. First aid in illness and injury; . ssues into the blood. Itis in these vessels that the final aim ofthe circulation is accomplished. Hereis extracted from the blood the nutritionbrought into the circulation by thearteries, and here the blood acquiresthe waste products which are to becarried away by the veins. The veins are formed by the unionof two or more capillaries, and contin-ually joining together, ultimately form large trunks, just as the little brooks and streamlets join toform larger streams, and, by continually uniting, at lastform the mighty river flowing down to the sea. Finally t
. First aid in illness and injury; . ssues into the blood. Itis in these vessels that the final aim ofthe circulation is accomplished. Hereis extracted from the blood the nutritionbrought into the circulation by thearteries, and here the blood acquiresthe waste products which are to becarried away by the veins. The veins are formed by the unionof two or more capillaries, and contin-ually joining together, ultimately form large trunks, just as the little brooks and streamlets join toform larger streams, and, by continually uniting, at lastform the mighty river flowing down to the sea. Finally theveins unite into two great venous trunks,—the inferior venacava, bringing the blood from the body and lower extremities,and the superior vena cava, bringing the blood from the headand the upper extremities into the right auricle. The pulmo-nary veins, four in number, bring the blood from the lungs tothe left auricle. Unlike the arteries, the veins collapse when empty, andenlarge when filled. This can readily be seen in the veins. AttcTU Fig. 41. — Capillary networkfrom the bowel, showinghow the capillaries con-nect the veins and arteries. 54 CONSTRUCTION OF THE HUMAN MACHINE in the hand and forearm, which are ordinarily small andinconspicuous, but after any exercise which tends to increasethe flow of venous blood, or when the arm is tightly boundso as to delay the flow of blood toward the shoulder, thevessels become large and prominent. Unlike the arteries, the flow of blood in which is caused by the pump-ing of the heart, the veins have no organ which directly forces theircontents to their destination. The movement of blood through theveins is due to four causes, (i) Thepressure behind of the blood pushed intothe capillaries from the arteries by theheart; this is the main cause. (2) Thepresence of valves at frequent intervals,which prevent the backward flow of provision in veins is of great surgicalimportance, for the blood can only flowfrom the smaller end of
Size: 1069px × 2337px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpubli, booksubjectphysiology
