COURSE OF THE BLOOD IN THE VEINS. 313 



which they receive is very much increased. It is then fur- 

 nished to supply material for the secretion, and not exclu- 

 sively 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 much increased, and the flow 

 is in an intermittent jet, as from a divided artery. This is 

 due to the relaxed condition of the arterioles of the part, and 

 the phenomenon thus observed is 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 

 the phenomenon 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 admissible. 

 Except in the veins near the heart, any pulsation which oc- 

 curs is to be attributed 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. ' Mr. T. M. King, in an article on the 

 " Safety-Valve of the Human Heart," 1 discussing the forces 

 which concur to produce the venous circulation, mentions 

 the fact that in some individuals, after a full meal, pulsation 

 can be observed in the veins of the hand or the median veins 

 of the forehead. This phenomenon is very delicate, and, to 

 make it more apparent, he employed a thread of black seal- 

 ing wax about two inches long, which was fixed across the 

 vein of the back of the hand with a little tallow, so as to 

 make a long and excessively light lever, capable of indicating 

 a very slight movement in the vessel. In this way he dem- 



1 Guy's Hospital Reports, 1837. 



