CIKCULATION 259 



The coats of the arteries are thick ; those of the veins are 

 thin. In the large arteries the muscular fibres of the media 

 are largely replaced by elastic fibres, so that the vessels may 

 better stand the strain of the charge of blood which is shot 

 from the heart at each contraction. In the veins double 

 flaps of the tunica intima form valves which prevent any 

 regurgitation of blood. 



The great characteristic of the walls of the large arteries 

 is the toughness and elasticity given by the abundance of 

 elastic fibrous tissue, of the small arteries the contractility 

 due to the preponderance of muscular fibres. 



Physiology 



The circulation of blood in the vessels is that of a fluid in a 

 closed system of elastic-walled tubes, at one end of which (the 

 great arteries) a high pressure, and at the other (the great veins) 

 a low pressure, is kept up. As a result of this distribution of 

 pressure there is a constant flow of blood from arteries to veins. 



Many points in connection with the circulation may be 

 conveniently studied on a model or scheme made of indiarubber 

 tubes and a Higginson's syringe. (Practical Physiology,) 



A. BLOOD PRESSURE 



The distribution of pressure is the cause of the flow of blood, 

 and must first be considered. 



1. General Distribution of Pressure 



That the pressure throughout the greater part of the blood 

 vessels is positive greater than the pressure of the atmos- 

 phere is indicated by the fact that if a vessel is opened, the 

 blood flows out of it. The force with ivhich blood escapes is 

 a measure of the pressure in that particular vessel. If an 

 artery be cut, the blood escapes with great force ; if a vein be 

 cut, with much less force (fig. 122). 



Arteries. If the pressure in the aorta, in the radial, in 

 the dorsalis pedis, and in one of the smallest arteries is 

 measured, it is found that while it is great in the great 

 arteries about 160 mm. Hg in the aorta it is much less 



