THE CIRCULATION OF THE BLOOD 153 



The changes in the blood flow through the skin are 

 accompanied by corresponding but inverse changes in the 

 internal organs. On a cold day the stomach and intestines, 

 the pancreas, the liver, the kidneys, etc. are richly supplied 

 with blood, while on a warm day their blood supply is 

 diminished. In the former case the blood withheld from the 

 skin finds its way into the internal organs ; in the latter case 

 the skin draws upon these organs for its needed supply. 

 The circulation in the internal organs compensates for that 

 in the skin. 



16. The reason for compensatory changes. We have seen 

 that it is the function of the heart to keep the arterial reser- 

 voir adequately distended with blood, thus supplying a steady 

 driving force for the flow of blood through the organs. When 

 the small arteries of the skin widen on a warm day, blood 

 escapes more rapidly into the skin from the arterial reservoir. 

 This alone would diminish the amount of blood in the reser- 

 voir unless the heart pumped more blood or unless the dila- 

 tion or widening of the cutaneous arterioles were compensated 

 by a constriction elsewhere, so that the total drain on the 

 reservoir remained the same. In the case in question it is 

 the latter of these alternatives which is adopted, and the 

 reservoir is kept filled without calling on the heart to pump 

 more blood. 



Conversely, on a cold day the diminution of the outflow 

 into the skin would lead to a backing up or accumulation 

 of blood in the great arteries, and so to their increased and 

 perhaps undesirable distention, if the dilation of the arte- 

 rioles of internal organs did not provide an outlet for the 

 surplus blood. 



Nowhere, perhaps, is this principle of compensatory dila- 

 tion or constriction of arteries in one region, to allow for the 

 effect of the opposite change in some other region, so highly 

 developed or so fully applied as in the reactions of the body 

 to changes in external temperature. 



