Various circumstances shew that in the living body the blood forms an uninterrupted column of fluid in the bloodvessels, and that the whole vascular system is kept in a state of forced dis tension by the reiterated impulses communi cated to the blood by the ventricular contrac tions. Besides the general fulness of the blood vessels and their connection with the heart, we may mention as proofs of the distended state of the vascular system, the facts, 1st, that, on opening any of the bloodvessels, the blood issues with greater force at the first moment than afterwards ; and 2d, that when we imitate the propulsion of the blood through the arteries and veins by artificial injection of fluids in a dead animal, we observe that the jet from an opened vessel continues to flow for some time after we have ceased to drive the piston of the syringe. The arteries being much stronger than the veins, re-act with greater povver than they do against the distending force of the heart. Were the arteries rigid tubes, it is ma nifest that in a given time just as much blood would pass from their remote extremities into the commencement of the veins, as enters them by the mouth of the aorta; .but the arteries must be fuller at one time than another, for the quantity of blood expelled from the ventricle at each systole, must pass suddenly into the first part of the aorta, while an equal quantity of blood, which must necessarily pass from the remote arteries into veins, as it moves uni formly, must employ the whole period of time occupied by a complete action of the heart in its passage ; and consequently it is manifest, that the arterial system must be fuller just after than immediately before the contrac tion of the ventricle. The arteries are distensi ble and elastic, they yield a little to every suc cessive stroke of the ventricle, and during the diastole they re-act by their elasticity, so as to keep up the flow of blood. We have already said, in speaking of the heart, that the muscular contraction of that organ is the chief, if not the only source of the power propelling the blood. It is only in those arteries which are nearest to the heart, however, that the blood can be said to be propelled by the direct impulse of the ven tricle, for in the rest of the arterial system, the progression of the blood is immediately effected by the elastic power of the arteries, called into operation in consequence of their distension by the action of the heart. In the experiments of artificial injection of the bloodvessels in dead animals already mentioned, as long as we con tinue to drive the piston of the syringe, and to propel fluids through the arteries into the veins, the arteries are kept in a state of forced disten sion ; in consequence of this, the fluid issues from an opened artery with a jet accelerated after each successive stroke of the pisto», and continues to flow for some time after the pro pelling power has ceased to act. The unifor mity of' the stream of fluid from the veins, which occurs in the same experiment, is a proof that the continued flow of blood in these tubes may, in the living body, be owing to an impul sion from the heart, transmitted by the arteries, and that it is caused by the elasticity of the coats of the vessels themselves.
a. Velocity of the blood in different arteries. The space of the aorta filled up by the blood propelled from the ventricle at each systole, divided by the time occupied in its propul sion, constitutes the velocity of the blood in the first part of the aorta. The diameter of the aperture of the aorta at the ventricle being taken as on an average 1.12 of an inch,* its area would be one square inch, and consequently 11 oz.
which equal 2.45 cubic inches of' blood, would occupy a little more than 2-5 inches of the aorta, supposing its size to be for such an extent of a uniform diameter. As it is satisfactorily ascer tained by actual measurement, that the blood contained in the smaller vessels is in much greater quantity than that in the larger trunks; or, in other words, as the capacity of the smaller vessels taken together is greater than that of the larger, it will at once be apparent, that the velocity of the blood must diminish in passing from the 'larger to the smaller vessels. The arterial and venous vessels may in fact be re garded as two hollow cones, curved so as to be joined at their apices to the heart, and at their bases to one another. The veins, being more numerous and wider than the arteries, must be represented by a wider cone. The section of these cones at any place is supposed to give the combined area of the section of the vessels at a corresponding distance from the heart.
The estimates made by different authors of the relative velocity of the blood in the larger and smaller vessels, differ in a great degree, and are exceedingly unsatisfactory. Haller, who fully admitted the greater capacity of the smaller arteries, and allowed that the flow of the blood must therefore, from hydraulic prin ciples, become less rapid in passing from the trunks to their branches,—a proposition which he illustrates by comparing the stream of blood in its passage to a river which enters a lake,— was yet inclined, from the result of his actual observations, to deny that the velocity is much less in the smaller than in the larger arteries. Spallanzani, although admitting more explicitly still than Haller the necessity of such a retarda tion, seems to have met with the same difficulty in reconciling theory with his attempts to mea sure the velocity of the blood in the small ves sels : and both these authors state, that although the circulation was in general comparatively slow in the web of the frog's foot, still in many instances in this situation, and more frequently in the mesentery, they were unable to detect any difference in the rapidity of the flovv of the blood in the larger and smaller arteries.1' Hales, again, states as the result of his ob servations and measurements, that the velocity of the blood in the smallest capillaries of the abdominal muscles of the frog, is so small as one or one and a half inch in a minute; and, from the attempts which we ourselves have made at these measurements, we feel inclined to agree with the statement of this able experi menter, having, upon several occasions, ascer tained that in those capillaries which admit only two globules of blood, the velocity is not greater than the hundredth part of an inch in a second ; but it seems doubtful vvhether in all the capillaries the velocity is so small as in those just alluded to, and in the larger capillary vessels of the diameter of six globules, when no unnatural obstruction to the circulation in the limb occurred, independently of the diffi culty of fixing the eye upon any globule in such a way as to trace its progress along the vessel, the velocity has always appeared so great as to prevent tbe possibility of measuring it ; and we are at a loss to conceive in what manner Haller made the comparison he_speaks of between the velocity in the.larger and smaller arteries. By means of the microscope, it is easy to see that the velocity is greater in the small arteries than in the corresponding veins, which are both more numerous and considerably larger than the arteries.