SEC. 9. A REVIEW OF SOME OF THE FEATURES OF 



THE CIRCULATION. 



§ 165. The facts dwelt on in the foregoing sections have 

 shewn us that the factors of the vascular mechanism may be 

 regarded as of two kinds: one constant, or approximately constant; 

 the other variable. 



The constant factors are supplied by the length, natural bore, 

 and distribution of the blood vessels, by the extensibility and 

 elastic reaction of their walls, and by such mechanical contrivances 

 as the valves. By the natural bore of the various blood vessels is 

 meant the diameter which each would assume if the muscular 

 fibres were wholly at rest, and the pressure of fluid within the 

 vessel were equal to the pressure outside. It is obvious, however, 

 that even these factors are only approximately constant for the 

 life of an individual. The length and distribution of the vessels 

 change with the growth of the whole body or parts of the body, 

 and the physical qualities of the walls, especially of the arterial 

 walls, their extensibility and elastic reaction change continually 

 with the age of the individual. As the body grows older, the once 

 supple and elastic arteries become more and more stiff and rigid, 

 and often in middle life, or it may be earlier, a lessening of arterial 

 resilience which proportionately impairs the value of the vascular 

 mechanism as an agent of nutrition, marks a step towards the 

 grave. 



The chief variable factors are on the one hand the beat of the 

 heart, and on the other the peripheral resistance, the variations in 

 the latter being chiefly brought about by muscular contraction or 

 relaxation in the minute arteries, but also, though to what extent 

 has not yet been accurately determined, by the condition of the 

 minute vessels according to which the blood can pass through 

 them with less or with greater ease, as well as by the character 

 of the circulating blood. 



These two chief variables, the beat of the heart and the width 

 of the minute arteries, are known to be governed and regulated by 

 the central nervous system, which adapts each to the circumstances 



