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Originally appearing in Volume V27, Page 929 of the 1911 Encyclopedia Britannica.
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VASCULAR SYSTEM. I. ANATOMY.—The circulatory or blood vascular apparatus consists of the central pump or heart, the arteries leading from it to the tissues, the capillaries, through the walls of which the blood can give and receive substances to and from the tissues of the whole body, and the veins, which return the blood to the heart. As an accessory to the venous system, the lymphatics, which open finally into the great veins, help in returning some of the constituents of the blood. Separate articles are devoted to the heart, arteries, veins and lymphatic system, and it only remains here to deal with the capillaries. The blood capillaries form a close network of thin-walled tubules from z-h to of an inch in diameter, permeating, with a few exceptions, the whole of the body, and varying somewhat in the closeness of its meshwork in different 'parts. In the smallest capillaries, in which the arteries end and from which the veins begin, the walls are formed only of somewhat oval endothelial cells, each containing an oval nucleus and joined to its adjacent cells by a serrated edge, in the interstices of which is a small amount of intercellular cement, easily demonstrated by staining the preparation with nitrate of silver. Here and there the cement substance is more plentiful, and these spots when small are known as stigmata, when large as stomata. As the capillaries approach the arteries on the one hand and the veins on the other they blend and become larger, and a delicate connective tissue sheath outside the endothelium appears, so that the transition from the capillaries into the arterioles and venules is almost imperceptible; indeed, the difference between a large artery or vein and a capillary, apart from size, is practically the amplification and differentiation of its connective tissue sheath. Embryology.—The first appearance of a vascular system is outside the body of the embryo in the wall of the yolk sac, that is to say, in the mesoderm or the middle one of the three embryonic layers. The process is a very early one and in the chick is seen to begin at the end of the first day of incubation. The first occurrence . is a network made up of solid cords of cells forming in certain places solid cell masses called the blood islands of Pander.' The central cells of these islands divide by karyokinesis and gradually float away into the vessels which are now being formed by fluid from the exterior, finding its way into the centre of the, cell cords and pressing the peripheral cells flat to form the endothelial lining. These free cells from the blood islands are known as erythroblasts and are the primitive corpuscles of the foetal blood. They have a large reticular nucleus and at first are colourless though haemoglobin gradually develops within them and the blood becomes red (see BLOOD). The erythroblasts continue to multiply by karyokinesis in early foetal life, especially in the liver, spleen, bone marrow and lymphatic glands, though later on their formation only occurs in the red bone marrow. In most of the erythroblasts the nucleus soon becomes contracted, and the cell is then known as a normoblast, while ultimately the general view is that the nucleus disappears by extrusion from the cell and the non-nucleated red blood plates or erythrocytes remain. The leucocytes or white blood corpuscles appear later than the red, and are probably formed from lymphoid tissue in various parts of the body. The blood vessels thus formed in the so-called vascular area gradually travel along the vitelline stalk into the• body of the embryo, and two vessels larger than the rest are formed one on each side of the stalk. These are the vitelline veins, which, as they pass towards the caudal end of the embryo, become the two primitive aortae, and these fuse later on to form the heart. After the inversion of the pericardial region and formation of the head fold (see COELOM AND SEROUS MEMBRANES) the front of the developing heart becomes the back, and the vitelline veins now enter it from behind. It must be understood that most of our knowledge of the early history of the blood vessels is derived from the study of lower mammals and birds, and that this is being gradually checked by observations on human embryos and on those of other primates. It seems probable that in these mammals, owing to the small size. of the yolk sac, the vessels of the embryo establish an early communication with those of the chorion before the vitelline veins are formed (see Quain's Anatomy, vol. i., London, 1908). The later stages of the embryology of the vascular system are sketched in the articles on Heart, Arteries, Veins and Lymphatic System (qv.). (F.'G. P.) II. HISTORY OF DISCOVERY Galen, following Erasistratus (ob. 280 B.C.) and Aristotle, clearly distinguished arteries from veins, and was the first to overthrow the old theory of Erasistratus that the arteries contained