Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/113

VASCULAR SYSTEM 97 recognized and figured the spiral arrangement of fibres in the ventricles. The question was further investigated by ill. James Keill, a Scottish physician (1673-1719), who in his Account of Animal Secretion, the Quantity of Blood in the Human Body, and Muscular Motion (1708) attempted to estimate the velocity of blood in the aorta, and gave it at 52 feet per minute. Then, allowing for the resistance of the vessels, he showed that the velocity diminishes to wards the smaller vessels, and arrived at the amazing con clusion that in the smallest vessels it travels at the rate of ^ inch in 278 days, a good example of the extravagant errors made by the mathematical physiologists of the period. Keill further described the hydraulic phenomena of the circulation in papers communicated to the Royal Society and collected in his Essays on Several Parts of the Animal (Economy (1717). In these essays, by estimating the quantity of blood thrown out of the heart by each contraction, and the diameter of the aortic orifice, he cal culated the velocity of the blood. He stated (pp. 84, 87) that the blood sent into the aorta with each contraction would form a cylinder 8 inches (2 ounces) in length and be driven along with a velocity of 156 feet per minute. Estimating then the resistances to be overcome in the vessels, he found the force of the heart to be " little above 16 ounces," a remarkable difference from the computation of Borelli. KeilFs method was ingenious, and is of his torical interest as being the first attempt to obtain quan titative results ; but it failed to obtain true results, because the data on which he based his calculations were inaccurate. These calculations attracted the attention not only of the anatomico-physiologists, such as Haller, but also of some of the physicists of the time, notably of Jurin and D. Bernoulli. Jurin (died 1750) gave the force of the left ventricle at 9 ft 1 oz. and that of the right ventricle at 6 fl> 3 oz. He also stated with remarkable clearness, considering that he reasoned on the subject as a physicist, without depending on experimental data gathered by himself, the influence on the pulse induced by variations in the power of the heart or in the resistance to be overcome. 1 The experimental investigation of the problem was supplied ales, by Stephen Hales (1677-1761), rector of Teddington in Middlesex, who in 1708 devised the method of estimating the force of the heart by inserting a tube into a large artery and observing the height to which the blood was impelled into it. Hales is the true founder of the modern experimental method in physiology. He observed in a horse that the blood rose in the vertical tube, which he had connected with the crural artery, to the height of 8 feet 3 inches perpendicular above the level of the left ventricle of the heart. But it did not attain its full height at once : it rushed up about half way in an instant, and afterwards gradually at each pulse 12, 8, 6, 4, 2, and sometimes 1 inch. When it was at its full height, it would rise and fall at and after each pulse 2, 3, or 4 inches ; and sometimes it would fall 12 or 14 inches, and have there for a time the same vibrations up and down at and after each pulse as it had when it was at its full height, to which it would rise again after forty or fifty pulses. 2 He then estimated the capacity of the left ventricle by a method of employing waxen casts, and, after many such experi ments and measurements in the horse, ox, sheep, fallow deer, and dog, he calculated that the force of the left ventricle in man is about equal to that of a column of blood 7 feet high, weighing 51 ^ K>, or, in other words, that the pressure the left ventricle has to overcome is 1 Jones, Abridgement of Phil. Trans., 3d ed. 1749, vol. v. p. 223. See also for an account of the criticisms of D. Bernoulli the elder and others, Haller s Klementa Physiologist, vol. i. p. 448. 2 Hales, Statical Essays, containing Haemastutics, &c. , 1733, vol. ii. p. 1. equal to the pressure of that weight. When we contrast the enormous estimate of Borelli (180,000 ft>) with the underestimate of Keill (16 oz.), and when we know that the estimate of Hales, as corroborated by recent investiga tions by means of elaborate scientific appliances, is very near the truth, we recognize the far higher service rendered to science by careful and judicious experiment than by speculations, however ingenious. With the exception of some calculations by Dan Bernoulli in 1748, there was no great contribution to hsemadynamics till 1808, when two remarkable papers appeared from Thomas Young (177 3- Thomas 1829). In the first, entitled "Hydraulic Investigations," Youn S- which appeared in the Phil. Trans., he investigated the friction and discharge of fluids running in pipes and the velocity of rivers, the resistance occasioned by flexures in pipes and rivers, the propagation of an impulse through an elastic tube, and some of the phenomena of pulsations. This paper was preparatory to the second, "On the Func tions of the Heart and Arteries," the Croonian lecture for 1808 in which he showed more clearly than had hitherto been done (1) that the blood-pressure gradually diminishes from the heart to the periphery ; (2) that the velocity of the blood becomes less as it passes from the greater to the smaller vessels ; (3) that the resistance is chiefly in the smaller vessels, and that the elasticity of the coats of the great arteries comes into play in overcoming this resistance in the interval between systoles ; and (4) that the con tractile coats do not act as propulsive agents, but assist in regulating the distribution of blood. 3 The next epoch of physiological investigation is charac- Use of terized by the introduction of instruments for accurate instni - measurement, and the graphic method of registering pheno mena, now so largely used in science.* In 1825 appeared E. and W. Weber s Wellenlehre, and in 1838 E. Weber s Ad Notat. Anatom. et Physiolog., i., both of which contain an exposition of E. H. Weber s schema of the circulation, a scheme which presents a true and consistent theory. In 1826 Poiseuille invented the hasmadynamometer. 5 This was adapted with a marker to a recording cylinder by Ludwig in 1847, so as to form the instrument named by Volkmann the kymograph. Volkmann devised the hasma- dromometer for measuring the velocity of the blood in 1850; for the same purpose Vierordt constructed the haimatachometer in 1858 ; Chauveauand Lortet first used their haemadromograph in 1860 ; and lastly, Ludwig and Dogiel obtained the best results as regards velocity by the "stream-clock" in 1867. As regards the pulse, the first sphygmograph was constructed by Vierordt in 1856 ; and Marey s form, of which there are now many modifications, appeared in 1860. In 1861 Chauveau and Marey ob tained tracings of the variations of pressure in the heart cavities (see p. 99 below) by an experiment which is of great historical importance. During the past twenty-five years vast accumulations of facts have been made through the instruments of precision above alluded to, so that the con ditions of the circulation, as a problem in hydrodynamics, have been thoroughly investigated. Since 1845, when the brothers Weber discovered the inhibitory action of the vagus, and 1858, when Claude Bernard formulated his researches showing the existence of a vaso-motor system of nerves, much knowledge has been acquired as to the relations of the nervous to the circulatory system. The Webers, John Reid, Claude Bernard, and Carl Ludwig may be regarded as masters in physiology equal in stand ing to those whose researches have been more especially alluded to in this historical sketch. The Webers took the first step towards recognizing the great principle of 3 See Miscellaneous Works, ed. Peacock, 2 vols., London, 1855. 4 See Marey, La Methode Graph, dans les Sc. Exper., Paris, 1S78. 5 Magendie s Journal, vol. viii. p. 272.

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