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



THE CIRCULATION. 



187. 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 contri- 

 vances 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 valvular mechanisms too also 

 shew signs of age, as years advance, and more or less marked 

 and increasing imperfections diminish the usefulness of the 

 machine. 



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 

 walls of the minute vessels according to which the blood can pass 



