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Vascular System

1911 Encyclopedia Britannica

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Vasco Nurez de Balboa
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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 2 0 1 -0-6to 3 o l ooof 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 tparts. 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.

1 Embryology

2 The Cardiac Nerves

3 (2) Malformations

4 (3) The Myocardium

5 (5) Valvular Lesions

6 (6) Functional Cardiac Disorders


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 new 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 (q.v.). (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 arose from the liver in two great trunks, the vena 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 structure of the valves of the heart. Andreas Vesale or Vesalius (1514-1564) contributed largely to anatomical knowledge, 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, 1880).

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 described 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. (Matthieu Reald Columbo, ob. 1560), 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 upholds the view of Galen, that all the veins originate in the liver; and he even denies the muscular structure of the heart.'. In 1553 Michael Servetus (1511-1553), a pupil or junior fellow-student of Vesalius, in his Christianismi Restitutio, described accurately the pulmonary circulation? 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-. covery of the circulation, and in Italy he is regarded as the real discoverer. 3 Cesalpino knew the pulmonary circulation. Further, he was the first to use the ' An interesting account of the views of the precursors df Harvey will be found in Willis's edition of the Works of Harvey, published by the Sydenham Society. Compare also P. Flourens, Histoire 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 circulationi ) 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 contractive 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 put forward claims on behalf of Carlo Ruini as being covery the true discoverer. Ruini published the first edition of circa- of his anatomical writings in 1598, the year William Harvey entered at Padua as a medical student. This claim has been carefully investigated by Gamgee, who has come to the conclusion that it cannot be maintained.4 The anatomy of the heart was examined, described and figured by Bartolomeo Eustacheo ( c. 1500-1574) 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 announced his views to the world by publishing his treatise De Motu Cordis 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.

Harvey. 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 universally adopted. The circulation in the capillaries circula- between the arteries and the veins was discovered by 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 1683.2 The idea that the same blood was propelled through the body in a circuit suggested that life might be sustained by renewing the blood in the event of some of it being lost. About 1660 Lower, a London physician (died 1681), 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. 3 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. 4 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 pressure of the blood in the vessels, its velocity, and the phenomena of the pulse wave. Giovanni velocity Alphonso Borelli (1608-1679) investigated the circulaton during the lifetime of Harvey. He early conceived the design of applying mathematical principles to the explana 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 (1680-85) 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 180,000 lb. He also recognized and figured the spiral arrangement of fibres in the ventricles. The question was further investigated by 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 ft.

1 See his Opera Omnia, vol. i. p. 328.

2 Lowthorp, Abridgement of Trans. Roy. Soc., 5th ed. vol. iii.

p. 230.

3 Ibid. p. 231.4 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 9 lb 1 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.' The experimental investigation of the problem was supplied 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 afterwards gradually at each pulse 12, 8, 6, 4, 2, and sometimes 1 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. 6 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 72 ft. high, weighing 512 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 (180,000 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 1808, when two remarkable papers appeared from Thomas Young (1773-1829). In the first, entitled " Hydraulic Investigations," which appeared You g s 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., 3749), vol. v. p. 223. See also for an account of the criticisms of D. Bernoulli the elder and others, Haller's Elementa Physiologiae, 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 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 contractile coats do not act as propulsive agents, but assist in regulating the distribution of blood.1 The next epoch of physiological investigation is characterized by the introduction 'of instruments for accurate measurement, and the graphic method of registering phenomena, now so largely used in science. 2 In 1825 appeared E. and Wilhelm Weber's (1804-1891) Wellenlehre, and in 1838 Ernest Weber's (1795-1878) 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 Jean Louis Marie Poiseuille invented the haemadynamometer. 3 This was adapted with a marker to a recording cylinder by Ludwig 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 Marey 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 Marey, La Methode graph. dans les sc. expel'. (Paris, 1878).

