COURSE OF THE BLOOD IN THE VEINS. 99 



intermittent impulse of the heart, which progressively diminishes as we recede from this 

 organ but is still felt even in the smallest arteries, is lost, as we have seen, in the capil- 

 laries. Here, for the first time, the blood moves in a constant current ; and, as the press- 

 ure in the arteries is continually supplying fresh blood, that which has circulated in the 

 capillaries is forced into the venous radicles in a steady stream. As the supply to the 

 capillaries of different parts is regulated by the action of the small arteries, and as this 

 supply is subject to great variations, there must necessarily be corresponding variations 

 in the current in the veins and in the quantity of blood which these vessels receive. As we 

 should anticipate, then, the venous circulation is subject to very great variations arising 

 from irregularity in the supply of blood, aside from any action of the vessels themselves 

 or any external disturbing influences. Great variations in the venous current are observed 

 in the veins which collect the blood from the intestinal canal. During the intervals of 

 digestion, these vessels carry a comparatively small quantity of blood ; but, during diges- 

 tion, they are laden with the fluids received by absorption, and the quantity is largely 

 increased. 



It often happens that a vein becomes obstructed from some cause which is entirely 

 physiological, as the action of muscles. The immense number of veins, as compared with 

 the arteries, and their free communications with each other, provide that the current, 

 under these circumstances, is simply diverted, passing to the heart by another channel. 

 When any part of the venous system is distended, the vessels react on the blood and exert 

 a certain influence on the current, always pressing it toward the heart, for the valves 

 oppose a flow in the opposite direction. 



The intermittent action of the heart, which pervades the whole arterial system, is 

 generally absorbed, as it were, in the passage of the blood through the capillaries ; but, 

 when the arterioles of any part are very much relaxed, the impulse of the central organ 

 may extend to the veins. Bernard has shown this in the most striking manner, in his 

 well-known experiments on the circulation in the glands. "When the glands are pouring 

 out their secretions, the quantity of blood which they receive is very much increased. It 

 is then furnished to supply material for the secretion, and not exclusively for nutrition. 

 If the vein be opened at such a time, it is found that the blood has not lost its arterial 

 character, that the quantity which escapes is increased, and that the flow is in an inter- 

 mittent jet, as from a divided artery. This is due to the relaxed condition of the arteri- 

 oles of the part, and the phenomenon thus observed constitutes the true venous pulse. 

 What thus occurs in a restricted portion of the circulatory system may take place in all 

 the veins, though in a less marked degree. Physicians have frequently noticed, after the 

 blood has been flowing for some time in the operation of venesection, that the color 

 changes from black to red, and the stream becomes intermittent, often leading the 

 operator to fear that he has pricked the artery. In all probability, this is due to 

 the relaxation of the arterioles as one of the effects of abstraction of blood, producing 

 the same condition that has been noted in some of the glands during their functional 

 activity. The hypothesis that it is due to an impulse from the adjacent artery is not ad- 

 missible. Except in the veins near the heart, any pulsation which occurs is to be attrib- 

 uted to the force of the heart, transmitted with unusual facility through the capillary 

 system. A nearly uniform current, however, is the rule, and a marked pulsation, the 

 rare exception. 



Pressure of Blood in the Veins. The pressure in the veins is always much less than 

 in the arteries. It is exceedingly variable in different parts of the venous system and in 

 the same part at different times. As a rule, it is in inverse ratio to the arterial pressure. 

 Whatever favors the passage of blood from the arteries into the capillaries has a tendency 

 to diminish the arterial pressure, and, as it increases the quantity of blood which passes 

 into the veins, must increase the venous pressure. The great capacity of the venous sys- 

 tem, its numerous anastomoses, the presence of valves which may shut off a portion from 



