360 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



The early determinations of the elasticity of the blood-vessels, 

 were made upon excised segments which were suspended from a 

 hook and loaded with different weights. In as much as the curve 

 obtained by this method resembled a hyperbole, the conclusion was 

 drawn that the coefficient of the elasticity does not possess a constant 

 value but increases with the distention. It seems, however, that the 

 degree of distensibility obtained under this condition, is not comparable 

 to the distensibility produced by an internal 

 pressure, but merely gives us an idea regarding 

 the compactness or strength of the blood- 

 vessels. Mareyi sought to establish more 

 perfect experimental conditions by placing seg- 

 ments of arteries in plethysmographs and by 

 subjecting their walls to a steady internal pres- 

 sure. This end he attained by connecting the 

 lumen of the segment with a bottle filled with 

 saline solution which he could raise to a certain 

 level above the preparation. Roy^ and others 

 state that a steadily rising pressure leads to a 

 gradual increase in the caliber of the blood- 

 vessel, but a limit is soon reached beyond which 

 the distention diminishes very rapidly. In 

 rabbits the normal distensibility is reached with 

 a pressure of 70 mm. Hg, in dogs at 75 to 125 

 mm. Hg, and in the ox at 100 to 150 mm. Hg. 

 A much higher pressure is required to cause a 

 , normal artery to rupture. In accordance with 



RANGEMENT FOR TESTING the determinations of Grehant and Quinquaud,* 

 THE Elastic Power op the carotid artery of the dog can withstand an 

 Blood-vessels. internal pressure of 600 mm. Hg, while the lowest 



pende^d InT gfass'%X Pressure necessary to burst the carotid artery of 

 fiUed with saline solution man amounts to 1.29 m. Hg. As the smaller 

 (T). Its ends are closed arteries are even stronger than the larger ones 



with discs of rubber and , ,, , • i i i • i 



its lumen connected with and as the arterial pressure seldom rises above 

 a pressure bulb (B). The 150 mm. Hg, the margin of safety is more than 



meniscus of the saline solu- Qjvjr^lp 



tion M in tube C indicates K ' i e • i i • 



the degree of distention of It IS also 01 interest to note that the opti- 

 the artery. mum degree of movability of the vessel wall is 



had at a pressure most closely approaching the 

 normal. At this time the most perfect elastic play is obtained. If the 

 pressure is raised much beyond this point, the distensibility becomes less 

 and less. Supposing, therefore, that the quantity of the circulating 

 blood is increased, the power of the vascular system to accommodate 



1 Trav. de Lab., iv, 1880, 253. 



2 Jour, of Physiol., iii, 1881, 125; also see: Zwardemaker (Neterl. Tijdschr. vor 

 Jencesk., xxiv, 1888, 61), and Frank (Ann. der Physik., 1906). 



3 Jour, de I'anat. et de la Physiol., xxvi, 1885. 



