CHAP, iv.] THE VASCULAR MECHANISM. 131 



I It cannot be too much insisted upon that the whole arterial 

 system is over-distended. This is what is meant by the higlr 

 arterial pressure. On the other hand, the veins are much less 

 distended. This is shewn by the low venous pressure. The dis- 

 tended arteries are continually striving to pass their surplus in 

 a continuous stream through the capillaries into the veins, so as 

 to bring both venous and arterial pressure to the same level. As 

 continually the heart by its beat is keeping the arteries distended, 

 and thus maintaining the difference between the arterial and 

 venous pressure, and thus preserving the steady capillary stream. 

 When the heart ceases to beat, the arteries do succeed in emptying 

 their surplus into the veins, and when the pressure on both sides of 

 the capillaries is thus equalized, the flow through the capillaries 

 ceases. 



In the facts just discussed, it makes no essential difference 

 whether the outflow on the far side of the resistance be an open 

 one, or whether, as is the case in the vascular system, the fluid be 

 returned to the pump, provided only that the resistance offered to 

 that return be sufficiently small. We shall see, in speaking of the 

 heart, that, so far from there being any resistance to the flow of 

 blood from the great veins into the auricle, the flow is favoured by 

 a variety of circumstances. We have seen moreover that, besides 

 the very sudden decrease in the immediate neighbourhood of the 

 capillaries, there is in passing along the whole vascular system from 

 the aorta to the venae cavse a gradual fall of pressure. A little 

 consideration shews that this must be the case. After what has 

 been said it is obvious that the movement of the blood may be 

 compared to that of a body of fluid, driven by pressure from the 

 ventricle through the vessels to its outflow in the auricle. Were the 

 pressure a continuous one, and were there no peripheral resistance, 

 there would be a gradual fall of pressure, from the part farthest 

 from the outfall, viz. the aorta, to the part nearest the outfall, viz. 

 the venaa cavae. The introduction of the peripheral resistance and 

 its attendant phenomena gives rise to the feature of a very sudden 

 and marked fall in the capillary region, but leaves untouched the 

 gradual character of the fall in the rest of the course, from the 

 aorta to the minute arteries, and from the minute veins to the 

 venae cavae. 



To recapitulate : there are three chief factors in the mechanics 

 of the circulation, (1) the force and frequency of the heart-beat, (2) 

 the peripheral resistance, (3) the elasticity of the arterial walls. 

 These three factors, in order to produce a normal circulation, must 

 be in a certain relation to each other. A disturbance of these 

 relations brings about abnormal conditions. Thus, if the peripheral 

 resistance be reduced beyond certain limits, while the force and 

 frequency of the heart remain the same, so much blood passes 

 through the capillaries at each stroke of the heart that there is 

 not sufficient left behind to distend the arteries, and bring their 



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