Elements of animal physiology, chiefly human . y forthe perpetual movement or circulation of the bloodfrom the first dawn of life to its ultimate close. Wenow proceed to discuss and explain the nature of the CIRCULATION. 87 mechanism and the forces which by this movement ofcirculation is initiated and sustained. 181. Circulation (from Lat. circulo, I encompass) is theprocess by which the blood is driven out from the heart,conveyed by the arteries and arterial capillaries, to thevarious parts of the body, and again returned to it by thevenous capillaries and the veins, the whole forming onecont
Elements of animal physiology, chiefly human . y forthe perpetual movement or circulation of the bloodfrom the first dawn of life to its ultimate close. Wenow proceed to discuss and explain the nature of the CIRCULATION. 87 mechanism and the forces which by this movement ofcirculation is initiated and sustained. 181. Circulation (from Lat. circulo, I encompass) is theprocess by which the blood is driven out from the heart,conveyed by the arteries and arterial capillaries, to thevarious parts of the body, and again returned to it by thevenous capillaries and the veins, the whole forming onecontinuous movement or journey. The circu-lation is some-times describ-ed under theterms greateror systemic cir-culation, lesseror pulmonarycirculation,and a subor-dinate branchof the systemiccirculation,termed theportal circu-lation. The greatercirculation isthat by whichthe blood issent out fromthe left side ofthe heart, dis-tributed bythe arteriesthrough thesystem, andreturned bythe veins tothe right side of the lesser circulation is. rifr. 40. Theoretic Circulation. that by which the blood is 88 ANIMAL PHYSIOLOGY. sent out from tlie right side of the heart to the lungs,and returned from them to its left side. The portal circulation comprises that portion of thesystemic circulation by which the blood, distributed tothe stomach, liver, spleen, pancreas, and the intestines,is collected by the vena 2^orta, and distributed throughthe substance of the liver, on its way to the inferiorvena cava, which it reaches by the hepatic veins. The heart, the arteries, the veins, and the capillariesconstitute the chief organs of circulation, and are de-signated the ^^circulatory system. 182. Position and Size of the Heart.—The heart issituated in the mediastinum or central part of thecavity of the thorax, immediately under the lowertwo-thirds of the sternum or breast-bone. It isalmost entirely surrounded by the two lungs. Itsbase is directed backwards and iipwards, being sup-ported c
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