air. According to him, the vein Galen. arose from the liver in two great trunks, the versa porta and vena cava. The first was formed by the union of all the abdominal veins, which absorbed the chyle prepared in the stomach and intestines, and carried it to the liver, wher it was converted into blood. The vena cava arose in the liver, divided into two branches, one ascending through the diaphragm to the heart, furnishing the proper veins of this organ; there it received the vena azygos, and entered the right ventricle, along with a large trunk from the lungs, evidently the pulmonary artery. The vena azygos was the superior vena cava, the great vein which carries the venous blood from the head and upper extremities into the right auricle. The descending branch of the great trunk supposed to originate in the liver was the inferior vena cava, below the junction of the hepatic vein. The arteries arose from the left side of the heart by two trunks, one having thin walls (the pulmonary veins), the other having thick walls (the aorta). The first was supposed to carry blood to the lungs, and the second to carry blood to the body. The heart consisted of two ventricles, communicating by pores in the septum; the lungs were parenchymatous organs communicating with the heart by the pulmonary veins. The blood-making organ, the liver, separates from the blood subtle vapours, the natural spirits, which, carried to the heart, mix with the air introduced by respiration, and thus form the vital spirits; these, in turn carried to the brain, are elaborated into animal spirits, which are distributed to all parts of the body by the nerves.' Such were the views of Galen, taught until early in the 16th century. Jacobus Berengarius of Carpi (ob. 1530) investigated the stricture of the valves of the heart. Andreas Vesale or Vesalius (1514–1564) contributed largely to anatomical know- Vesalius. ledge, especially to the anatomy of the circulatory organs. He determined the position of the heart in the chest; 1 See Burggraeve's Histoire de l'anatomie (Paris, 188o) he studied its structure, pointing out the fibrous rings at the bases of the ventricles; he showed that its wall consists of layers of fibres connected with the fibrous rings; and he de-scribed these layers as being of three kinds—straight or vertical, oblique, and circular or transverse. From the disposition of the fibres he reasoned as to the mechanism of the contraction and relaxation of the heart. He supposed that the relaxation, or diastole, was accounted for ,principally by the longitudinal fibres contracting so as to draw the apex towards the base, and thus cause the sides to bulge out; whilst the contraction, or systole, was due to contraction of the transverse or oblique fibres. He showed that the pores of Galen, in the septum between the ventricles, did not exist, so that there could be no communication between the right and left sides of the heart, except by the pulmonary circulation. He also investigated minutely the internal structure of the heart, describing the valves, the columnae corneae and the musculi papillares. He described the mechanism of the valves with much accuracy. He had, however, no conception either of a systemic or of a pulmonary circulation. To him the heart was a reservoir from which the blood ebbed and flowed, and there were two kinds of blood, arterial and venous, having different circulations and serving. different purposes in the body. Vesalius was not only a great anatomist: he was a great teacher; and his pupils carried on the work in the spirit of their master. Prominent among them was Gabriel Fallopius (1523-1562), who studied the anastomoses of the blood vessels, without the art of injection, which was invented by Frederick Ruysch (1638-1731) more than a century later. Another pupil was Columbus Columbus. (Matthieu Reald Columbo, ob. 156o), first a prosector in the anatomical rooms of Vesalius and afterwards his successor in the chair of anatomy in Padua; his name has been mentioned as that of one who anticipated Harvey in the discovery of the circulation of the blood. A study of his writings clearly shows that he had no true knowledge of the circulation, but only a glimpse of how the blood passed from the right to the left side of the heart. In his work there is evidently a sketch of the pulmonary circulation, although it is clear that he did not understand the mechanism of the valves, as Vesalius did. As regards the' systemic circulation, there is the notion simply of an oscillation of the blood from the heart to the body and from the body to the heart. Further, he up-holds the view of Galen, that all the veins originate in the liver; and he even denies the muscular structure of the heart.' Servetus. In 1553 Michael Servetus (1511-1553), a pupil or junior fellow-student of Vesalius, in his Christianismi Restilutio, described accurately the pulmonary circulation.2 Servetus perceived the course of the circulation from the right to the left side of the heart through the lungs, and he' also recognized that the change from venous into arterial blood took