G02 



CIRCULATION. 



lumn of blood of seven feet and a half high 

 into the area of the inner surface of the heart : 

 he hence calculates the pressure on the in- 

 ner surface of the human heart to be nearly 

 51 Ibs. The pressure on the interior of the 

 horse's heart he estimates at 113 Ibs. upon 

 similar principles. 



As pressure applied in any direction to a 

 fluid column is equally transmitted through all 

 its parts, and as the blood in the arteries forms 

 continuous columns which all branch off from 

 the aorta, it might a priori have been con- 

 cluded that the force of the blood must be the 

 same in all the arteries of any considerable 

 size. Hales, though he does not state this 

 proposition very explicitly, seems yet to have 

 taken it for granted ; for, in estimating the 

 pressure of the heart, he takes into account 

 merely the height of the column without re- 

 ference to the size of the artery. We shall 

 find this proposition to be satisfactorily proved 

 to be correct by direct experiments subse- 

 quently performed. 



The experiments of Hales were liable to two 

 principal objections: 1st, that the coagulation 

 of the blood in the long glass tube adapted to 

 the artery must have prevented its free motion ; 

 and, 2nd, that the length of the tube, besides 

 giving rise to the necessity of frequently re- 

 moving it and various other inconveniences, 

 must have occasioned a considerable loss of 

 blood in filling from the arteries of small ani- 

 mals. Both these sources of fallacy have been 

 provided against most successfully by M. 

 Poiseuille,* an ingenious ex- 

 Fig, 329. perimenter of Paris, who, by 

 the adoption of a simple con- 

 trivance, has been enabled to 

 measure with great accuracy 

 the arterial pressureof 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, (Jig. 329,) 

 consists of abent 

 glass tube of the 

 form here repre- 

 sented, filled with mercury in 

 the lower bent part (, d, e). 

 The horizontal part (6), 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. 



f 



Poiseuille' 's He- 

 madynamometer , 



* Magendie's Journal, vols. viii. & ix. Breschet's 

 Repert. d'Anat. et de Physiol. 1826. 



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- 

 mo meter 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 which the blood is 

 propelled in the commencement of the aorta 

 in man at 4 Ibs. 3 oz., a result which agrees 

 remarkably with that obtained by Hales.* 



Poiseuille, however, considers the pressure 

 backwards within the heart to amount to 13 Ibs. 

 only, as he calculates this in a different way 

 from that followed by Hales, 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 Hemastatics, he proposes it as the 

 " 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 Haller, were demon- 

 strated experimentally by Magendie, and re- 

 ceive a still more decided confirmation from 

 the experiments of Poiseuille made with the 

 hemadynamometer.f 



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 

 parities. We shall afterwards see that the 

 case is very different in the smaller vessels. 



We would also call the attention of the 

 reader 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 



* The power of the heart has also been calcu- 

 lated from the force supposed necessary to raise 

 the foot of one of the legs thrown across the other 

 in the pulsatory movement which is then seen to 

 occur, one of the most inaccurate methods that 

 could be adopted. 



t See Part IV, of this article. 



