THE CIRCULATION. 295 



The Venous Circulation. 



The veins, like the arteries, are composed of three coats an inner, 

 middle, and exterior ; but they contain a smaller quantity of muscular 

 and elastic fibres, and a larger proportion of condensed connective 

 tissue. They are consequently more flaccid and compressible than 

 the arteries, and less elastic and contractile. They are furthermore 

 distinguished, in the limbs, neck, and external parts of the head and 

 trunk, by being provided with valves, in the form of festoons, so placed 

 that they allow the blood to pass from the periphery toward the heart, 

 but prevent its reflux in the opposite direction. 



Though the walls of the veins are thinner and less elastic than those 

 of the arteries, yet their capacity for resistance to pressure is equal, or 

 even superior. Milne Edwards* has collected the results of various 

 experiments, which show that the veins will sometimes bear a pressure 

 sufficient to rupture the arteries. In one instance the jugular vein 

 supported a pressure equal to a column of water 148 feet in height ; 

 and in another, the iliac vein of a sheep resisted a pressure of more 

 than four atmospheres. The portal vein resisted a pressure of six 

 atmospheres : and in one case, in which the aorta of a sheep was rup- 

 tured by a pressure of 72 kilogrammes, the vena cava of the same 

 animal supported a pressure of 80 kilogrammes. 



This property of the veins is due to the white fibrous tissue in their 

 composition ; the same tissue which forms nearly the whole of the 

 tendons and fascia, and which is distinguished by its density and 

 unyielding nature. 



The elasticity of the veins, on the other hand, is much less than that 

 of the arteries, and there is consequently but little variation in their 

 calibre. When filled with blood, they swell to a certain size ; when 

 empty, their sides collapse, and remain in contact with each other. 



Another peculiarity of the venous system consists in its numerous 

 communicating channels. 



In injected preparations, two, three, or more veins are often seen 

 coming from the same region, with frequent transverse communica- 

 tions. The deep veins accompanying the main arteries of the limbs 

 inosculate freely with each other, and also with the superficial veins. 

 Among those coming from the head, the external jugulars communi- 

 cate with the thyroid, the anterior jugular, and the brachial veins. The 

 external and internal jugulars communicate with each other, and the 

 two thyroid veins form an abundant plexus in front of the trachea. 



Thus the blood, coming from the periphery toward the heart, flows 

 in a number of communicating channels; through which it passes, 

 under different conditions of pressure, by a variety of routes, but 

 always in the same direction. 



Movement of the Blood through the Venous System. The flow of 



Lefons sur la Physiologie. Paris, 1859, tome iv., p. 301. 



