315 



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 -g-Vrd of an inch in a second (the diameter of these ves- 

 sels being about T1 ' 00 th of an inch). The resistance that would be 

 opposed to water circulating under the same circumstances is calcu- 

 lated to be equivalent to a pressure of a column of twenty inches ; 

 but the resistance to the motion of the blood is supposed in conse- 

 quence of its viscidity to be about four times as great, and is conse- 

 quently stated as eighty inches. 



Theeffectsof curvature inincreasing 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 sy- 

 stem ; 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 propa- 

 gation 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 progres- 

 sive 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 di- 

 ameter 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 experi- 

 ment 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- 



