THE CIRCULATION 167 



for example, during sudden muscular effort and if the 

 heart tends to be overwhelmed by it, the depressor 

 fibres of the vagus come into action, and the vasomotor 

 centre is reflexly inhibited, with the result that the 

 blood-pressure suddenly falls and the heart is relieved. 



Seeing that such efficient arrangements exist for 

 counteracting abnormally high blood-pressure, it is 

 difficult to understand how a continuously abnormal 

 degree of tension can exist unless there be some de- 

 rangement of the compensating mechanism. Abnormal 

 lowering of the blood-pressure is not so easily compen- 

 sated for, but it is noteworthy that anaemia of the 

 medulla stimulates the general vasomotor centre, and 

 in this way a fall of blood-pressure from haemorrhage 

 is counteracted. When the pressure is low, too, the 

 action of the heart is more rapid, and the total output 

 of blood from it in a given time greater, and this also 

 tends to raise the pressure again towards the normal 

 level. 



Distribution of Blood in the Body. 



Intimately associated with blood - pressure is the 

 question of the distribution of the blood in the body. 

 We have seen that the existence of blood-pressure is in 

 part the result of the necessity for accommodating the 

 vascular 'bed' to the volume of the circulating blood. 

 The capacity of the vascular system is so great that it 

 could accommodate one-half or one-third as much blood 

 as it already contains, and the volume of the blood can 

 be increased by this amount without the pressure in the 

 aorta being affected, for the vessels simply relax in order 



