Page:The American Cyclopædia (1879) Volume IV.djvu/620

 608 CIRCULATION physician estimates the rapidity of the heart's action. As soon, howerer, as the cardiac con- traction which caused the dilatation of the arteries comes to an end and is followed by relaxation, then the elastic reaction of the arteries themselves, compressing the blood, drives it onward toward their terminal ramifi- cations and into the capillary system. Under these circumstances the blood would be partly forced backward into the relaxed ventricle, were it not for three thin but strong membra- nous valves which guard the orifice of the aorta at its junction with the heart, and which shut backward at the instant of the heart's relaxa- tion, and thus prevent any backward regurgi- tation. Thus the blood, forced into the arte- ries by the muscular power of the heart, is driven onward to the capillaries by the elastic force of the arteries themselves. Neverthe- less the heart is the essential cause even of the arterial circulation ; for the elasticity of the arteries, which is a passive physical property, like that of India rubber, is called into exer- cise only by the distention of these vessels under the cardiac impulse ; and the blood would soon cease flowing were it not, at each successive contraction of the ventricles, thrown into the arterial system in superabundant quantity. The arterial circulation, carried on under the combined influence of the two forces just described, presents certain peculiarities which are worthy of notice. First, its rapid- ity varies at different periods during a single pulsation. At the instant of the heart's con- traction the blood moves with a maximum velocity ; in the intervals of the heart's action, when it is compressed by the arterial elasticity alone, its velocity is diminished. Consequent- ly, if an artery of medium size be wounded, the blood escapes from it in jets. At each car- diac pulsation the stream of blood rises and flows more rapidly ; at each relaxation it falls to a lower level and runs more slowly. The stream is never entirely interrupted. On the contrary, it flows always with a considerable degree of force ; but its increased velocity at each period of the heart's action is abundantly visible. It is this pulsatile character of the haemorrhage from arteries, as well as the florid color of the arterial blood, which enables us at once to distinguish it from haemorrhage which takes place from veins or capillaries. Chau- veau has succeeded in measuring these varia- tions of rapidity in the arterial circulation, by an ingenious instrument adapted to the carotid artery of the horse. He finds that at the in- stant of the heart's contraction the blood is suddenly put in motion with a comparative- ly high degree of rapidity, amounting on the average to 20 inches per second ; the move- ment of the blood is then diminished or brought to a standstill immediately before the closure of the aortic valves; on the closure of the valves the blood again moves forward with a velocity of about 8 mches per second ; and it then diminishes gradually, falling to a little less than 6 inches per second by the end of the period of relaxation. Secondly, the dif- ference between the two conditions of disten- tion and collapse of the artery diminishes from the heart outward ; for at a distance from the heart the force of the cardiac pulsation has been partly expended in distending the larger vessels hi its neighborhood, and consequently the distention of the remote artery is less ; and at the time of cardiac relaxation the smaller ajtery is still kept partly distended by the elas- tio reaction of the larger ones behind. Thus, as in the case of water driven by the strokes of a force pump through an elastic air chamber, the intermittent action of the heart upon the blood becomes more and more equalized, from the centre of the circulation outward ; and in the smallest arterial ramifications the pulsating character of the flow is hardly visible to the naked eye, though still perceptible under the microscope. The pressure exerted upon the blood in the arterial system may be measured by attaching the carotid artery of the living animal to a reservoir of mercury provided with an upright open tube or pressure gauge. When the cavity of the artery is allowed to commu- nicate freely with the interior of the reservoir, the mercury, under the pressure of the blood, rises in the upright tube, and the height of the mercurial column thus becomes an indica- tion of the pressure to which the blood itself is subjected within the artery. The arterial pressure, measured by this test, is found to equal on the average a column of mercury 150 millimetres (6 inches) in height. Various os- cillations of the level of the mercury show the variations in pressure upon the blood during the different periods of a pulsation ; but in the healthy condition it seldom or never falls be- low 130 millimetres. Movement of the blood in the capillaries. In the capillary vessels the movement of the blood at once loses all trace of the pulsating character which it presented in the arteries, and becomes perfectly steady and uniform. This is because, on leaving the arteries and passing into the minute and ex- cessively abundant capillary vessels, the same quantity of blood at once becomes subjected to the influence of a much greater extent of elas- tic surface ; for the walls of the capillaries are themselves elastic, and the substance of the or- gans in which they ramify has also to a greater or less degree the same property. The effect of this increased elasticity of the surrounding parts upon a given quantity of blood is to equalize its rate of movement, and convert it into that of a completely uniform current. Nevertheless, the physical cause of the passage of the blood through the capillary vessels is simply the pressure from behind to which it is subjected in the arterial system. This is fully evident from the following considerations : If the nozzle of an injection pipe be placed in the femoral artery of a recently killed animal, and liquid blood be forced by it through the termi- nal ramifications of the artery and the capillary