THE NERVOUS REGULATION OF THE BLOODVESSELS 187 



varies with the individual, the external temperature, and other 

 circumstances, and then as a rule rather suddenly the vaso-con- 

 striction gives way and the flow begins again to increase, even while 

 the left hand is still kept in the cold water. When the left hand is 

 transferred from the cold to warm water (at 43 or 44 C), the first 

 effect is a transient diminution in the blood-flow in the right hand. 

 This soon gives place to an increase (vaso-dilatation). As an ex- 

 ample, the following table gives the condensed results of three, 

 experiments on two young men. Experiments II. and III. were on 

 the same man at an interval of three days. 



Such facts enable us to some extent to understand the manner in 

 which the distribution of the blood is adjusted to the requirements 

 of the different parts of the body, so that to a certain degree of 

 approximation no organ has too much, and none too little. The 

 blood-supply of the organs is always shifting with the calls upon 

 them. Now, it is the actively-digesting stomach and the actively- 

 secreting glands of the alimentary tract which must be fed with a 

 full stream of blood, to supply waste and to carry away absorbed 

 nutriment. Again, it is the working muscles of the legs or of the 

 arms that need the chief blood-supply. But wherever the call may 

 be, the vaso-motor mechanism is able, in health, to answer it by 

 bringing about a widening of the small arteries of the part which 

 needs more blood, and a compensatory narrowing of the vessels 

 of other parts whose needs are not so great. 



It is also through the vaso-motor system, and especially by the 

 action of that portion of it which governs the abdominal vessels, and 



