114 THE HUMAN BODY. 



exchange the disassimilated particles which are to be rejected 

 from the system or submitted to a fresh elaboration. A 

 living and nourishing fluid, it carries to the organs life, heat, 

 and the elements of nutrition. 



Venous circulation. After passing through the capillary 

 vessels, the blood passes into the venous radicles. At its 

 entrance into the aorta and during its course through the 

 arterial system, it was of a brilliant red colour, now it has 

 become dark; the arterial or red blood is changed to 

 venous or black blood; deprived of the greater portion of 

 its constituent elements it returns to their source for a fresh 

 supply. 



The blood moves in the veins from the impulsion received 

 originally from the heart; this force is designated by the term 

 vis a tergo, because it is exerted behind the fluid column. 

 The elasticity of the veins, and their contractility also, con- 

 tributes to urge the blood along in its return towards the 

 heart, but it is the valves which most effectually second the 

 cardiac impulse by opposing the reflux of the blood toward 

 the arteries. If a moderately tight ligature be placed around 

 the arm, the veins begin to swell because of the afflux of 

 blood, which can neither go on toward the heart because of 

 the ligature, nor return toward the arteries because of the 

 valves which oppose it in that direction. If the finger be now 

 passed lightly over the course of a vein in a direction op- 

 posed to the circulation, the little nodules which mark the 

 projections caused by the distended valves are easily dis- 

 tinguished. Thanks to the play of these valves, any pressure 

 upon the veins, from muscular contraction, or whatever 

 cause, can only carry the blood toward the heart, while with- 

 out them it would be impelled indiscriminately one way or 

 the other. Therefore the valves are more numerous in the 

 veins which are connected with the muscles; in the deep 

 veins of the limbs, for example, than in those which creep 

 along under the skin. 



Gravitation affects the movement of the venous blood, 

 which is much less rapid than that of arterial blood. When 

 the hands hang down for a long time as in walking, they 

 swell so much that the flexion of the fingers is difficult, and 



