THE BLOODVESSELS 67 



If a manometer be placed on the venous side of the model, 

 it will show a very low pressure at the time when the arterial 

 pressure is high. If the arterial tubes be felt it will be observed 

 that at each stroke of the pump they expand, producing what 

 is known in living tubes as the pulse ; this expansion of the tube 

 is greatest nearest to the syringe, dying out entirely at the 

 peripheral resistance. It is evident that if we loosen the clamps, 

 and so reduce the resistance and lower the mean pressure, that 

 pulsatile waves will pass over to the venous side of the model, 

 and these can again be obliterated by screwing up the clamp. 

 Lastly, our model, if working at mean pressure, will show the 

 effect of injury to the arterial tubes ; if these be pricked, a con- 

 tinuous jet of water shoots out, the strength of the jet varying 

 with each stroke of the syringe, whilst an injury to the venous 

 side produces no jet of water, but only a trickling flow. 



Practically this embraces our knowledge of the main facts of 

 the circulation, for all we have found true of syringe, elastic 

 tubes, and clamps, will be found true of heart, bloodvessels, and 

 peripheral resistance. The heart has to keep the arteries full ; 

 the innumerable smaller arteries with their muscular coat supplv 

 the peripheral resistance. Under the influence of this and the 

 contraction of the left ventricle, the pressure in the arteries rises 

 so high, and their distension is so great, that as much blood 

 passes through the periphery during the contraction of the 

 heart and the ensuing pause as enters the aorta during the 

 contraction of the left ventricle. The elastic sj*stem of arteries 

 insures that an intermittent is converted into a continuous flow, 

 and thus a perpetual pressure is kept up on the mass of blood 

 during the heart's pause. By a contraction of the arterioles 

 the peripheral resistance is increased and the blood-pressure 

 raised ; by a relaxation of the arterioles the peripheral resistance 

 is reduced and the blood-pressure falls. We have stated that a 

 contraction of the arterioles by increasing the resistance raises 

 arterial pressure and as a rule lowers that in the veins. This 

 holds equally true for the pressure conditions in the vessels of 

 any locally circumscribed area of the body as for the vascular 

 system generally. It must not, however, be forgotten that 

 local effects may and do produce general effects. If, for instance, 

 one artery alone contracts, this must lead to an increase of arterial 

 pressure, which produces an increased flow of blood through all 

 the simultaneously uncontracted arteries on into the veins. 

 When the contracted artery is small, so that the area it supplies 

 is limited, the local effects are more marked than the general 

 effects. If, on the other hand, the local area affected is at all 

 large, the influence of changes in the arteries of this area on the 

 general blood-pressure may be very obvious. We shall meet 



