234 THE BODY AT WORK 



the capillaries in general are the changes of pressure in par- 

 ticular localities, brought about by the mediation of vaso- 

 constrictor and vaso-dilator nerves. In almost all organs and 

 parts of the body the automatic tone of arteries is enhanced 

 by impulses which flow continuously down vaso-constrictor 

 nerves. These impulses start from, or, to speak more accu- 

 rately, pass through, the vaso-motor centre in the medulla 

 oblongata. From every part of the body impulses ascend to 

 this centre, urging it to keep up the blood-pressure by universal 

 constriction. Yet no separate organ would be interested in 

 sending such a message if it were not open to it to ask at the 

 same time that the constriction of its own vessels might be 

 relaxed. Hence it may be said that every individual in the 

 community is crying out for universal economy, with more 

 generous treatment of himself. The response made by the 

 State to the latter part of his demand is in proportion to the 

 vehemence with which it is presented. 



If the spinal cord of an animal be cut across near the medulla 

 oblongata, respiration being maintained by pumping air into 

 and out of the lungs, the heart continues to beat with un- 

 diminished force, but the pressure in the large arteries falls to 

 one-third of its normal height. Constricting impulses no longer 

 pass down the spinal cord from the vaso-motor centre. This 

 experiment also illustrates the truth of the statement that 

 models of the vascular system arrangements of pumps and 

 indiarubber tubes are more likely to mislead than to inform. 

 In an artificial schema the relaxation of the constriction of the 

 small tubes on the proximal side of the capillary vessels would 

 reduce friction. Fluid would reach the capillaries in larger 

 quantity, and pass through them more quickly. The pressure 

 in the tubes which represented veins would consequently ap- 

 proach more nearly to that on the arterial side. But when the 

 spinal cord is divided the pressure falls in the veins, as well as 

 in the arteries. This is due to another factor, and one of very 

 great importance in the regulation of the circulation. The 

 blood from the digestive organs is collected by the " portal 

 system " of veins. These do not join the inferior vena cava ; 

 they go to the liver, where they again break up into capillaries. 

 It is not until after this second distribution through minute 

 vessels that the blood is re-collected by the hepatic veins and 



