. Comparative anatomy. Anatomy, Comparative. CHAPTER lo THE VASCULAR SYSTEM The Vascular System. While the blood of many invertebrates fills intercellular spaces without specialized walls, the circulation in verte- brates is a closed system, the essential components of which are a circulat- ing fluid, a heart with receiving and propulsive chambers and valves so arranged as to permit the blood to flow in one direction only, arteries to carry blood away from the heart, veins to bring blood back again, and microscopic capillaries to connect arteries and veins. The walls of the capillaries are so
. Comparative anatomy. Anatomy, Comparative. CHAPTER lo THE VASCULAR SYSTEM The Vascular System. While the blood of many invertebrates fills intercellular spaces without specialized walls, the circulation in verte- brates is a closed system, the essential components of which are a circulat- ing fluid, a heart with receiving and propulsive chambers and valves so arranged as to permit the blood to flow in one direction only, arteries to carry blood away from the heart, veins to bring blood back again, and microscopic capillaries to connect arteries and veins. The walls of the capillaries are so thin that they permit passage of plasma from the blood DORSAL AORTA PRECARDINAL V | POSTCARDINALV MESENTERIC CAUDAL ARTERV. CAUDAL VEIN MOUTH VENTRAL AORTA I CARDINAL V. 1 SUBINTESTINALV. AORTIC ARCH 6 ABDOMINAL V Fig. 291.—Diagram of the primitive chordate circulation. The arteries are shown in black, the veins are stippled. The similarity of this circulatory system to that of annelids has suggested to morphologists a common genetic origin. (After Kingsley modified.) into the tissues. This fluid in the form of lymph is restored again to the veins by way of special vessels, the lymphatics, which like veins, permit flow in only one direction. It was William Harvey (1616) who first demonstrated the circulation of the blood. Before Harvey's day it had been assumed that the blood ebbs and flows in the arteries and veins Hke water in tidal streams. Har- vey was able to demonstrate that the valves in the heart and blood vessels permit a one-way movement only; that a cut artery spurts blood from the cut end nearer the heart, while a cut vein bleeds most from the end farther from the heart; that pressure of a finger on a vein results in distension on the side farther from the heart; and that in a dead body Uquid injected into an artery will return to the heart by a vein, while the Hquid injected into a vein wiU not return to the heart by an artery. Later, following the inven
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