238 THE VASCULAR MECHANISM. 



between a high blood-pressure and fulness of blood or plethora, since an 

 enormous quantity of blood may be driven into the vessels without any 

 marked rise of pressure. 



A REVIEW OF SOME OF THE FEATURES OF THE CIRCULATION. 



173. The facts dwelt on in the foregoing sections have shown 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 distri- 

 bution of the bloodvessels, by the extensibility and elastic reaction of their 

 walls, and by such mechanical contrivances as the valves. By the natural 

 bore of the various bloodvessels 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 toward the grave. The 

 valvular mechanisms, too, also show 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 deter- 

 mined, by the condition of the walls 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. 



174. 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 cen- 

 tral nervous system, which adapts each to the circumstance of the moment, 

 and at the same time brings the two into mutual interdependence ; but the 

 central nervous system is not the only means of government ; there are other 

 modes of regulation, and so other safeguards. 



Thus while undoubtedly the two prominent governors of the heart are 

 the inhibitory fibres of the vagus and the augmentor fibres from the splanch- 

 nic system, the one slowing the rhythm and weakening the stroke, the 

 other quickening the rhythm and strengthening the stroke, other causes may 

 vary the beat in the absence of any action of either of these two nerves. 

 Mere distention of the ventricle, by increasing the tension of the ventricular 

 fibres, and so increasing the force of the contraction of each fibre (see 

 148), brings about a more forcible beat. As we shall see in dealing with 

 respiration, a powerful inspiration leads to a larger flow of blood into the 

 heart, and forthwith the ventricle, out of its very fulness, gives stronger 

 beats for the time. So also when by valvular disease or otherwise an un- 

 usual obstacle is presented to the outflow from the ventricle, increased vigor 

 in the strokes of the distended organ strives to compensate the mischief. As, 

 however, in the case of the skeletal muscle, the tension, if too great, may be 

 injurious. In a similar manner the auricle, by a stronger or a weaker con- 