3 Magendie's Journal, vol. viii. p. 272.

xxv11.30 Physiology The unicellular animal immersed in water absorbs nurittive matter and oxygen, and excretes waste materials with its whole surface. Owing to the small mass of the protozoa the metabolic products can penetrate throughout the whole. With the evolution of the multicellular organs of the metazoa and the division of physiological labour a circulatory mechanism became of immediate need. A double-layered animal like the common water polype Hydra can exist, it is true, without such a mechanism, but communities of polypes, such as the sponges, form channels for the circulation of water. With the development of the three-layered animal the coelom or body cavity arose by the splitting of the mesoderm, and it was in this body cavity that the evolution of the circulatory system took place, an evolution which finally became perfected in the higher members of the metazoa into a closed vascular system filled with red blood. The evolution of the red matter, haemoglobin, as a special carrier of oxygen was necessitated by the increasing mass and muscular activity of the higher animal, in comparison with the size of the oxygenabsorbing surface - the gill or lung. The blood vascular system of the invertebrata such as the Arthropoda and Insecta, is not generally a closed system, but consists of a pulsatile heart whence proceed arteries which open into lacunar spaces forming part of the coelom. The lacunae exist between the organs and tissues of the body, and the blood from these spaces is returned to a venous sinus whence the heart draws its supply through valved openings. The movements of the animal help to return the blood from the tissue spaces to the heart, while the heart by its rhythmic contraction drives the blood into the arteries. Somewhere in the course of this system are placed the gills and renal organs, and it appears to be a matter of indifference whether the gills be placed on the arterial or venous side of the system, both arrangements being found in different types. In some types (mussel, earthworm), the whole blood passes through the renal organs at each circulation, in others (crayfish) only parts. In the earthworm the vascular system is closed, the arteries and veins being connected by capillaries in place of lacunae. The movement of tissue juices may be maintained by physico-chemical forces alone, e.g. by the forces of osmosis and adsorption, as is seen in the movements of sap in the vascular bundles of plants, in the streaming of protoplasm in the plant cell and in the marvellous rhythmic to-and-fro movements of the richly granular juice contained in the veins of the spreading protoplasmic sheet of myxomycetes. Such agencies come into play in the lacunar or capillary part of the circulation of the metazoa and are assisted by the movements of the body wall and of the alimentary organs. The evolution of a special pumping organ, the heart, associated with the aeration of the body fluids in the gills, led to the perfection of the efficient system of circulation which is found in the vertebrata.

The blood is to be regarded as alive in as strict a sense as any other component of the living body. It is a tissue consisting of mobile elements - the blood corpuscles - and a plasma - a colloidal albuminous fluid which is analogous to the more solid intercellular material of other tissues. The primary sources of its elements are the blood-forming organs - the bone marrow, the haemolymph and lymphatic glands and other lymphatic tissue, and the spleen. It circulates as the middleman between the tissues, conveying from the alimentary canal the products of digestion - sugar, fat, aminoacids and salts; oxygen from the lungs; carbonic acid, urea and other waste products of the tissues to the lungs and kidneys; internal secretions from one organ to another; and acts not only as a carrier, but deals with the material remitted to it on the way. One other function of the blood, a most important one, must not be omitted, that of defence against the invasion of bacteria and their toxins, and other parasites.

The blood is contained in a continuous system of vessels; arteries lead from the heart and divide into a multitude of capillary vessels, and these lead into the veins which finally pass back to the heart. The heart is to be regarded as a double organ, each half consisting of an auricle and a ventricle. The fight half contains dark venous blood which has been returned from the body and is sent to the lungs: the left heart contains the bright oxygenated blood which has been returned from the lungs and is distributed to the body. There are thus two circulations - the one pulmonary, from the right side of the heart to the pulmonary artery and thence to the capillaries of the lungs and to the left heart by the pulmonary veins - the other systemic, from the left side of the heart, by the aorta, to the arteries and capillaries of the body tissues and organs, whence the blood returns by the veins to the right side of the heart. A schematic representation is given of the circulatory system in the accompanying diagram. The venous blood flows into the right auricle (RA) from the superior vena cava and the inferior vena cava. The right ventricle (RV) drives through the lungs the blood received from the right auricle. The right auriculo-ventricular valve, or tricuspid, and the pulmonary semilunar valve are represented directing the flow of blood in this direction. From the pulmonary capillaries the blood returns by the pulmonary veins (PV) into the left auricle (LA), and so through the left auriculo-ventricular or mitral valve into the left ventricle (LV). By the left ventricle the blood is driven through the aortic semilunar valve, and is distributed to the systemic arteries, and so to the capillaries of the various organs and back to the veins. The muscular wall of the auricles and that of the right ventricle are much thinner than that of the left ventricle. This is so, because the energy required of the left ventricle must exceed that of the right ventricle, inasmuch as the resistance in the systemic system exceeds that in the pulmonary circuit.

