BLOOD, Circulation of. The highly dif ferentiated and marvelously regulated circula tory system in man furnishes striking evidence of evolutionary development in response to growing complexity of environmental de mands. Unicellular animals and the lowest metazoa obtain from the surrounding fluid in which they live oxygen and nutritive material and excrete waste materials into it directly, the whole surface performing these functions. The movement of the fluid is maintained by the movements of the cell or body of the ani mal itself and by the influence of nutritive ab sorption. In the Polypi the movement receives aid besides from the action of cilia on the inner walls of the body. The division of physiologi cal labor in the more complex and differen tiated metazoa requires a special circulatory mechanism and a fluid which contains more specifically respiratory and nutritive elements. Therefore in the three-layered animal a cir culatory system was evolved within a celom or body cavity. The fluid within this cavity is still, however, in some coelenterates merely water containing organic elements, but this gradually acquires a greater diversity of ele ments making for coagulability, more complete respiratory function and better fitting it for supplying varying tissues in their nutritive and excretory needs. It is only, however, in the vertebrates that the blood exhibits a constancy of composition and circulates in a completely closed system. Birds and mammals alone have reached a complete separation of the arterial and venous blood. The Arthropods and In sect° have a circulatory system which consists of a pulsating heart which sends its current through the arteries into open spaces or la cuna within the body cavity where the blood and tissues directly interchange their contents. The muscular movements of the body help to return the blood into a venous sinus from which it reaches the heart by slitlike aper tures in the latter provided with valves. Dur ing its course the blood passes through the rcnals and gills. Capillaries are found Instead of the lacuna in the annelids, as the earth worm. The Molluscs have the heart provided with an auricle and a ventricle, as in the snail and whelk; two auricles, one on either side of the ventricle, as in the fresh-water mussel; or two auricles and two ventricles, as in the ark shells. Among the ascidians, which stand low in that division of animals to which the mollusks belong, the remarkable phenomenon is encountered of an alternating current, which is rhythmically propelled for equal periods in opposite directions. All vertebrated animals,
except Amphioxus, have a heart which in most fishes consists of an auricle. and a ventricle, but in the mud fishes (Lepidosiren) there are two auricles and one ventricle; and this tri locular heart is found in the amphibians and in most reptiles except the crocodiles, which, like birds and mammals, have a four-cham bered organ consisting of two auricles and two ventricles. In the crocodiles, however, the two blood streams are not yet wholly sep arated because of a communication between the right aorta containing arterial blood and the left aorta containing venous blood. In the trilocular hearts the two currents are mixed in the single ventricle but a separation is gradually established by the extension of the ventricular septum which finally completely divides the ventricle.
History.— The correct knowledge of the circulation of the blood in the human organ ism dates from the investigations and discov eries of Harvey published in 1628. In reality the discovery of the circulation was a gradual accretion but it was Harvey who finally placed this knowledge upon an accurate basis, re moving certain mistaken theories which were still accepted and demonstrating at last the cir culatory process. Galen's physiological teach ings had long held sway even though they had come to be considerably modified. This an cient physiologist distinguished, as others had already done, arteries from veins and exploded the still older theory that the arteries con• tamed air. The veins, he believed, had their origin in the liver, proceeding from it in two great trunks; the portal vein carrying to the liver the food absorbed from the alimentary canal which was there converted into blood containing nutritive "natural spirits,') and the vena cava which divided into an ascending and a descending branch. The blood was carried to the right side of the heart where commu nicating pores in the septum united the two ventricles and where the action of the innate heat of the heart and the air obtained through respiration united to produce the ((vital which make the blood fit for higher functions. The veins further carried the crude blood in an ebb and flow from the right side of the heart for the lower nutritive functions, while similarly the purer arterial blood from the left side supplied to the tissues their vital func tions. From this arterial blood in the brain spirits) were elaborated which were carried along the nerves to produce movements and the highest functions of the body.