. The structure and life of birds . ourishing allthe tissues of the body. The capillaries unite to formlarger vessels called veins, and these finally form twogreat trunk veins which carry the blood into the rightauricle. From the right auricle it passes to the rightventricle. Thence it is driven into the lungs, from thelungs it passes into the left auricle, and thence intothe left ventricle where the same process beginsagain. Thus the blood in, the right chambers ofthe heart can reach the left only through thelungs : that in the left can find its way to theright only through the arteries and v
. The structure and life of birds . ourishing allthe tissues of the body. The capillaries unite to formlarger vessels called veins, and these finally form twogreat trunk veins which carry the blood into the rightauricle. From the right auricle it passes to the rightventricle. Thence it is driven into the lungs, from thelungs it passes into the left auricle, and thence intothe left ventricle where the same process beginsagain. Thus the blood in, the right chambers ofthe heart can reach the left only through thelungs : that in the left can find its way to theright only through the arteries and veins of thebody. The pure arterial blood is all on the left side,the impure venous blood on the right. The formermay be known by its bright red colour, the latter isblue-black. The following diagram will make clearthe course of circulation. When the blood has passed through the arteriesinto the capillaries and from them into the veins, itfinds a new contrivance to assist in driving it on. In jo THE STRUCTURE AND LIFE OF BIRDS vi FORM AND FUNCTION 71 the veins of the limbs arc valves which prevent anybackward flow. Every movement must tend bypressure to move the blood forward or backward, andit will be urged forward since no other course is places where these valves are in the human arm orhand can be seen if a finger be pressed upon a veinand then passed downwards along it in the directionof the capillaries, thus tending to cause a backwardcurrent. Little knots will be seen at intervals,marking the places where the passage of the bloodis checked by the pouchlike valves. Birds havefewer of these valves than mammals, but more thanreptiles. Besides the veins there are other channels in allparts of the body along which a current is settingtowards the heart. These are the lymphatics, socalled because they contain a pale watery fluid. Theydiffer from veins (1) in that the capillaries from whichthey spring end blindly, do not connect witharteries, (2) in having in
Size: 1082px × 2310px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookidstructurelif, bookyear1895