The heart fills with venous blood during its expansion or diastole, and forces the blood into the arteries during its contraction or systole. The large arteries are of less capacity than the corresponding veins, and their walls are essentially extensile and elastic. The pulmonary arteries are especially extensile structures. The small arteries and arterioles are essentially muscular tubes and can vary considerably in diameter. The arterioles open into the capillaries, and these are so numerous that each organ may be regarded as a sponge full of blood. The skeletal muscles and the muscular walls of the viscera at each contraction express the blood within them, and materially influence the circulation. The whole muscular system, as well as the heart, must therefore be regarded as a pump to the vascular system. The capillary wall is composed of a single layer of flattened cells, separating the blood within from the tissues without. Through this layer, which is of extraordinary tenuity, there takes place an exchange of material between the blood and the tissues, an exchange which depends on the physico-chemical conditions which characterize the living state of the cells. The phenomena of adsorption and osmosis come into play here, but the conditions still await complete elucidation. The veins are of larger calibre than the corresponding arteries, and have tough and inextensile walls. Their walls are muscular, and contract on local stimulation. The veins are not, as a rule, distended with blood to their full potential capacity. The latter is so great that the whole blood of the body can collect within the veins.

The heart and lungs are placed within the thoracic cavity (T), the floor of which is formed by the muscular diaphragm (D); the heart is itself enclosed in a tough inextensile bag, the pericardium (P ), the function of which is to check overdilatation of the heart. The pericardium bears to the muscular wall of the heart the same relation as the leather case of a football does to the bag within. In particular, it prevents over-distension of the heart during muscular efforts.

The abdominal organs and blood vessels are encompassed by the muscular wall of the abdomen (A), and may be regarded as enclosed in a sphere of muscle. Above is the dome of the diaphragm (T), and below the basin-like levator ani, closing the outlet of the pelvis; in front are the recti muscles, behind the quadrati lumborum and the spine; while the oblique and transverse muscles complete the wall at either side. The brain is enclosed in a rigid and unyielding box of bone - the cranium, while the limbs are encompassed by the extensile and, in health, taut and elastic skin.

The heart's energy is spent in maintaining a pressure of blood in the elastic arteries, and by the difference of pressure in the arteries and veins the blood is kept flowing through the capillaries into the veins. The movements of the body and particularly of respiration help to return the blood from the capillaries and veins back to the heart, valves being set in the veins to direct the blood in this direction. The blood is a viscous fluid and its viscosity varies; it is propelled by a heart which varies both in rate and energy; it circulates through a system of muscular and elastic arteries and veins, which varies in capacity and may alter in elasticity. The width of bed through which it flows varies greatly at different parts of the circuit, and the resistance offered to the moving blood is very much greater in the capillary-sized vessels than in the large arteries and veins. The blood continually varies, both in quantity and in quality, as it effects exchanges through the capillary walls with the tissues. The problems of the FIG. I. - General Cou rse of Circulation and some of the Principal Vessels. H', right ventricle; H, left ventricle; A, A, A, aorta; h, part of left auricle; P, pulmonary artery, going to lungs; P, pulmonary veins; v, ascending or lower vena cava; e, trachea or wind-pipe; p, p', bronchial tubes; a', a, right and left carotid arteries; v, v', veins from root of neck (internal jugular and subclavian), joining to form descending or upper vena cava; i, hepatic artery 1, hepatic vein; I, superior mesenteric artery, going to mesentery and bowels; L, portal vein, going to liver; k', renal artery; k, renal vein; V, inferior vena cava, splitting into the two iliac veins, v, v.

