This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Pulse In The Arteries is caused by their distension and elongation under the pressure exerted by the rush of blood with each beat of the heart, but can only be felt in those positions in which an artery lies near some Arm structure against which it can be pressed. With the aid of instruments it can be shown that it is communicated with great rapidity to the whole arterial system, the smallest arteries pulsating within a sixth of a second after the largest. The blood already in the arteries is pushed on by each new quantity thrown in from the heart; the velocity with which the blood travels being far less than that with which the pulse is communicated. The actual rate of movement of the blood can be observed in the arteries of animals by the insertion of an instrument for the purpose between the ends of a divided vessel, and it is calculated from such experiments, that in man the mean velocity is about ten inches per second in the carotid, and about two and a quarter in the foot, the rate of flow being much slower in small arteries than in large. The reason why it is slower is to be found in a peculiarity in the branching of arteries; for in almost all instances in which an artery divides, the united areas of the divisions are greater than the area of the parent trunk; and, consequently, the total area of the combined arterial channels increases rapidly the farther the distance from the heart.
While the frequency of the pulse corresponds with that of the heart's contractions, its character depends on a variety of circumstances, of which the chief are the amount of blood in the body, the vigour and regularity of the heart's action, and the degree to which the muscular fibres of the arterial wall are contracted, and so offer resistance to the heart. Some of the peculiarities of the pulse, which cannot be appreciated by pressure of the finger on the wrist, are exhibited with the aid of the sphygmograph (Marey), an instrument which is fastened to the wrist, and in which a Spring, pressed against the radial artery, causes a light lever, carrying a pen, to move up and down. The pen is in contact with a slip of paper or smoked glass set in motion by clock-work, and produces a tracing which indicates the pulsations by elevations, and the element of time by horizontal distance. Such a tracing shows, that in health the distension of the vessel takes place with almost instantaneous suddenness, commencing and finishing abruptly; while the gradual character of the recoil is shown by its making a long sloping line. When the arterial resistance is great, as it is in the most robust health, it counteracts the distending impulse given by the heart, so that the rise of the tracing is not so considerable as it would otherwise be; and in these circumstances there is a moment's continuance of the distension, then a gradual and but slightly undulating descent. But when the arterial resistance is slight, the secondary distending impulses given by the elastic recoil of the larger vessels produce more effect on the tracing, and one particular rise becomes prominent, which appears to be caused by the walls of the commencement of the aorta, redistended by the blood thrown back on the aortic valves, again recoiling. Such a pulse is said to be dicrotous.
Fig. 68. Sphygmographic Tracings of the pulses of three persons all healthy. In 1, the arterial resistance is greatest; in 2, the dilatation of the vessel has taken place with such force as to jerk the lever of the instrument from its rest, and hence the sharp points at the tops of the waves; 3, is a distinctly dicrotous pulse.