Poiseuille,* an ingenious ex perimenter of Paris, who, by the adoption of a simple con trivance, has been enabled to 'measure with great accuracy the arterial pressure of the blood, and has thus confirmed and extended the interesting re searches of Hales.
The instrument employed by Poiseuille, to which he gives the name of Hemadynamome ter, (fig. 329,) consists of a bent glass tube of the form here repre sented, filled with mercury in the lower bent part (a, d , e). The horizontal part (b) , provided with a brass head, is fitted into the artery, and a little of a solu tion of carbonate of soda is interposed between the mercury and the blood which is allowed to enter the tube for the pur pose of preventing its coagula tion. When the blood is al lowed to press upon the fluid in the horizontal limb, the rise of the mercury towards (c) measured from the level to which it has fallen towards (d) gives the pressure under which the blood moves.
One of the most important facts established by Poiseuille's experiments is, that the pressure of the blood is within certain limits nearly the same in arteries of very different calibre and at different distances from the heart; as proved by the rise of the mercury of the hemadyna mometer to nearly an equal height when this instrument was connected with the iliac, caro tid, radial, facial, and other arteries in some of the lower animals. It is hence apparent, that, in order to ascertain the whole amount of force with which the blood is propelled. in the aorta; or the statical force of the heart itself, it is sufficient to measure by means of the tube the momentum of the blood in any one of the arteries. Poiseuille estimates the force with vvhich the blood is propelled in the commencement of the aorta in man at 4 lbs. 3 oz.,—a result which agrees remarkably with that obtained by Hales.* Poiseuille, however, considers the pressure backwards within the heart to amount to 1 3 lbs. only, as he calculates this in a different way fiom that followed by Ilales, viz. by multi plying the pressure of the blood in the aorta into the surface of a plane passed through the base and apex of the left ventricle,—a mode of calculation which it appears that Dr. Hales
had not lost sight of; for, at page 21 of the work on IIemastatics, he proposes it as the l4 means of estimating the force of the blood which the muscular fibres of the ventricle must resist." Poiseuille estimates the force with which the blood moves in the radial artery of man at four drachms.
Hales had remarked that the blood in the tube connected with an artery rose regularly a little way at each systole of the ventricle, and remained always somewhat higher during the straining of the animal, that is, while the muscles of expiration were in action. These phenomena, known to IIaller, were demon strated experimentally by Magendie, and re ceive a still more decided confirmation from the experiments of Poiseuille made with the hemadynamometent We would here remark that, it having been shewn by the above-mentioned experiments that the force of the heart is sensibly the same in the trunks and larger branches of the arte ries, it is manifest that the angles of rami fication and the friction of the blood against the sides of the vessels can give rise to very little if any diminution in the force of the heart transmitted by the elasticity of the arterial parietes. We shall afterwards see that the case is very different in the smaller vessels.
We would also call the attention of the readcr to an interesting application of the fact of the complete transmission of pressure through the fluid contained within the bloodvessels in all directions, in the immense force which the blood occasionally appears to exert within an aneurisrnal tumour; giving rise to its peculiarly hard pulsation on every side, and assisting the ravages by absorption which are frequently the consequence of the larger internal aneurisms. The pressure in an aneurism is obviously to be measured by the extent of its internal surface multiplied into the force with which the blood moves in the part of the artery where it opens into the aneurismal sac.
c. Arterial pulse.—The arterial pulse, or suc cession of beats felt by the finger placed over an artery, depends upon the impulse of the left ventricle being communicated along the arterial tube and the column of blood which it contains.