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FIG. 2. - Scheme of the Circulation of the Blood in Man, standing erect. The venous system is stippled. C, rigid cranial wall; N, muscles and cutaneous wall of neck; T, thoracic wall; A, muscular and cutaneous wall of abdomen; D, diaphragm; L, muscles and cutaneous wall of limbs; P, pericardium; AO, aorta; S. V. C, I. V. C, venae cavae; P.V, portal vein; V, valves in veins of neck, or legs; RA, LA, right and left auricles; RV, LV, right and left ventricles.

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circulation are thus far from simple. They resolve themselves mainly into a consideration of (1) the physiology of the heart; FIG. 3. - The Thoracic Viscera. In this diagram the lungs are turned to the side, and the pericardium removed to display the heart. a, upper, a', lower lobe of left lung; b, upper, b', middle, lower lobe of right lung; c, trachea; d, arch of aorta; e, superior vena cava; f, pulmonary artery; g, left, and h, right auricle; k, right, and 1, left ventricle; m, inferior vena cava; n, descending aorta; I, innominate artery; 2, right, and 4, left common carotid artery; 3, right, and 5, left subclavian artery; 6, 6, right and left innominate vein; 7 and 9, left and right internal jugular veins; 8 and 10, left and right subclavian veins; II, 12, 13, left pulmonary artery, bronchus and vein; 14, 15, 16, right pulmonary bronchus, artery and vein; 17 and 18, left and right coronary arteries.

(2) the physical characters of the circulation; (3) the control of the heart and vessels by the nervous system.

A. Keith, in Journal of Anatomy and Physiology. FIG. 5. - Showing the Attachments of the Heart. a, a, auricular base of ventricle; c, c, aortic base of ventricles; d, d, arterial mesocardium; e, e, venous mesocardium; f,ascending aorta; g, pulmonary aorta; h, superior vena cava; i, inferior vena cava, perforating diaphragm and pericardium; 1, m, n, structures at the root of the lung - bronchus, pulmonary artery, and pulmonary veins; o, vortex at apex; p, pectinate musculature of right auricle; r, superficial musculature of right ventricle.

The Action of the Heart. The permanent position and general arrangements of the heart are described in a separate article, and it is only necessary here to allude to certain points of physiological importance. The substance of the heart is composed of a special kind of muscular tissue which must be regarded as a syncytium in which no distinct and separate cells occur, a complex plexus of branching and anastomosing fibres, forming one functional whole. The fibres are nucleated, have a cross-striated structure and are surrounded by delicate connective tissue sheaths. The cross-striations are due to the primitive fibrils which as in skeletal muscle are differentiated into alternate doubly and singly refracting substances. These fibrils are embedded in a granular nucleated sarcoplasm. Between the bundles of fibres are thin layers of connective tissue containing closely spun networks of capillaries. The muscle of the auricles consists of a circular layer common to both and a deeper layer separate for each chamber. The auriculo-ventricular ring consists of connective tissue surrounding the auriculo-ventricular orifices and separating the auricular from the ventricular muscle with the exception of an important band, the auriculo-ventricular bundle. The superficial fibres of the ventricles appear to have origin in the auriculo-ventricular ring, to wind about the heart spirally and to end in the tendons of the papillary muscles or pass up to the ring again on the inner surface of the heart. The middle layers consist of bundles of fibres running more or less circularly round the ventricles.

