THE MECHANISM OF THE CIRCULATION. 



demonstration of arterial pressure. Nevertheless the method of Hales 

 is open to two objections first, the inconvenience of a long tube; 

 second, the speedy coagulation of the blood. 



By Poiseuille l both these sources of difficulty in the method of ex- 

 periment were removed, when he substituted for Hales' glass tube a 

 mercurial manometer, and to prevent coagulation filled the tube of 

 connection between the artery and the manometer with a solution of 

 sodium carbonate. 



Ludwig 2 completed the work of Poiseuille by adding to the mercury 

 manometer a float carrying a writing style, and 

 by recording thereby on a revolving drum the 

 variations of arterial pressure. To this device 

 of Ludwig (kymographiori) the graphic method 

 in physiology owed its origin. 3 



Owing to the inertia of the mercury, the 

 mercurial manometer is not fitted to give a true 



ii l j record of the pressure changes which occur 



|| during the cardiac cycle. When the heart is 



beating slowly, the maximal aortic pressure, 

 when read by this instrument, is too high, 

 and the minimal pressure is too low; with a 

 rapid heart the reverse obtains. Thus, no 

 knowledge as to the true systolic and diastolic 

 variations of pressure can be gained by it. 



The mercurial manometer is nevertheless the 

 most convenient and useful instrument for obtain- 

 ing records of the mean arterial pressure. It is 

 stated by v. Kries 4 that the manometer should be 

 constructed of tubing 4 mm. in diameter. The 

 error in the mean pressure is then not greater than 

 3 per cent. With the use of wider or narrower 

 tubes the error becomes greater. If the mano- 

 meter tube be very considerably constricted in one 

 part of its course, then the instrument, whatever 

 be its diameter, gives the true mean pressure, for 

 the oscillations of the mercury are almost entirely 

 damped down by the constriction. The speed of the instrument Is thereby 

 sacrificed, for it takes some time to rise to the mean pressure. The instru- 

 ments of precision now largely employed for investigations on the systolic 

 and diastolic variations of arterial pressure, are Fredericq's modified form of 

 sphygmoscope and the Fick, Hiirthle, or v. Basch manometers (see intracardiac 

 pressure). Before use these manometers must be graduated against a mercury 

 manometer. 



For the purpose of obtaining continuous records of arterial pressure, the 

 writing style of the arterial manometer is brought to bear on a band of paper, 

 covered with lamp-black. By means of clockwork this band can be made to 

 revolve at varying speeds round two drums placed at either end of a long 

 frame. There are also brought to write on the paper an electric signal, by 

 which points of excitation can be marked during the course of an experiment, 



1 "Rech. sur la force du coeur aortique," These, Paris. 1828. 



2 Arch.f. Anat., Physiol. u. wisscnsch. Med., 1847, S. 261. 



3 For an improved method of constructing the float of Ludwig's kynm 

 Anderson Stuart, Journ. Physiol. , Cambridge and London, 1891, vol. xii. p. 147. 



4 Arch.f. Physiol., Leipzig, 1878, S. 430. 



FIG. 50. Poiseuille's 

 hsemodynamorneter. 



