658 



CIRCULATION. 



mates the capacity of the left ventricle at 

 1 oz. fluid measure, and that of the right at 

 2 oz. The contemplation of the muscular 

 structure of the left ventricle, and the great 

 diminution in size it undergoes during its sys- 

 tole, would induce us to conclude that it must 

 be completely emptied during the contraction, 

 and that there cannot remain any blood even 

 among the columnae carneae. The right ven- 

 tricle does not appear from the quantity of its 

 muscular substance to be so well suited to be 

 emptied, but its position round the left must 

 assist considerably in the diminution of its 

 size during its systole. In some cases of sud- 

 den death in healthy persons, both ventricles 

 have been found completely empty. 



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 Hales 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 Ibs. ; and let us sup- 

 pose that \\ oz. of blood is expelled from each 

 ventricle into the great arteries connected with 

 them, then 112 oz. or 7 Ibs. of blood would 

 pass through each ventricle in a minute, and 

 28 Ibs. 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 of the arterial circulation. 

 In proceeding to consider the phenomena and 

 causes of the flow of blood through the arterial 



\^j 



system, we 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 