The greater part of the hear', lies free in the pericardial sac. The pericardium is reflected from the wall of the sac on to the wall of the heart and attaches the heart at the point where the venae cavae and aorta leave the sac. This part of the pericardium gives a fixation point to the auricles, for it is attached to the roots of the lungs and thereby to the thoracic wall, to the diaphragm and to the structures at the root of the neck. On opening the chest the normal fulcra for the movements of the auricles are lost, and this renders it difficult to record the exact movements of the heart. The attached part of the heart is called the base, and the venous part of the base is the beginning and the arterial part the end of the tube, coiled on itself, from which in the embryo the heart develops. The longitudinal and circular muscle fibres of the ventricles are antagonists. The circular fibres by their contraction tend to lengthen the apex-base diameter, the longitudinal fibres resist this and the two together wring the blood out of the heart. The apex is maintained as a fixed point by this antagonistic action, and thus the longitudinal fibres are enabled to expand the auricles by pulling down the floor of these chambers. This action is important, as it contributes to the filling of the auricles simultaneously with the emptying of the ventricles. Tracings of the jugular pulse give evidence of such action.

In the case of the auricles the longitudinal musculi pectinati not only help the circular fibres to expel the blood, but draw up the base of the ventricle to meet its load of blood. Thus the base of the ventricular part (or floor of the auricles) is pulled up during auricular systole, and down during ventricular systole. The posterior and upper borders of the left auricle lie against the unyielding structures of the posterior mediastinum, the pulmonary artery and bronchi, the floor and anterior part in contact with the base of the ventricle and ascending aorta respectively. The latter parts alone are free to move during systole. Thus the left ventricular base is drawn up and the aorta back on auricular systole (A. Keith).

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As regards the valves of the heart - (i) the tricuspid guards the right auriculo-ventricular opening, and consists of three flaps of fibrous tissue, covered, like all the internal surfaces of the heart, with the smooth shining membrane, the endocardium. The flaps are continuous at their base, forming an annular From Hill's Manual of Physiology, by permission of Edward Arnold.

FIG. 4. - Diagram of Chambers of Heart and Large Vessels.

A, Vena cava, superior.

B, Vena cava, inferior.

C, Pulmonary artery.

D, Aorta.

E, Right auricle.

F, Right ventricle.

G, Left auricle, into which open the four pulmonary veins.

H, Left ventricle.

The arrows point the course of the blood.

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membrane surrounding the opening. The bicuspid or mitral the consists of two cusps and guards the left auriculo-ventricular opening. The under surface and free edge of each cusp of these FIG. 6. - Cavities of the Right Side of the Heart. a, superior, and b, inferior vena cava; c, arch of aorta; d, pulmonary artery; e, right, and f, left auricular appendage; g, fossa ovalis; h, Eustachian valve; k, mouth of coronary vein; 1, m, n, cusps of the tricuspid valve; o, o, papillary muscles; p, semilunar valve; q, corpus Arantii; r, lunula.

valves are attached by chordae tendinae to two papillary muscles; these are pillars of muscle which rise up from the inner surface of the ventricles.

The edges of these valves which come into opposition are exceedingly thin and delicate, while the outer parts, which bear the full systolic pressure of the blood, are tough. The cardiac muscle, by its contraction, limits the size of the auriculo-ventricular orifices and so maintains the competency of the valves. It is the papillary muscles and chordae tendineae which pull down the diaphragm formed by the closed valves (the floor of the auricles), thus expanding the auricles and enabling the valvular as well as the muscular parts of the wall of the ventricles to approach together and wring out the blood. The thin, moist, film-like edges of the valves of the heart come into perfect apposition and prevent all leakage, while the fibrous parts give strength and support. The ventricles are never completely emptied, for some blood remains in contact with the auriculo-ventricular valves up to the end of systole and ensures Left anterior cusp of pulmonary valve Left posterior cusp of pulmonary valve Left posterior cusp of aortic valve Left coronary artery Anterior cusp of mitral valve Posterior cusp of mitral valve_, Left ventricle a ^ From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 7. - The Bases of the Ventricles of the Heart, showing the au aortic and pulmonary orifices and their valves.