place in the lungs and not in the left ventricle. Not so much the recognition of the pulmonary circulation, as that had been made previously by Columbus, but the discovery of the re- spiratory changes in the lungs constitutes Servetus's claim to be a pioneer in physiological science. Andrea Cesalpino (1519-1603), a great naturalist of this period, also made important contributions. towards the dis- Cesalptno. covery of the circulation, and in Italy he is regarded as the real discoverer a Cesalpino knew the pul- monary circulation. Further, he was the first to use the ' An interesting account of the views of the precursors of Harvey will be found in Willis's edition of the Works of Harvey, published by the Sydenham Society. Compare also P. Flourens, Histosre de la decouverte de la circulation du sang (Paris, .1854), and Professor R. Owen, Experimental Physiology, its Benefits to Mankind, with an Address on Unveiling' the Statue of W. Harvey, at Folkestone, 6th August 1881. ' See Willis, Servetus and Calvin (London, 1877). ' A learned and critical series of articles by Sampson Gamgee in the Lancet, in 1876, gives an excellent account of the controversy as to whether Cesalpino or Harvey was the true discoverer of the circulation; see also the Harveian oration for 1882 by George Johnston (Lancet, July 1882), and Professor G. M. Humphry, Journ. Anat. and Phys., October 1882.term "circulation," and he went far to demonstrate the systemic circulation. He experimentally proved that, when a vein is tied, it fills below and not above the ligature. The following passage from his Quaestiones Medicae (lib. v. cap. 4, fol. 125), quoted by Gamgee, shows his views: " The lungs, therefore, drawing the warm blood from the right ventricle of the heart through a vein like an artery, and returning it by anastomosis to the venal artery (pulmonary vein), which tends towards the left ventricle of the heart, and air, being in the meantime transmitted through the channels of the aspera arteria (trachea and bronchial tubes), which are extended near the venal artery, yet not communicating with the aperture as Galen thought, tempers with a touch only. This circulation of the blood (huic sanguinis circulations) from the right ventricle of the heart through the lungs into the left ventricle of the same exactly agrees with what appears from dissection. For there are two receptacles ending in the right ventricle and two in the left. But of the two only one intromits; the other lets out, the membranes (valves) being constituted accordingly." Still Cesalpino clung to the old idea of there being an efflux and reflux of blood to and from the heart, and he had confused notions as to the veins conveying nutritive matter, whilst the arteries carried the vital spirits to the tissues. He does not even appear to have thought of the heart as a con-tractive and propulsive organ, and attributed the dilatation to " an effervescence of the spirit," whilst the contraction—or, as he termed it, the " collapse "—was due to the appropriation by the heart of nutritive matter. Whilst he imagined a communication between the termination of the arteries and the commencement of the veins, he does not appear to have thought of a direct flow of blood from the one to the other. Thus he cannot be regarded as the true discoverer of the circulation of the blood. More recently Ercolani has Dls- put forward claims on behalf of Carlo Ruini as being co very the true discoverer. Ruini published the first edition of circa-of his anatomical writings in 1598, the year William tatlon of Harvey entered at Padua as a medical student. This blood. claim has been carefully investigated by Gamgee, who has come to the conclusion that it cannot be maintained.' The anatomy of the heart was examined, described and figured by Bartolomeo Eustacheo (c. 150o-1574) and by Julius Caesar Aranzi or Arantius (c. 1530-1589), whose name is associated with the fibro-cartilaginous thickenings on the free edge of the semilunar valves (corpora Arantii). Hieronymus Fabricius of Acquapendente (1537-1619), the immediate predecessor and teacher of Harvey, made the important step of describing the valves in the veins; but he thought they had a subsidiary office in connexion with the collateral circulation, supposing that they diverted the blood into branches near the valves; 'thus he missed seeing the importance of the anatomical and experimental facts gathered by himself. At the time when Harvey arose the general notions as to the circulation may be briefly summed up as follows: the blood ebbed and flowed to. and from the heart in the arteries and veins; from the right side at least a portion of it, passed to the left side through ' the vessels in the lungs, where it was mixed with air; and, lastly, there were two kinds of blood—the venous, formed originally in the liver, and thence passing to the heart, from which it went out to the periphery by the veins and returned by those to the heart; and the arterial, containing " spirits " produced by the mixing of the blood and the air in the. lungs—sent out from the heart to the body and returning to the heart,by the same vessels. The pulmonary circulation was understood so far, but its relation to the systemic circulation was unknown. The action of the heart, also, as a propulsive organ was not recognized. It was not until 1628 that Harvey }tarvey. announced his views to the world by publishing his treatise De Motu Gordis et Sanguinis. His conclusions are given in the following celebrated passage: " And now I may be allowed to give in brief my view of the circulation of the blood, and to propose it for general adoption. Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the auricles and 4 Gamgee, "Third Historical Fragment," in Lancet, 1876. ventricles, and is sent for distribution to all parts of the body, where it makes its way into the veins and pores of the flesh, and then flows by the veins from the circumference on every side to the centre, from lesser to the greater veins, and is by them finally discharged into the vena cava and right auricle of the heart, and this in such a quantity, or in such a flux and reflux, thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingestor, and is much greater than can be required for mere purposes of nutrition, it is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion, that this is the act or function which the heart performs by means of its pulse, and that it is the sole and only end of the motion and contraction of the heart " (bk. x. ch. xiv. p. 68). Opposed to Caspar Hofmann of Nuremberg (1571-1623), Veslingius (Vesling) of Padua (1598-1649), and J. Riolanus the younger, this new theory was supported by Roger Drake, a young Englishman, who chose it for the subject of a graduation thesis at Leiden in 1637, by Werner Rolfinck of Jena (1599-1673), and especially by Descartes, and quickly gained the ascendant; and its author had the satisfaction of seeing it confirmed by the discovery of the capillary circulation, and uni-Capillary versally adopted. The circulation in the capillaries circula- between the arteries and the veins was discovered by don. Marcellus Malpighi (1628-1694) of Bologna in 1661. He saw it first in the lungs and the mesentery of a frog, and the discovery was announced in the second of two letters, Epistola de Pulmonibus, addressed to Borelli, and dated 1661.1 Malpighi actually showed the capillary circulation to the astonished eyes of Harvey. Anthony van Leeuwenhoek (1632-1723) in 1673 repeated Malpighi's observations, and studied the capillary circulation in a bat's wing, the tail of a tadpole and the tail of a fish. William Molyneux studied the circulation in the lungs of a water newt in 1683.E The idea that the same blood was propelled through the body in a circuit suggested that life might be sustained by renewing Transfer the blood in the event of some of it being lost. About of blood. 166o Lower, a London physician (died 1691), succeeded in transferring the blood of one animal directly from its blood vessels into those of another animal. This was first done by passing a " quill " or a " small crooked pipe of silver or brass " from the carotid artery of one dog to the jugular vein of another? This experiment was repeated and modified by Sir Edmund King (1629-1709), Thomas Coxe (1615-1685), Gayant and Denys with such success as to warrant the operation being performed on man, and accordingly it was carried out by Lower and King on the 23rd of November 1667, when blood from the arteries of a sheep was directly introduced into the veins of a man.' It would appear that the operation had previously been performed with success in Paris. The doctrine of the circulation being accepted, physiologists next directed their attention to the force of the heart, the Force of pressure of the blood in the vessels, its velocity, heart and and the phenomena of the pulse wave. Giovanni velocity Alphonso Borelli (1608-1679) investigated the circulaof brood, tion during the lifetime of Harvey. He early conceived the design of applying mathematical principles to the explana- eoreul. tion of animal functions; and, although he fell into many errors, he must be regarded as the founder of animal mechanics. In his De Motu Animalium (168o-85) he stated his theory of the circulation in eighty propositions, and in prop. lxxiii., founding on a supposed relation between the bulk and the strength of muscular fibre as found in the ventricles, erroneously concluded that the force of the heart was equal to the pressure of a weight of 18o,000 lb. He also recognized and figured the spiral arrangement of fibres in the ventricles. The question was further investigated by James Keel 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 ft. 1 See his Opera Omnia, vol. i. p. 328. ' Lowthorp, Abridgement of Trans. Roy. Soc., 5th ed. vol. iii. p. 230. i Ibid. p. 231. * Ibid. p. 226.per minute. Then, allowing for the resistance of the vessels, he showed that the velocity diminishes towards the smaller vessels, and arrived at the amazing conclusion that in the smallest vessels it travels at the rate of 4 in. 