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198 Dr . Southwood Smith on the Animal Economy .
Untitled Article
whole mass of the blood in an adult man is about twenty-iive pounds ; different currents of it complete the circulation at different times , in proportion to the length of the course they have to make , and the degree of resistance they have to encounter ; as , for instance , a part of the stream has only to circulate through the muscles of the heart itself , while other parts have to supply organs widely removed from it ; but it is thought that the entire ^ circulation is completed , on an average , in two minutes and a half . A quantity of blood , therefore , equal to the whole mass must pass through the heart twenty-eight times in an hour . * Consider , ' said Dr . Smith , ' what an affair this must be in very large animals . The aorta of the whale is larger in the bore than the main pipe of the water-works that supply London with water . Ten or fifteen gallons of blood are thrown out of its huge heart at half-stroke with an immense velocity into a tube of a foot diameter /
All the arteries of the system take their rise from two great trunks . One , the pulmonary artery , springing from the right ventricle to ramify through the lungs ; the other , the aorta , springing from the left ventricle to supply the whole body . These two main-trunks , each following its own course , divide and subdivide , every branch becoming smaller and smaller , till they reach a degree of minuteness which is ill-described by the term capillary arteries , for they are much smaller than the finest hair . A more accurate idea of their real size will be
conceived by the recollection that some of them are too small to admit a single red particle of the blood , estimated at about - $ -fev of an inch in diameter . These capillary arteries pervade ' every organ and every tissue in such numbers , that , as before stated , the point of the finest needle can penetrate nowhere without wounding some of
them . They terminate in the capillary veins . The veins go on in the inverse order of the arteries , uniting together , forming larger and larger branches , till gradually they become veins of considerable magnitude , and , at length , form two great trunks , the superior and inferior venae cavae , pouring the blood into the right auricle of the heart . We have here described the systemic veins . The pulmonic form four trunks , and return the blood renovated and ready for the systemic arteries to the left auricle . The artery has three distinct coats . The external one is composed of cellular tissue , the substance of which all the membranes of the
body are formed . The middle one is formed of fibres , arranged in rings round the vessel ; it is the strongest and thickest of the three , and is highly elastic , especially longitudinally , possessing also the power of enlarging and diminishing the caliber of the tube , a power truly vital , and extremely analogous to muscular contractility . The inner covering of the artery is called the serous coat ; it is strong , but thin , smooth , and polished , in order to offer as little resistance
as possible to the now of the blood . In the capillaries the structure 19 considerably modified . The coats become gradually thinner till , at length , they disappear entirely , and the blood flows through membraneless canals in the substance of the tissues . The disappearance of the membranous coats of the capillaries has been only recently discovered by observations with the microscope . With its assistance the currents of Wood have beea see # flowing through the tissue ? . .
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), March 2, 1833, page 198, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct2610/page/54/
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