their closure. Incompetency of the valves may arise when the right heart is greatly dilated. The aortic and pulmonary valves consist of three semilunar, pocket-shaped cusps. A fibrous nodule is placed centrally in the free edge of each cusp, whence numerous tendinous fibres radiate to the attached borders of the cusp. The rest of the free edges which come into apposition are thin and delicate. Opposite the cusps are bulgings of the aortic walls - the sinuses of Valsalva. From the anterior one arises the right coronary artery and from the left posterior, the left coronary artery, these vessels supply the substance of the heart with blood. Eddies formed in the sinuses during the period of systolic output bring the semilunar valves into apposition, so that they close without noise or jar at the moment when the intraventricular becomes less than the aortic pressure. The auriculo-ventricular valves are likewise floated up by eddies, and brought into apposition at the moment the intraventricular pressure surmounts that in the auricles.

The heart in size is about equal to the closed fist of a man. The average weight of the heart in the new-born baby is about 24 grms., in the adult 300 grms. The percentage which the heart weight bears to the whole body is. in the new-born and 0.46 in the adult. While the whole body increases in weight 21 -fold, the heart increases only 12.74-fold (Vierordt, Karl, 1818-1884). The average weight of the male and female heart is almost the same. The average volume of the whole heart is about 270 c.c. The capacity, estimated by filling the heart with wax, is for each auricle about 100-150 c.c., and 150-230 c.c. for each ventricle. There are considerable sources of error in such measurements. The muscle of the left ventricle is about 1.6 cm. in thickness, and of the right ventricle 0.5 cm. The left ventricle has twice From Hill's Manual of Physiology, by permission of the muscular mass of Edward Arnold. the right. The cirFIG. 8. - Position of the Valves of the Heart cumference of the left in Systole and Diastole. auriculo-v e n t r i c ul a r orifice is about 14.0 cm.; of the right, about 12.5 cm.; of the aortic orifice, 8

o cm.; of the pulmonary orifice, 9

o cm. The average diameter of the vena cava superior is about 23 mm.; of the vena cava inferior, 34 mm.; of each of the four pulmonary veins about 13-14 mm. of the pulmonary artery, 28 mm.; of the aorta, 32 mm.

The physiologist or physician has many means at his disposal of examining the heart's action. By palpation with the hand over the region of the heart, its stroke, the cardiac impulse, can be felt. By auscultation with the ear directly, or with use of the stethoscope the sounds of the heart can be heard. By percussion the anatomical limits of the organ can be defined. The cardiac impulse can be recorded by tambour methods of registration, the heart sounds by means of the microphone and capillary electrometer, while the volume and movements of the heart can be studied with the help of the Röntgen rays The impulse is caused by the sudden hardening of the muscular mass of the ventricles against the wall of the thorax. It is synchronous with the beginning of systole. The position at which the impulse is felt varies with changing posture cardiac of the body, as different parts of the mpu s. Conus arteriosus thorax come in turn in contact with the ventricle. In the supine position it is usually to be felt in the fifth intercostal space 32 inches from the midsternal line. The chest wall is driven out by the systole only where the heart muscle touches it; at other places it is slightly drawn in. This indrawing is attributed valve to the expulsion of the blood out of the thorax by the left ventricle. The thorax is a closed cavity and the vacuum therein produced by systolic output into the arteries of the head, limbs and abdomen is filled by (I) the drawing of air into the lungs, (2) the drawing of venous blood into the great veins and right auricle, (3) the slight indrawing of the chest wall. The impulse is recorded by placing small cup, or receiving tambour, over the spot where it is most evident, and connecting the inside of the cup by a tube to a recording tambour. The cup can be closed by a rubber dam, or an air-tight junction can be effected by pressing it upon the skin. The stroke of the heart is transmitted as a wave of compression to the air within the system of tambours. The recording tambour is brought to write on a drum, moved by clockwork, and covered with a paper smoked with lamp-black. From the record so obtained we can obtain information as to the time relations of the heart-beat, but no accurate information as to its energy or amount of contraction.

