The whole of the blood issuing from the ventricles into the first parts of the great arte ries is retained within these arteries by the action of the semilunar valves, and it would appear that in the healthy condition the adap tation of these valves is such that very little if any blood regurgitates or flows backwards into the ventricles. At the time that the auri cles contract, a very different phenomenon presents itself, for while a certain quantity of the blood from the auricles passes onwards into the ventricles, some is driven back into the orifices of the great veins. This venous re gurgitation is particularly evident in the veins connected with the right side of the heart, the orifices of which have no valves or very im perfect ones; and it gives rise to a pulsation in their larger branches, synchronous with the systole of the auricle, as may be seen in most thin persons in the jugular vein at the lower part of the neck. It would appear that upon some occasions, even in the state of health, a certain back stroke from the ventricles also is perceptible in the veins, and Ilales was of opinion that some of the blood (half an ounce) from the right ventricle flowed back into the auricle during each systole of the ventricle. It must be apparent that immediately after the auricle has ceased to propel its contents into the ventricle, and just when the systole of the ventricle begins, the column of blood extend ing from the ventricle into the auricle through the auriculo-ventricular orifice must be con tinuous, and the pressure of' the ventricular systole must thus be transmitted upwards until the valves flap together and close that opening. Accordingly, in some persons in health, a ve nous pulse, synchronous with the ventricular systole, is occasionally seen or felt in the jugu lar veins, but this appearance is much more commonly a sign of disease ; for the venous pulse which is synchronous with the ventri cular systole is much increased when an ob stacle presents itself to the free flow of blood through the pulmonary artery, or when from ossification or other morbid alteration, the auri culo-ventricular valves do not close accurately the passage in which they are placed.
We may conclude, from the observations above alluded to, that on an average each of the ventricles of the heart gives out nearly one ounce and a half at each stroke; and we may now state the general calculation of the time that the blood takes to move through the heart, which is generally founded upon the above data. Let us suppose the heart to beat seventy five times in a minute, which is nearly the ave rage number of pulsations in a healthy man in the prime of life, and assume the quantity of _blood in the body at 28 lbs. ; and let us sup pose that 11 oz. of' blood is expelled from each ventricle into the great arteries connected with them, then 112 oz. or 7 lbs. of blood would pass through each ventricle in a minute, and 28 lbs. in four minutes ; or in three minutes, if the quantity of blood passing through the ventricles at each systole be estimated at two ounces, i. e. a quantity of blood equal to that which we conceive to be contained in the whole body, would flow through the heart in the short space of four minutes, and this quan tity would run the same course fifteen times in an hour. We must guard against conceiving, on the one hand, that this calculation affords any accurate measure of the quantity of blood which actually passes through the ventricles in a given time, for there are innumerable circum stances which tend to cause this quantity to vary to a considerable extent; and on the other hand, it must at all times be borne in mind that we can, from such calculations, estimate only the velocity of the blood in the heart itself, or the time which a certain quantity of blood takes to pass through its cavities, but that we are not furnished with any measure of the time that the whole of the circulating quantity of blood actually takes to pass through its course, for the length of the courses through which the blood has to pass in different parts of the vas cular system varies to such a degree, that in some places, as for example in the bloodves sels of the heart itself, the return to the heart must be effected in less than half the time employed by that which is transmitted to the extremities. On comparing the longest or
shortest calculations of this kind made by dif ferent authors, we shall find that the time of a circulation is made to vary from six minutes and a half to one minute.
We shall not at present enter upon the con sideration of the force with which the blood issues from the left ventricle of the heart, as the experiments by which this force is deter mined being made upon the arteries, come more suitably to be treated of under the arte rial circulation.
2. Phenomena qf the arterial circulation.— In proceeding to consider the phenomena and causes of the flow of blood through the arterial system, vve purpose to treat of, 1st, the velocity; 2d, the force of the blood in the arteries ; 3d, the nature of the arterial pulse ; 4th, the vital properties of the arteries; and 5th, the influence exerted by this class of bloodvessels on the cir culation. We shall find that, in this part of our subject, the difficulty of becoming ac quainted with the immense variety of circum stances capable of modifying- the flow of the blood, has prevented the explanation of pheno mena which are in themselves sufficiently sim ple and apparent. In our remarks upon the above-mentioned topics, we shall endeavour to refer the phenomena of the circulation, as far as we can, to hydraulic principles, which, when correctly applied, must form the only. sure guide in conducting a physiological inquiry of this nature.
The flow of the blood, as it is expelled from the left ventricle, may be said to be intermit tent, for it moves only at the time of the ventn cular systole. Farther on in its course, in the larger as well as the middle sized arteries, the flow of blood is remittent, or is more rapid after each beat of the heart, and by the time it arrives at the capillary vessels and commence ment of the veins, the velocity is rendered per fectly uniform. The effect, therefore, produced by the arterial tubes is to convert an intermittent, first into a remittent, and afterwards into a uni form force. When an opening is made into one of the larger arteries, the jet of blood which issues is regularly increased in velocity at every systole of the ventricle. In the very small ar teries, this acceleration of the stream becomes less perceptible. We know that it has altoge ther disappeared in the smallest vessels or :ca pillaries, from microscopic observation of the flow of the blood in them, and the uniformity of the velocity of the stream in the veins is clearly shewn in all instances in which a vein is opened, as in the common operation of bleeding from the arm.