PHYSIOLOGICAL ANATOMY OF THE VEINS. 303 



The arteries are always full, and their tension is subject to 

 comparatively slight variations. Following the blood into 

 the capillaries, there are the immense variations in the circu- 

 lation with varying physiological conditions of the parts, 

 which we have already noted. As should naturally be ex- 

 pected, the condition of the veins varies with the changes in 

 the capillaries, from which the blood is taken. In addition 

 to this, there are independent variations, as in the erectile 

 tissues, in the veins of the alimentary canal during absorp- 

 tion, in veins subject to pressure, etc. 



Following the veins in their course, it is observed that 

 anastomoses with each other form the rule and not the 

 exception, as in the arteries. There are always a number of 

 channels by which the blood may be returned from a part ; 

 and if one vessel be obstructed from any cause, the current 

 is simply diverted into another. The veins do not present a 

 true anastomosing plexus, such as exists in the capillary sys- 

 tem, but simply an arrangement by which the blood can 

 easily find its way back to the heart, and by which the ves- 

 sels may accommodate themselves to the immense variations 

 in the quantity of fluid contents to which they are liable. 

 This, with the peculiar valvular arrangement in all but the 

 veins of the cavities, provides against obstruction to the flow 

 of blood through, as well as from, the capillaries, in which 

 it seems essential to the proper nutrition and function of 

 parts, that the quantity and course of the blood should be 

 regulated exclusively through the arterial system. Special 

 allusion to the different venous anastomoses belongs to de- 

 scriptive anatomy. Physiologically, the communication be- 

 tween the different veins is such* that the blood can always 

 find a way to the heart, and 'once fairly out of the capillaries, 

 it cannot react and influence the circulation of fresh blood in 

 the tissues. 



Collected in this way from all parts of the body, the blood 

 is returned to the right auricle, from the head and upper 

 extremities, by the superior vena cava, from the trunk and 



