If any factor fail, the mechanism of the circulation is at once impaired. For example, the heart's beat may be stopped by the stimulation of the inhibitory nerve fibres of the vagus, in which case the blood pressure rapidly falls, as shown by the curve taken by the graphic method. Or weakness of the heart beat may arise from disease (fatty degeneration) of the muscle, when signs of low arterial tension can be recognized in the human subject.

Mercurial Manometer for measuring and recording the blood pressure.

Fig. 131. Mercurial Manometer for measuring and recording the blood pressure.

(a) Proximate limb of manometer, (b) Union of two limbs of manometer, (c) The rod floating on mercury and carrying the writing point, (d) Stop-cock through which the sodium bicarbonate can be introduced between the blood and mercury of manometer.

Valves

Any insufficiency of the aortic valves that permits the blood to flow backward into the ventricle, allows the arterial pressure to fall between each ventricular systole, and gives rise to the characteristic "pulse of unfilled arteries," as it is called by the physician.

Elasticity Of Arteries

The resiliency of the arterial coats may also be destroyed to a certain extent by degeneration of the tissue, in which case the large arteries become greatly distended, and unable to exert their normal steady pressure on the blood.

The ordinary modern form of rotating blackened cylinder (R).

Fig. 132. The ordinary modern form of rotating blackened cylinder (R), which is moved by clockwork in the box (A) by means of the disc (D) pressing upon the wheel (n), which can be raised or lowered by the screw (L), so as to come in contact with any part of the disc more or less near the centre, and thus rotate at different rates. The cylinder can be raised by the screw (v), which is turned by the handle (U). {Hermann).

Contractile Arterioles

Injuries of the nervous centres are often associated with paralysis of the muscular arterioles and fall of blood pressure; but the effect upon the blood pressure of dilatation of the small arteries can be best seen by experimenting on the nerves that control their contraction. If paralysis or inhibition of the vasomotor mechanisms be experimentally produced, the result on the arterial pressure is the same, a sudden fall, which may reach that of the atmosphere. The chief opposition to the outflow of blood from the arteries being removed, they cease to be tense, even though the ventricle continue to beat and pump the blood into them.