From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 9. - The Relation of the Heart to the Anterior Wall of the Thorax.

I, II, III, Iv, v, vi, the upper six costal cartilages.

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The movements of the heart consist of a series of contractions which succeed each other with a certain rhythm. The period of contraction is called the systole and that of relaxation the diastole. The two auricles contract and relax synchronously, and these movements are followed by the synchronous contraction and relaxation of the ventricles. the Finally, there is a short period when the whole heart is in heart. diastole. The whole series of movements is known as the cardiac _ Right posterior cusp of aortic valve _Anterior (infundibular) cusp of tricuspid valve _Right (marginal) cusp of tricuspid valve _Posterior (septal) cusp of tricuspid valve Right ventricle riculo-ventricular, Right anterior cusp of pulmonary valve Right coronary artery Anterior cusp of aortic cycle. Taking 75 as the average number of heart-beats per minute, each cardiac cycle will occupy. 8 seconds. Of this period auricular systole occupies .1 second auricular diastole occupies. 7 „ ventricular systole occupies

3 „ ventricular diastole occupies. 5 „ In 1861 Chauveau and Marey obtained direct records of the heart of a horse, and determined the sequence and duration of the events happening in the heart, and measured the endo-cardiac pressure by an instrument termed the cardiac sound. The sound - a twoway tube - was pushed down the jugular vein until the orifice of one tube lay in the right ventricle and of the other in the right auricle. The tubes were connected with recording tambours which wrote on a moving drum covered with smoked paper.

Another tambour was used to record the cardiac impulse. The tracings so obtained (fig. to) teach us the following facts: (1) The auricular contraction is less sudden than the ventricular, and lasts only a very short time, as indicated by the line ab. The ventricle, on the other hand, contracts suddenly and forcibly and remains contracted a considerable time, as shown by the line c' d' and by the flat top to the curve which succeeds d'. (2) The auricular movement precedes the ventricular, and the latter coincides with the impulse of the apex against the wall of the chest. (3) The contraction of the auricle influences the pressure in the ventricle as shown by the small rise a'b', and that of the ventricle influences the pressure in the auricle somewhat as shown by the waves cd. Much labour has been spent in the contrivance of rapidly acting spring pressure gauges, freed as far as possible from inertia, in order to investigate more exactly the changes of intracardiac pressure, which were first described by Chauveau and Marey. As the intraventricular pressure FIG. 10. - Tracings from the Heart of a Horse, by Chauveau and Marey. The upper tracing is from the right auricle, the middle from the right ventricle, and the lowest from the apex of the heart. The horizontal lines represent time, and the vertical amount of pressure. The vertical dotted lines mark coincident points in the three movements. The breadth of one of the small squares represents one-tenth of a second.

may rise 150 mm. of mercury in one-tenth of a second, it is no easy matter to contrive an instrument which will respond as rapidly and yet yield an accurate result without overshooting the mark. The final result of a most careful inquiry is the confirmation in almost every point of Chauveau and Marey's pressure curves. Karl Hi.irthle's differential manometer has proved to be an instrument of great value and precision. A double-bored tube cannula is introduced so that one tube reaches the right auricle and the other the right ventricle. In observations on the left side of the heart, one tube is placed in the left ventricle and the other in the aorta, and each of these tubes is brought into connexion with a tambour. The two tambours are placed one on either side of the fulcrum of a lever. This lever works against a light spring, which in its turn sets in motion a writing-style. The style records the pressure changes on a drum covered with smoked paper. By this means there can be recorded the exact moment at which the auricular pressure exceeds that in the ventricle, that is to say, the moment when the auriculo-ventricular valves open; likewise the moment when the ventricular pressure becomes greater than that in the auricles, and the auriculo-ventricular valves shut. Similarly, there can be recorded the moment when the intraventricular pressure exceeds that in the aorta and the semilunar valves open, and the moment at which the diastole of the ventricle begins, when the aortic pressure becomes the greater, and the semilunar valves shut. The smoothness with which the heart works is shown by the fact that neither the opening nor the closing of the valves is marked by any peak or point on the pressure curves.

