536 The Measurement of Arterial Pressure in Man. 



2. A schema has been constructed by us in which artery, tissue vessels, 

 and vein are represented. 



3. This schema demonstrates some of the principles which govern the 

 circulation in the brain or other encapsulated organ, and the effect of 

 compression upon circulation in such organs. 



4. Two such schemata arranged in series imitate the conditions which 

 pertain respectively in the arm and forearm, and enable us to demonstrate 

 the effects of compression, and in particular the conserving effect on the 

 pulse of the tissue vessels. 



5. The schemata demonstrate and elucidate these well-known clinical facts : 

 (1) that the pulse may reappear on decompression at a lower pressure than 

 it disappears on compression ; (2) that the radial pulse is reinforced when the 

 compressive force applied to the upper arm is below the diastolic pressure. 



6. The schema demonstrates (1) that the maximal pulse occurs when the 

 diastolic pressure is just overtopped by the compressive force, and is a good 

 index of diastolic pressure ; (2) that the diastolic pressure is raised towards 

 the systolic pressure in proportion as the peripheral resistance is increased 

 by compression or obstruction of venous outflow ; (3) that this rise of 

 pressure occurs throughout arteries, tissue vessels, and veins ; (4) that the 

 pulse is damped down by the lability of the vessel wall, and it is owing to 

 lability that the pulse on decompression returns at a lower pressure level 

 than it disappears on compression. 



7. The explanation of the difference which pertains between the arm and 

 leg systolic readings, taken in the horizontal posture, in cases of aortic 

 regurgitation, is to be sought in the difference of the lability of the artery 

 and the conditions of the peripheral circulation. The upper limb, as a whole, 

 is more labile than the leg. In the normal person the difference of lability 

 is brought out by exercise, which produces a big systolic wave. The leg is, 

 so to speak, a tighter drum-head, and responds better to the bigger stroke. 



It must always be borne in mind that the support of the column of blood 

 in the artery is not only formed by the tissues of the arterial wall, but also 

 by the surrounding tissues of the whole limb. 



