74 THE MECHANISM OF THE CIRCULATION. 



output of the heart must increase the blood pressure by a very signi- 

 ficant amount. The arteries reach their maximum extensibility at the 

 normal blood tension, 90 to 120 mm. Hg. Beyond this point the 

 arteries become more and more rigid, and the heart must then at each 

 beat push on a longer and still longer column of blood. 



The elasticity of an artery is very complete. On removing the 

 tension the volume returns exactly to its previous condition. On raising 

 the internal tension up to the normal blood pressure the cubic volume 

 of an artery very greatly increases, e.g. four to six times. 



If the heart-beat be brought to a standstill by vagus inhibition, the 

 arteries almost entirely empty themselves of blood into the veins. 



When an artery is dilated by an injection of fluid, it reacts, accord- 

 ing to John Hunter, with a force greater than that of the injection 

 pressure. 1 This resilience is much greater in the living artery than in 

 the dead artery. It is probable that when the heart, on vagus inhibi- 

 tion, ceases to beat, the arteries empty themselves by contraction of 

 their muscular coat. Some arteries exhibit a propulsive wave of con- 

 traction, as is seen in the contractile bulbus arteriosus of amphibia and 

 some fishes. The same phenomenon is to be seen in the rhythmic con- 

 tractility of the arteries of the ear, in the saphenous artery of the rabbit, 

 and in the vessels of the frog's web. 2 



The breaking strain of a healthy artery is very great. In some 

 experiments of Volkmann, the carotid of a goat successfully withstood a 

 pressure of 2250 mm. Hg, i.e. about fourteen times the normal blood 

 pressure. It takes an internal pressure of 3000 to 8500 mm. Hg to 

 rupture the carotid artery of a dog. In the case of the human carotid 

 the smallest rupturing pressure was found to be 1290 mm. Hg. The 

 larger arteries rupture more easily than the smaller, and thus the aorta 

 breaks asunder at a lower tension than the radial. 3 



According to Nicolls, to attain the greatest strength with a given 

 amount of material, a cylinder should be constructed of layers the 

 elastic coefficient of which gradually increases from within outwards. 

 Now, this is exactly the arrangement of the different layers of the 

 arterial wall ; thus the greatest strength is attained by the smallest 

 amount of material consistent with safety. Similar reasoning applies 

 to the walls of a sphere, from which it appears that the main source of 

 weakness in an aneurysm is, that the wall practically is composed of only 

 one coat. 4 As the sac enlarges the total strain increases as the cube of 

 the radius of curvature. 



The influence of the amount of blood in the body on the systolic 

 output and arterial pressure. — The average amount of blood in the 

 body, estimated by Welcker's method, can be taken as ecp^ivalent to 

 6 to 9 per cent, of the body weight in dogs, 5 to 9 per cent, in cats, 4 to 

 8 per cent, in rabbits. In man it has been estimated as equal to 7 per 

 cent, of the body weight. 5 On transfusion of fluid, the striking fact 

 becomes apparent, that the arterial tension cannot possibly be raised by 

 such means above that height which the tension can reach when the 

 vessels contain a normal amount of blood. This was so even when 



1 "Works of John Hunter," London, 1837, vol. iii. p. 157. 



2 Riegel, Arch. f. d. ges. Physiol., Bonn, 1871, Bd. iv. S. 350. 



3 Stephen Hales, "Statical Essays," vol. ii. ; Grtshant and Quinquaud, Journ. de Vanat. 

 et physiol., etc., Paris, 1885, tome xxi. p. 287. 



4 Nicolls, Journ. Physiol., Cambridge and London, 1896, vol. xx. p. 410. 



5 Bischoff, Ztsehr.f. wissenselt, Zool., 1856, Bd. vii. S. 331. 



