MOVEMENT OP THE BLOOD IN THE ARTERIES. 487 



elasticity is to distribute the pressure of the blood upon the walls of the arteries, 

 much more equally than would be the case if they formed a system of rigid 

 tubes. For, according to Volkmann, 1 since the lateral pressure of a liquid 

 moving through tubes of uniform caliber with rigid walls, is proportional to the 

 resistance to be overcome at each point, and since this resistance depends upon 

 the adhesion and friction between the liquid and the parietes of the tube, the 

 lateral pressure at each point will vary inversely with the distance of that point 

 from the discharging orifice. Consequently, if the arteries constituted a system 

 of rigid tubes, the pressure on their walls would decrease very rapidly in passing 

 from the heart towards their peripheral extremities. Such, however, is far from 

 being the case ( 519). 



517. The distension of the Arteries consequent upon the intermittent injec- 

 tion of blood into their trunks, and the subsequent contraction which results from 

 the elasticity of their walls, give rise to the pulsation which is perceptible to the 

 touch in all but the smallest arteries, and is visible to the eye when they are 

 exposed. This pulsation involves an augmentation of the capacity of that por- 

 tion of the artery in which it is observed; and it would seem to the touch as if 

 this were chiefly effected by an increase of diameter. It seems fully proved, 

 however, that the increased capacity is chiefly given by the elongation of the 

 artery, which is lifted from its bed at each pulsation, and, when previously 

 straight, becomes curved; the impression made upon the finger by such displace- 

 ment not being distinguishable from that which would result from the dilatation 

 of the tube in diameter. A very obvious example of this upheaval is seen in 

 the prominent temporal artery of an old person. The total increase of capacity 

 was estimated by Flourens, from experiments upon the carotid artery, at about 

 l-23d part; but it is affirmed by Volkmann (Op. cit., xiv.) that this must not 

 be considered by any means a constant ratio, since it varies in different arteries 

 and in the same artery under different circumstances. 3 The distension of the 

 arteries does not take place at the same moment over the whole body, but is 

 propagated as a wave from the commencement to the point of discharge. The 

 passage of this wave was considered by Prof. E. H. Weber to be distinct from 

 the act of propulsion of the fluid; but it has been shown by Volkmann (Op. 

 cit., chap, x.) that they are one and the same. He has further shown that two 

 systems of waves arise, when a fluid is driven through an elastic tube by inter- 

 mitting impulses; one of these being in the fluid, and the other in the walls of 

 the tube. These may propagate themselves with different velocities, and thus 

 two undulations may result from one impulse. This want of coincidence be- 

 tween the two waves is probably the explanation of the dichrotous pulse, often 

 observable in convalescence from fevers and other diseases after the subsidence 

 of vascular excitement. That a certain time is required for the transmission of 

 the pulse-wave from the heart to the periphery of the circulation, is proved by 

 the want of synchronism between the ventricular systole and the pulsation of 

 the arteries in various parts of the body, the difference varying according to their 

 distance from the heart. A considerable diversity in the amount of this interval 



1 "Hamodynamik," p. 38. 



2 The experiments of Volkmann have led him to believe that the transverse dilatation 

 is greater than the longitudinal ; but these experiments were made under conditions so 

 different from those of the living artery, that but little weight can, in the Author's opinion, 

 be attached to them. It is to be remembered, however, that every increase in length 

 augments the capacity only in a simple ratio ; thus a tube of 21 inches in length will only 

 contain one-twentieth more than a tube of twenty inches long, of the same diameter. On 

 the other hand, every increase in diameter augments the capacity of the tube in the ratio 

 of the square of that increase; thus the capacity of a tube of 21 lines in diameter will be 

 to that of a tube of 20 lines as 441 : 400, or one-tenth more. Consequently, supposing the 

 increase of capacity to take place equally in both directions, the increase in longitudinal 

 dimension will be far more apparent than the transverse enlargement. 



