GENERAL PRINCIPLES OF THE CIRCULATION. 



73 



obtained by equal increments of tension from such a strip is hyperbolic 

 in form. 1 



The variations in the volume of an artery examined under different 

 degrees of internal tension has been measured by Marey, 2 Roy, 

 and others. This can be done by the apparatus shown in 

 Fig. 47. 



In healthy arteries, placed under observation immediately after 

 death, the volume by equal increments of internal tension will be found 

 to increase by equal 

 increments up to a 

 certain point. Under 

 continued increase of 

 tension the extensi- 

 bility decreases, or, 

 in other words, the 

 coefficient of elasticity 

 increases. Thus the 

 elasticity curve of a 

 healthy artery is a 

 true hyperbola. In 

 the rabbit's aorta the 

 maximum extensibil- 



FIG. 47. R, pressure bottle ; a, artery ; M, plethysmo- 

 graph. Marey. 



ity, or the point at which the change to a decreased ratio of extensibility 

 takes place, is reached at about 70 mm. Hg; in the cat's, at 110 to 120 

 mm. Hg. In the living dog, by measurements of the diameter, the 

 maximum extensibility of the aorta has been found at a pressure of 75 

 to 100 mm. Hg. 3 In many arteries, taken from the post-mortem room, 

 the extensibility is found to be greater at lower increments of tension. 

 This is the case with all wasting diseases ; and although, on microscopical 

 examination, no structural change is to be found, yet the more advanced 

 the marasmus preceding death, the lower is the pressure which corre- 

 sponds to the maximum extensibility of the arteries. With the advance 



FIG. 48. The height between the horizontal lines represents extension 

 of artery under different pressures. The pressures are noted at the 

 end of the lines. The difference in length of the lines is merely for 

 the convenience of placing the numerals. Roy. 



of age the arteries become less and less extensile. When the arterial 

 pressure is high, it follows from the above fact that any increase in the 



1 Wertheim, Ann. de cMm. etphys., Paris, 1847, tome xxi. p. 385. 



2 "Trav. du labor, de Marey," 1880, tome iv. p. 78 ; Roy, Journ. PhysioL, Cambridge 

 and London, 1881, vol. iii. p. 125; 1888, vol. ix. p. 227. 



3 Zwaardemaker, Nederl. Tijdschr. v. Geneesk., Amsterdam, Tweede Reeks, 1888, vol. 

 xxiv. p. 61. 