The absence of a mechanism for preventing regurgitation of blood from the auricles of birds and mammals is remarkable, for in fishes, amphibia and reptiles this is effected by valves guarding the sino-auricular junction. In the warm-blooded vertebrata with the appearance of the diaphragm the sinus becomes merged into the right auricle, and the venous cistern formed by the superior and inferior venae cavae, the innominate, iliac, hepatic and renal veins takes the place of the sinus.

Six pairs of valves prevent regurgitation from this cistern, viz. those placed in thee common. femoral, the sub-clavian and jugular veins. The cistern when filled holds some 400 c.c. of blood; in the liver there is some 500 c.c. of blood, and this can be expressed into the cistern by abdominal pressure; in the portal venous system, when distended, another 500 c.c. may be held, which can be expressed through the liver into the cistern. A large volume of blood is thus at the disposal rena cave of the heart for it to draw on during diastole. Respiration by the aspirating action of the thorax sucks this blood into the heart, while the inspiratory descent of the diaphragm squeezes the abdominal contents and forces blood from the liver and cistern into the heart. These forces take the place of the sinus and are far more efficient. The intra-abdominal pressure may be raised on bending or straining till it becomes equivalent to the pressure of a column of mercury 80-10o mm. high (Keith). Under such conditions the pericardium prevents the right side of the heart being over-distended with venous blood.

With these facts in view, we can now describe the complete course of a cardiac cycle. We will start at the moment when the blood is pouring from the venae cavae and pulmonary veins into the two auricles. The auricles are relaxed and their cavities open into the ventricles by the funnelshaped apertures formed by the dependent segments of the tricuspid and mitral valves. The blood passes freely through these apertures into the ventricles. The small positive pressure which is always present in the venous cistern (aided by the respiratory forces) From Diseases of the Heart, by James Mackenzie, M.D., by permission.

FIG. 12. - Tracings of the Jugular Pulse Apex Beat, Carotid and Radial Pulses. The perpendicular lines represent the time of the following events. 1, the beginning of the auricular systole; 2, the beginning of ventricular systole; 3, the appearance of the pulse in the carotid; 4, the appearance of the pulse in the radial; 5, the closing of the semilunar valves; 6, the opening of the tricuspid valves.

is at this time filling the right heart, while the positive pressure in the pulmonary veins is filling the left heart. The auricular systole now takes place. The circular muscle bands compress the blood out of the auricles into the ventricles, while the longitudinal bands aid in this and pull up the base of the ventricles to meet the load of blood. As the contraction starts from the mouths of the venae cavae, and sweeps towards the ventricles, there can NHN?aiN? ' 'MEW  ?[?

Nnnn((/O¦. ??

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'NM MN?r¦?1 EMU 'A ' 'Wwii I¦mNill ' 'ME M .t?._?N A. Keith, in Journal of Anatomy and Physiology. 'FIG. 11. - Diagram of the Venous Cistern from which the Heart is filled. The abdominal or infradiaphragmatic part of the cistern is indicated in black; the thoracic or supra-diaphragmatic is stippled.

occur but little regurgitation of blood into the venous cistern, but the cessation of flow into the auricle during its systole does produce a slight rise of pressure in the cistern, as is shown by tracings taken from the jugular pulse. The function of the auricles is to rapidly complete the filling of the ventricles.

The auriculo-ventricular valves are floated up and brought into apposition by eddies set up in the blood which streams into the ventricles, and close without noise or jar at the moment when the intra-ventricular pressure exceeds in the least that in the auricles. The systole of the ventricles immediately following that of the auricles closes the auriculo-ventricular valves, and as the intra-ventricular pressure rises above that in the pulmonary artery and aorta respectively the semilunar valves open and the blood is

Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Vascular System'. 1911 Encyclopedia Britanica. https://www.studylight.org/​encyclopedias/​eng/​bri/​v/vascular-system.html. 1910.
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