1912.] Resilience of Arterial Wall on Blood-Pressure. 185 



walls, it is obvious that, the more resilient or yielding are those supplying 

 any part, the more closely will the blood stream at the threshold of the 

 capillary area supplied approach a uniform pressure (roughly the mean 

 between the systolic and diastolic pressures, less, of course, what has been 

 lost by friction), while the harder or less resilient the arterial wall, the 

 more closely will the variations approach those in the aorta. 



Now all the arteries, and to a greater extent the arterioles, are contractile, 

 and under the influence of the nervous system, and with an increased tone 

 or contraction, they become not only narrower as to lumen, but also thicker 

 as to wall, that is less resilient, so it may happen that the organism can with 

 the same heart force vary the pressure at the threshold of a particular 

 capillary area between an intermittent pressure with a high systolic beat 

 and an almost continuous one, with a lower systolic pressure. In the one 

 case there would be a hammer-like percussive wave beating open the 

 capillaries, the blood would be hammered in ; in the other there would be a 

 more continuous pressing in of the blood at a lower tension. 



It may be that narrowing of lumen and lessened total flow goes with the 

 more percussive wave due to hardening, but this does not necessarily follow, 

 for it is possible that a tightening of the muscular coat, and a lessening of 

 the resilience, may take place before actual narrowing occurs. It is further 

 possible that the great arteries and the arterioles act differently, or 

 independently in some cases. 



We advance the view that the throbbing and capillary pulse observed in 

 an acutely inflamed area is due to an increased tone of the arterial walls, a 

 lessening of their resilience ; this throbbing is often relieved by hot fomenta- 

 tions which act by relaxing the contraction of the vessel walls. In cases of 

 aortic regurgitation the hammer-like pulse propelled through the harder leg 

 arteries secures to the legs an adequate supply of blood, compensating as it 

 does for the diastolic fall due to the regurgitation ; there, again, hot water 

 baths relax the arterial wall. 



One of us, L. H., with Martin Flack,* has shown that in the case of the 

 salivary gland each alveolus is surrounded by a tough membrana propria 

 which resists expansion and allows the secreting cells to draw fluid from the 

 capillaries and raise the secretory pressure to almost double the height of 

 the arterial blood-pressure, without obliterating the surrounding capillaries 

 or interrupting the venous outflow. Under such conditions the veins are 

 narrowed, by the expansion of the alveoli up to their limiting membranes, 

 and the blood vessels, arteries, capillaries, and veins form a system of rigid 

 vessels with a rapid rate of flow, the pulse even coming through into the 

 * 1 Roy. Soc. Proc..' 1912, B, vol. 85, p. 312. 



