488 OF THE CIRCULATION OF THE BLOOD. 



is observable in different states of the arterial system; for, as Dr. C. J. B. 

 Williams has pointed out, 1 when the tonicity is in excess, the arteries approach 

 the condition of rigid tubes, and the pulse at the wrist is almost exactly syn- 

 chronous with the heart's beat; whilst, if the tonicity be defective, the radial 

 pulse is felt at a long interval after the heart's beat, and the difference is still 

 more perceptible when the pulse is examined in the feet. The longest interval 

 in a state of health seems to be between l-6th and l-7th of a second. 



518. The rate of movement of the blood in the Arteries can only be guessed- 

 at, as regards the Human subject, from the comparative results of experiments 

 upon the lower animals. It is stated by Volkmann (Op. cit., p. 196) that the 

 average velocity of the current in the carotids of a considerable number of 

 Mammals which he examined, was about 300 millim., or nearly 12 inches, per 

 second; that the velocity is greater in the arteries lying near, than in those at a 

 distance from the heart; that it is not increased by an augmentation in the num- 

 ber of pulsations ; 3 but that it is greatly augmented by an increase in the volume 

 of the blood, and lessened by its diminution. [The instrument devised by 

 Volkmann, and which he calls the haemodrometer, consists of a glass tube, fifty- 

 two inches long, bent into the form of a hair-pin, and containing water, which is 

 substituted for a segment of the bloodvessel, in which it is required to measure 

 the velocity of the blood's stream. The column of blood which comes from 

 the heart pushes the column of water before it, without any great mixture of 

 the two fluids taking place, and in passing through a determined space it takes 

 a measurable time, whence it may be calculated how far the blood moves in a 

 second. 



The following description will explain the instrument and the mode of using 

 it. At A (Fig. 141*) is a metal tube, an inch and a half in length ; the ends of 

 this (a a') are conical, and fit into two corresponding conical tubes (&, A/), made 

 like the pipes of an injecting syringe, so that they can be readily fitted into an 

 artery. A stopcock (&') commands the channel of this tube, not only at a', 

 but also by two cogged wheels, at a. The mechanism of this arrangement 

 may be readily understood, by reference to the adjoining sections of this por- 

 tion of the instrument at B and C, and the view of its other surface at D 

 (r, / D). At h, Ti' are two short tubes, also of metal, which are fitted into 

 the horizontal tube below the stopcock, and so that their channels (as shown at 

 C) may communicate with, and be exactly equal to, that of the horizontal tube. 

 The stopcock (If) commands this communication likewise. These short tubes 

 (A, h f ) fit exactly upon the bent glass tube (p, p), and complete the communi- 

 cation between its channel, and that of the horizontal tube at its extremities. 

 When the stopcock is turned so as to open the channel of the horizontal tube 



1 "Principles of Medicine," 3d Am. Ed., p. 75. Dr. Williams mentions, what the 

 Author has himself noticed, that the radial pulse, in cases of deficient tonicity, is some- 

 times felt after the second sound of the heart is heard ; a fact that negatives the doctrine 

 of the pulse put forward by Mr. Colt ("Medical Gazette," vol. xxxvi. p. 456), which was 

 founded on the assumption that the pulse is perceived in every part of the arterial system 

 previous to the occurrence of the second sound of the heart, that is, before the closure of 

 the aortic valves. The Author has a very distinct recollection of a case which he wit- 

 nessed when a student in the Middlesex Hospital, in which the radial pulse, though actu- 

 ally synchronous with the heart's beat, was really dependent upon the preceding ventricular 

 systole ; the whole of the interval between one systole and another being required for the 

 transmission of the pulse-wave from the heart to the wrist, as was proved by tracing it 

 from the centre towards the periphery of the arterial system. Now in this case, if the 

 marked want of synchronism between the pulse at the wrist and in the neck had not 

 excited attention, the synchronism between the radial pulse and the heart's beat would 

 have passed as an ordinary occurrence, instead of being a very extraordinary phenomenon. 



2 On this very important point, the observations of Volkmann are in full accordance 

 with the results of some of Hering's experiments performed with special reference to it 

 (2 509). 



