If the blood pressure be recorded with Ludwig's Kymograph, a tracing will be obtained which shows that the pressure undergoes periodic elevations and depressions of two different kinds. The smaller oscillations are found to correspond with the heart beat, the larger waves have the same rhythm as the respiratory movements, and the average elevation of the mercurial column is spoken of as the 7nean pressure. In the large arteries of the warm-blooded animals this mean pressure varies with the size of the animal from 90 mm., mercury, to more than 200 mm. In cold-blooded animals it is comparatively low, from 22 mm. in the frog (Volkmann) to 84 mm. in a large fish.

The general mean pressure in the arteries is increased by (1), increased action of the heart; (2), increased contraction of the muscular coat of the arteries; (3), sudden increase in the quantity of blood. When the change is gradual, the vessels adapt themselves to the increase.

The opposite of these conditions may be said to have a contrary effect.

The character of the change in pressure which accompanies the heart's systole is not shown exactly in the tracing obtained by the mercurial manometer, owing to the sluggishness of the movement of the mercurial column, which, as it were,'rubs off the apices of the curves. But with the spring manometer of Fick, the details of these oscillations are marked. They are of course synchronous with the arterial pulse, and follow the variations of tension, as will be described when treating of that subject. (See Figs. 136 and 137).

Blood pressure Curve, drawn by mercurial manometer.

Fig. 135. Blood-pressure Curve, drawn by mercurial manometer. O - x = zero line, y - y = curve with large respiratory waves and small waves of heart impulse. A scale is introduced to show height of pressure.