VENOUS CIRCULATION. 291 



This resistance of the veins is to be attributed to the large pro- 

 portion of white fibrous tissue which enters into 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, however, is much less than that of the 

 arteries. When they are filled with blood, they enlarge to a certain 

 size, and collapse again when the pressure is taken off; but they do 

 not react by virtue of an elastic resilience, or, at least, only to a 

 slight extent, as compared with the arteries. Accordingly, when 

 the arteries are cut across, and emptied of blood, they still remain 

 open and pervious, retaining the tubular form, on account of the 

 elasticity of their walls; while, if the veins be treated in the same 

 way, their sides simply fall together and remain in contact with each 

 other. 



Another peculiarity of the venous system is the abundance of 

 the separate channels, which it affords, for the flow of blood from 

 the periphery towards the centre. The arteries pass directly from 

 the heart outward, each separate branch, as a general rule, going 

 to a separate region, and supplying that part of the body with 

 all the blood which it requires ; so that the arterial system is kept 

 constantly filled to its entire capacity with the blood which passes 

 through it. But that is not the case with the veins. In injected 

 preparations of the vascular system, we have often two, three, 

 four, or even five veins, coming together from the same region of 

 the body. There are also abundant inosculations between the dif- 

 ferent veins. The deep veins which accompany the brachial artery 

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

 of the arm. In the veins, coming from the head, we have the ex- 

 ternal jugular communicating with the thyroid veins, the anterior 

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

 communicate 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 many channels ; and as these 

 channels communicate freely with each other, the blood passes some- 

 times through one of them, and sometimes through another. 



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 forces. 



