660 SPECIAL PHYSIOLOGY. 



from a living animal, and subjected to continued cold. The elasticity 

 of the artery is, however, also, incessantly at play in the natural state 

 of the vessel, which is always in a condition of moderate and constant 

 tension, permitting and explaining its slight contraction and retraction 

 into its sheath, when it is divided. 



As already stated, by the successive contractions of the ventricles, 

 and by the closure of the auriculo-ventricular valves, the blood is not 

 only directed from the heart into the great arterial trunks, but is also 

 projected into them by successive jerks. If the arteries had rigid in- 

 elastic walls, this intermittent motion of the blood in them, would be 

 propagated even to the capillary system. Owing, however, to their 

 elasticity, and to the successive closure of the semilunar valves across 

 the mouths of those vessels, the separate impulses caused by the ven- 

 tricular contractions, are gradually rendered less distinct, and, finally, 

 before the stream of blood enters the capillary vessels, its motion be- 

 comes continuous. The elastic coats of the aorta, near the heart, 

 having been distended by the force of the left ventricle, recoil on the 

 contained blood; this fluid being practically incompressible, transmits 

 the pressure on itself, backwards, so as to close the semilunar valves, 

 and forwards, so as also to urge onward the column of blood in the 

 systemic arteries. But the intermittent effect of the heart's strokes, 

 is propagated onwards through all the main arteries of the body, in 

 which it is manifested by the pulse, and by the escape of the blood 

 per saltum, or in jets from any of those vessels when they are wounded. 

 The motion of the blood from the ventricle is truly intermittent that 

 is, it ceases absolutely at intervals; the jet from a large artery, when 

 wounded, is not quite intermittent; that from the smaller arteries, 

 though the stream is jerking, is distinctly remittent, i. e., the jet never 

 ceases altogether, but is alternately stronger and weaker; finally, in 

 the smallest or microscopic arteries, the flow of the blood, under ordi- 

 nary circumstances, loses even the remittent character, and becomes 

 perfectly equable and continuous, and remains so in the capillary ves- 

 sels. This effect of the elastic recoil of the previously distended arte- 

 rial walls, may be illustrated by the action of a vulcanized india-rubber 

 tube, which, if of sufficient length, changes the jerking flow of water, 

 forced into it by a syringe or pump, into a flowing stream. The force 

 of the ventricle, transmitted through the column of blood, acts most 

 powerfully on the vessels nearest to the heart, in which the elastic 

 tissue is most abundant ; whilst the effect of the ventricular force is 

 gradually weakened in the more distant vessels, the elastic coat of 

 which becomes proportionally thinner. On the contrary, as already 

 mentioned, the muscular fibres are relatively least abundant in the 

 largest, and most so in the smallest arteries; and it is improbable that 

 their contractility is called into play, to resist the distending effect of 

 the heart's force. Although the arteries, by their resilience, at length 

 convert the intermittent stroke of the heart into a uniform propulsive 

 force, yet the heart itself is still the moving agent of the arterial blood; 

 for the recoiling force exercised by the arteries, is itself due to their 

 previous distension by the force exerted by the heart. When, indeed, 

 this force is too weak to distend the arteries as usual, the remittent 