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 Oeconomy (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 calculated 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 in. (2 oz.) in length and be driven along with a velocity of 156 ft. per minute. Estimating then the resistances to be overcome in the vessels, he found the force of the heart to be " little above 16 oz.," —a remarkable difference from the computation of Borelli. Keill's method was ingenious, and is of historical interest as being the first attempt to obtain quantitative 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 q lb i oz., and that of the right ventricle at 6 lb 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.5 The experimental investigation of the problem was supplied Bates. 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 ft. 3 in. 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 after-wards gradually at each pulse 12, 8, 6, 4, 2, and sometimes. i in. When it was at its full height, it would rise and fall at and after each pulse 2, 3 or 4 in.; and sometimes it would fall 12 or 14 in., 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 .° He then estimated the capacity, of the left ventricle by a method of employing waxen casts, and, after many such experiments 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 71 ft. high, weighing 511 lb, or, in other words, that the pressure the left ventricle has to overcome is equal to the pressure of that weight. When we contrast the enormous estimate of Borelli (18o,000 lb) with the under-estimate of Keill (16 oz.), and when we know that the estimate of Stephen Hales (1677-1761), as corroborated by recent investigations 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 (1700-1782) in 1748, there was no great contribution to haemadynamics till 18o8, when two remarkable papers ap- peared from Thomas Young (1773-1829). In the first, om= entitled " Hydraulic Investigations," which appeared You in the Phil. Trans., he investigated the friction and dis- charge of fluids running in pipes and the velocity of rivers, the 5 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, Hailer's Elementa Physialogiae, vol. i. p. 448. " Hales, Statical Essays, containing Haemastatics, &c. (1733), vol. ii. p. I. 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 Functions of the Heart and Arteries,"—the Croonian lecture for ,8o8—in which he showed more clearly than had hitherto been done (I) 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 contractile coats do not act as propulsive agents, but assist in regulating the distribution of blood.' The next epoch of physiological investigation is characterized by the introduction :of instruments for accurate measurement, use of and the graphic method of registering phenomena, maim- now so largely used in science.2 In 1825 appeared ments. E. and Wilhelm Weber's (18(4–1891) Wellenlehre, and in 1838 Ernest Weber's (1795–1878) Ad Notat. Ana-tom. 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 Jean Louis Marie Poiseuille invented the haemadynamometer.3 This was adapted with a marker to a recording cylinder byLudwig in 1847, so as to form the instrument named by Alfred Volkmann (1801–1877) the kymograph. Volkmann devised the haemadromometer for measuring the velocity of the blood in 1850; for the same purpose Vierordt constructed the haematachometer in 1858; Chauveau and Pierre Lortet (1792–1868) 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 Karl Vierordt (1818–1884) in 1856; and Etienne Marey's form, of which there are now many modifications, appeared in 1860. In 1861 Jean Chauveau (b. 1827) and Maxey obtained tracings of the variations of pressure in the heart cavities (see 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 conditions 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 (1813–1878) 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 (1816–1895), Claude Bernard and Carl Ludwig (1809–1849) may be regarded as masters in physiology equal in standing 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 inhibitory action; John Reid showed how to investigate the functions of nerves by his classical • research on the eighth pair of cranial nerves; Claude Bernard developed the fundamental conception of vaso-motor nerves; and Ludwig showed how this conception, whilst it certainly made the hydraulic problems of the circulation infinitely more complicated than they were even to the scientific imagination of Thomas Young, accounted for some of the phenomena and indicated at all events the solidarity of the arrangements in the living being. Further, Ludwig and his pupils used the evidence supplied by some of the phenomena of the circulation to explain even more obscure phenomena of the nervous system, and they taught pharmacologists how to study in a scientific manner the physiological action of drugs. (J. G. M.) ' See Miscellaneous Works, ed. Peacock (2 vols., London, 1855)- 2 See Marev, La Methode graph. dans les sc. exper. (Paris, 1878). Magendie's Journal, vol. viii. p. 272.
GIORGIO VASARI (1511-1571)

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