THE VENOUS CIRCULATION. 341 



results of various experiments, which show that the veins will some- 

 times resist a pressure which is sufficient to rupture the walls of the 

 arteries. 1 In one instance the jugular vein supported, without breaking, 

 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 atmos- 

 pheres. The portal vein was found capable of resisting a pressure of 

 six atmospheres ; and in one case, in which the aorta of a sheep was 

 ruptured by a pressure of 12 kilogrammes, the vena cava of the same 

 animal supported a pressure equal to 80 kilogrammes. 



This property of the veins is to be attributed to the abundance of 

 white fibrous tissue in their composition ; the same tissue which forms 

 nearly the whole of the tendons and fasciae, 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. When filled with blood, they enlarge to a certain size ; 

 and when cut across and emptied, their sides simply collapse and remain 

 in contact with each other. 



Another peculiarity of the venous system consists in its numerous 

 independent and communicating channels. 



In injected preparations, two, three, or more veins are often to be 

 seen coming, together, from the same region of the body, and presenting 

 frequent transverse communications. The deep veins accompanying the 

 brachial artery inosculate freely with each other, and also with the 

 superficial veins of the arm. In the veins coming from the head, the 

 external jugulars communicate with the thyroid veins, the anterior 

 jugular, and the brachial veins. The external and internal jugulars 

 commuicate with each other, and the two thyroid veins also form an 

 abundant plexus in front of the trachea. 



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

 not in a single channel, but in several; and as these channels communi- 

 cate freely with each other, the blood passes most abundantly some- 

 times through one of them, and sometimes through another. 



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

 blood through the veins is less powerful and regular than that through 

 the arteries. It depends on the combined action of three different physi- 

 cal forces. 



I. The most constant and important of these forces is the pressure 

 of the blood from the capillary circulation. The blood moves from the 

 arteries into and through the capillary vessels, under an impulse derived 

 originally from the contractions of the heart, and converted by the elas- 

 ticity of the arterial walls into a more or less steady and uniform pres- 

 sure. This pressure is not entirely exhausted in carrying the blood 

 through the narrow channels of the capillary system ; and it accord- 

 ingly emerges from these vessels and enters the commencement of the 

 veins with a certain amount of force sufficient to fill the venous rootlets 



1 Legons sur la Physiologie. Paris, 1859, tome iv. p. 301. 



