68 CIRCULATION OF THE BLOOD. 



exposed a considerable extent of the primitive carotid in a horse, he enclosed a portion 

 in a tin tube filled with water and connected with a small upright graduated tube of glass. 

 The openings around the artery, as it passed in and out of the apparatus, being carefully 

 sealed with tallow, it is evident that any dilatation of the vessel would be indicated by 

 an elevation of the water in the graduated tube. This experiment invariably showed a 

 marked dilatation of the artery with each contraction of the heart. 



It being fully established that the arteries are dilated with each ventricular systole, it 

 becomes important to study the influence of their elasticity upon the current of blood. 

 Division of an artery in a living animal exhibits one of the important phenomena due to 

 the elastic and yielding character of its walls. We observe, even in vessels of consider- 

 able size, as the carotid or femoral, that the flow of blood is not intermittent but remit- 

 tent. With each ventricular systole, there is a sudden and marked impulse ; but, during 

 the intervals of contraction, the blood continues to flow with considerable force. As we 

 recede from the heart, the impulse becomes less and less marked ; but it is not entirely 

 lost, even in the smallest vessels, the flow becoming constant only in the capillary system. 

 That the force of the heart is absolutely intermittent, is shown by the following experi- 

 ment : If the heart be exposed in a living animal, and a canula be introduced through the 

 walls into one of the ventricles, we have a powerful jet at each systole, but no blood is 

 discharged during the diastole. The same absolute intermittency of the current will be 

 seen if the aorta be divided. It is evident that we must look to the arteries themselves 

 for the force which produces a flow of blood during the intervals of the heart's action. 

 The conversion of the intermittent current in the largest vessels into a nearly-constant 

 flow in the smallest arterioles is effected by the physical property of elasticity. This may 

 be illustrated in any elastic tube of sufficient length. If we connect with a syringe a series 

 of rubber tubes progressively diminishing in caliber and discharging by a very small orifice, 

 and inject water in an intermittent current, if the apparatus be properly adjusted, the 

 fluid will be discharged at the end of the tube in a continuous stream. Nearer the 

 syringe, the stream will be remittent; and, directly at the point of connection of the 

 syringe with the tube, the stream will be intermittent. The intermittent impulse may be 

 said, in this case, to be progressively absorbed by the elastic walls of the tube. Each 

 impulse first distends that portion of the tube nearest to it, and farther on the distention 

 is diminished until it becomes inappreciable. If the syringe be connected with two 

 tubes, one elastic and the other inelastic, the current will be either remittent or contin- 

 uous in the one, and intermittent in the other. This modification of the impulse of the 

 heart has great physiological importance ; for it is evidently essential that the current of 

 blood, as it flows into the delicate capillary vessels, should not be alternately intermitted 

 and impelled with the full power of the ventricle. After all, it is in the capillaries that 

 the blood performs its functions ; and here we should have a constant supply of the fluid 

 in proper quantity and in proper condition to meet the nutritive and other requirements 

 of the parts. 



The elasticity of the arteries favors the flow of blood toward the capillaries by a 

 mechanism which is easily understood. The blood discharged from the heart distends the 

 elastic vessel, which reacts, after the distending force ceases to operate, and compresses 

 its fluid contents. This reaction would have a tendency to force the blood in two direc- 

 tions, were it not for an instantaneous closure of the valves, which renders regurgitation 

 with the heart impossible. The influence, then, can only be exerted in the direction of the 

 periphery ; and, if we can imagine as divided an action which is propagated with such 

 rapidity, the reaction of that portion of the vessel immediately distended by the heart 

 distends a portion farther on, which, in its turn, distends another portion, and so the wave 

 passes along until the blood is discharged into the capillaries. In this way we can see 

 that, in vessels removed a sufficient distance form the heart, the force exerted on the 

 blood by the reaction of the elastic walls is competent to produce a very considerable 

 current during the intervals of the heart's action. This theoretical view is fully carried 



