CHAPTER XVI. 



ARTERIAL PRESSURE. THE PULSE. 



Weber's Schema. It is clear from the statements made 

 in the last chapter that it is the pressure exerted by the elas- 

 tic arteries upon the blood inside them which keeps up the 

 flow through the capillaries, the heart serving to keep the 

 big arteries tightly filled and so to call the elastic reaction of 

 their walls into play. The whole circulation depends 

 primarily of course upon the beat of the heart, but this 

 only indirectly governs the capillary flow, and since the 

 latter is the aim of the whole vascular apparatus it is of 

 great importance to know all about arterial pressure; not 

 only how great it is on the average but how it is altered in 

 different vessels in various circumstances so as to make the 

 flow through the capillaries of a given part greater or 

 less according to circumstances; for, as blushing and pallor 

 of the face (which frequently occur without any change in 

 the skin elsewhere) prove, the quantity of blood flowing 

 through a given part is not always the same, nor is it 

 always increased or diminished in all parts of the Body at 

 the same time. Most of what we know about arterial pres- 

 sure has been ascertained by experiments made upon the 

 lower animals, from which deductions are then made con- 

 cerning what happens in man, since anatomy shows that the 

 circulatory organs are arranged upon the same plan in all 

 the mammalia. A great deal can, however, be learnt by 

 studying the flow of liquids through ordinary elastic tubes. 

 Suppose we have a set of such (Fig. 87) supplied at one 

 point with a pump, c, possessing valves of entry and exit 

 which open only in the direction indicated by the arrows, 

 and that the whole system is slightly overfilled with liquid so 



