THE VASCULAR MECHANISM. 



233 



The influence of gravity is antagonised completely in man, and to a 

 lesser extent in most animals, by means of a compensating action on the 

 part of the vaso-motor centre. When a man rises from the horizontal 

 to the standing position, the blood tends to accumulate in his 

 abdomen, and the supply of blood to the brain is diminished. This 

 diminution at once stimulates the extremely sensitive vaso-motor 

 centre, which sends out impulses constricting the arterioles of the 

 splanchnic area, thereby forcing the blood out of this area into the rest 

 of the body, including the brain. Conversely, the splanchnic arterioles 



FIG. 91. Aortic blood pressure. Effect of posting. (L. Hill.) From Practical 



Physiology, by Pembrey and others. 

 A-B, vertical, head up ; B, horizontal ; C, vertical, head down ; D, horizontal. 



relax to some extent whenever an individual changes from the vertical 

 to the horizontal position. 



This reaction on the part of the vaso-motor centre to any change 

 in the position of the body as regards gravity, is so rapid and complete 

 that we are not normally aware of its existence. The temporary giddi- 

 ness, which is often noticed by individuals who are anaemic or run 

 down on changing suddenly from' a horizontal to a standing position, 

 is due to the fact that the response of the vaso-motor centre to the 

 change of position is slower than usual, and that for a few moments the 

 brain is inadequately supplied with blood. In the same way " fainting " 

 is in many cases caused by temporary diminution of the activity of 

 the vaso-motor centre, so that the blood pressure falls and the blood 

 supply to the brain is deficient, causing loss of consciousness. The 



