by their special 

 arrangement, cause a state of tension in the coats of the vessels 

 and in their sheath, which is favorable to the transmission of 

 sounds: the tone of these sounds differs very much (whistling 

 sound, musical sound, bruit de diable) ; they are sometimes 

 continuous, and sometimes intermittent ; some are produced 

 in the arteries, and others in the veins. Weber supposes 

 them to be caused by the coats of the vessels being made to 

 vibrate by the motion of the blood, but these sounds are 

 more generally attributed, as is done by Chauveau and 

 Potain, to the blood passing rapidly through a narrow and 

 then through a wider passage, through which it flows more 

 slowly. Chauveau has shown, indeed, that vibrations are 

 produced under these circumstances, by means of a fluid 

 vein, which causes a sort of eddy at the point where the 

 narrow part joins the wider (fluid veins of Savart). This 

 arrangement may be carried out in different ways; normally, 

 as at the opening of the jugular vein into the subclavian ; 



