182 THE MOVEMENT OF THE BLOOD IN THE VEINS. 



these processes are going on, the migration of the leukocytes and rarely also of 

 the red cells takes place. Under favorable conditions the stasis may be relieved, 

 generally with a reversal in the order of the phenomena that have attended its 

 development. The escape of blood-corpuscles through the intact wall of the vessel 

 is designated diapedesis. The swelling of inflamed parts is due in part to the 

 dilatation of the vessels, but chiefly to the escape of plasma into the tissues. 



THE MOVEMENT OF THE BLOOD IN THE VEINS. 



In the smallest veins, which are formed by the union of capillaries, 

 the velocity of the blood-current is greater than in the capillaries, but 

 slower than in the smallest arteries. At the same time, the current 

 is everywhere uniform, and according to hydrodynamic laws the venous 

 current would continue with absolute regularity to the heart, if it were 

 not subject to other disturbances. Such disturbances, however, are 

 operative in various directions. Among special peculiarities of the 

 veins to which interference with the uniformity of the current is attrib- 

 utable the following may be mentioned: 



i. The relative relaxation, the great distensibility and compress- 

 ibility of even the larger trunks; 2, the incomplete distention, which 

 does not increase to any considerable degree the elastic tension of the 

 walls; 3, the numerous and at the same time free anastomoses among 

 neighboring trunks, both in the same tissue-plane and from above down- 

 ward. By this means it is possible for the blood, when the venous 

 area is partly compressed, to escape through numerous readily distensible 

 channels, and thus the occurrence of actual stasis is prevented; 4, the 

 presence of numerous valves, which permit the blood-current to move 

 only in a centripetal direction. These are wanting in the smallest 

 veins, and they are most numerous in the medium-sized veins. The 

 valves are of great hydrostatic significance, inasmuch as they divide 

 long columns of blood, as, for example, in the crural vein when the body 

 is in the erect position, into sections, thus preventing the entire column 

 from exerting its hydrostatic pressure down to the lowest portions of 

 the vein. 



As soon as pressure is exerted on a vein, the nearest valves below the 

 point of pressure close and those next above open, thus leaving a free 

 passage for the blood to the heart. The pressure on the veins may be 

 of varied character: in the first place from without, by contact with 

 various objects. Further, thickened and contracted muscles may com- 

 press the veins, especially in the movements of the extremities. That 

 the blood escapes in a stronger stream from an opened vein when the 

 muscles are moved at the same time can be seen whenever venesection is 

 practised. If the muscles are permanently contracted, the venous 

 blood, escaping from the muscles, collects in the parts that are not 

 moved, especially in the cutaneous veins. The pulsatory pressure in 

 the arteries accompanying the veins also tends to accelerate the venous 

 current. 



Direct observations have been made as to the velocity of the venous blood- 

 current with the hemodromometer and the rheometer. Thus, Volkmann found 

 a velocity of 225 mm. in a second for the jugular vein; but in view of the low 

 pressure that prevails in the venous system, the employment of instruments for 

 measuring the velocity is necessarily attended with marked deviations from the 

 normal. Reil observed that the quantity of blood escaping from an opening in an 

 artery was two and a half times as great as the quantity of blood escaping from 

 a similar opening in a vein. 



