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 to the heart, 

 and microscopic capillaries connecting arteries and veins. The walls of 

 the capillaries are so thin that they permit passage of plasma from the 

 blood into the tissues. This fluid, in the form of lymph, is restored to the 



DORSAL AORTA 

 PRECARDINAL V I POSTCARDINAL V 



CAUDAL ARTERY 



'CAUDAL VEIN 



VENTRAL AORTA 1 L.COMMON CARDINAL V. 



AORTIC ARCH 6 



Pig. 243. — 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.) 



veins by way of special vessels, the lymphatics which, Hke 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 may pass to the heart by a vein, while the liquid injected into a 

 vein will not enter the heart from an artery. Later, following the inven- 

 tion of the microscope, Malpighi (1661) discovered the interconnexion of 

 arteries and veins by way of the capiUaries. 



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