THE CIRCULATION 169 



the occurrence of cold feet in many persons or a general 

 feeling of coldness during the early stages of digestion. 



The splanchnic system may be described as the 

 ' resistance-box ' of the circulation (Hill), by means of 

 which variations in the systemic blood-pressure are 

 regulated, and examples of its operation have already 

 been given. In addition to this, the circulation possesses 

 two regulating ' reservoirs ' in the liver and lungs. As 

 Stolnikow has pointed out, the liver acts as a blood 

 reservoir on the systemic circulation, so that the right 

 heart may take a greater or less volume of blood from 

 the great veins without materially modifying the systemic 

 pressure. The lungs play a similar part to the left 

 heart, acting as a reservoir in which blood may be 

 stored if the left heart for some reason diminishes its 

 output, and, conversely, contains a reserve of blood upon 

 which the left heart draws when its output increases 

 (Brodie). Overfilling of the pulmonary and hepatic 

 reservoirs are prominent features among the ' back- 

 pressure ' signs of a failing heart. 



Local variations in blood distribution are controlled 

 by the tissues or organ concerned themselves. By means 

 of afferent impulses the tonic constriction normally 

 exerted on the arterioles of the part by the vasomotor 

 centre is cut off, -the arterioles dilate like stop-cocks 

 which have been suddenly turned on, and the general 

 arterial pressure drives more blood into the dilated area. 

 In the case of organs in which a sudden and very 

 copious blood-supply is often demanded vasodilator 

 nerves are present as well, by means of which an active 

 dilatation of the arterioles is brought about. 



