Hales, the pressure may be considered as equivalent to a column of
seven feet. In order to calculate the resistance, the author employs
the theorems contained in his former communication, and adopts the
measurements of Keill and others, for the diameters of the aorta, and
of its successive subdivisions. The quantity of blood in the arteries
is estimated at nine or‘ten pounds; its velocity in the aorta about
eight inches and a half in a second ; while that in the capillary arte-
ries is about fird of an inch in a second (the diameter of these vessels being about “Anyth of an inch). The resistance that would be
opposed to water circulating under the same circumstances is calculated to be equivalent to a pressure of a column of twenty inches;
but the resistance to the motion of the blood is supposed in consequence of its viscidity to be about four times as great, and is consequently stated as eighty inches.
The effects of curvature in increasing the resistance are not neglected, but they form a very small part, in comparison to the difference that would arise from assuming different dimensions for the arterial system; or different allowance for the resistance of vessels that are too small for direct experiment, or a different ratio for the assumed effect of viscidity.
The author next examines the nature and velocity of the propagation of the pulse, which he considers analogous to the motion and waves on the surface of Water, or of sound transmitted through the air; the elasticity of the arteries being, in this case, substituted for the elasticity of the fluid. Since‘the blood in the human arteries is subjected to a pressure, which is measured by a column of about seven feet and a half, the velocity with which the impulse is transmitted will be about fifteen feet and a half in a second; but as the progressive motion of the blood itself is about eight inches in the same time, the aggregate velocity of a pulsation is considered as sixteen feet in a second.
The greatest velocity of the blood during the contraction of the heart being about one eighth part of that quantity, the area of the artery must be proportionally dilated during its passage, and the diameter must increase in the ratio of fifteen to sixteen.
The force of the heart necessary to occasion this distension, must be proportionally greater than the average, and must be equal to a column of 101 inches, which agrees extremely well with an experiment of Hales. on the ascent of blood in a tube connected with the artery of a horse.
The author acknowledges, however, that though the calculations agree perfectly with each other, and with experiments on the power of the heart, and affections of the smaller arteries, yet they do not correspond With the apparent alteration in the diameter of an artery exposed to view; and he infers that the velocity of the pulse in the larger arteries, is really much more considerable than has been stated.
With respect to the functions of the muscular fibres of the arteries, Dr. Young apprehends that it will appear demonstrable that they are much less concerned in the motion of the blood than is almost uni-
