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 prima- 

 rily, of course, upon the beat of the heart, but this only in- 

 directly 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 circum- 

 stances; for, as blushing and pallor of the face (which fre- 

 quently 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 pressure has been ascertained by experi- 

 ments made upon the lower animals, from which deductions 

 are then made concerning what happens in man, since An- 

 atomy shows that the circulatory organs are arranged upon 

 the same plan in all the mammalia. A great deal can, how- 

 ever, be learnt by studying the flow of liquids through ordi- 

 nary elastic tubes. Suppose we have a set of such (Fig. 98) 

 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 that its elastic walls are slightly stretched. 

 These will in consequence press upon the liquid inside them 

 and the amount of this pressure will be indicated by the 

 gauges; so long as the pump is at rest it will be the same 

 everywhere (and therefore equal in the gauges on B and A), 



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