550 



ANATOMY OF VERTEBRATES. 



outer mass of areolar tissue, in which such fibres as can be traced 

 run lengthwise, c. This forms the greatest proportion of the 

 venous coat, and the transition to the mid-stratum, b, is closer 

 than that between b and the thinnest layer, a, which is lined by 

 the epithelial tissue. 



The elastic tissue is never so developed as to give the yellow 

 colour which characterises the middle coat of arteries, or to main- 

 tain the tubular form in the empty vein. The valves are not 

 confined to the place of union of the venous trunks with the heart, 

 as is the case with arteries, but occur, usually in pairs, fig. 426, in 

 . 96 the major part of the venous 



system after the vessels have 

 gained, in returning from the ca- 

 pillary area, a conspicuous size. 

 They e consist,' as Harvey de- 

 scribed, 6 of raised or loose por- 

 tions of the inner membrane of 

 these vessels, of extreme deli- 

 cacy, and a sigmoid or semilunar 

 shape. They are situate at dif- 

 ferent distances from one an- 

 other, and diversely in different 

 individuals ; they are connate at 

 the sides of the veins ; they are 

 directed upward or toward the 

 trunks of the veins ; the two — 

 for there are, for the most part, 

 two together — regard each other, 

 mutually touch, and are so ready 

 to come into contact by their 

 edges, that if anything attempt 

 to pass from the trunks into the 

 branches of the veins, or from 

 the greater veins into less, they 

 completely prevent it ; they 

 are further so arranged, that the horns of those that succeed 

 are opposite the middle of the convexity of those that pre- 

 cede, and so on alternately.' He further writes, ' In many places 

 two valves are so placed and fitted, that, when raised, they come 

 exactly together in the middle of the vein, and are there united 

 by the contact of their margins ; and so accurate is the adapta- 

 tion, that neither by the eye, nor by any other means, can the 

 slightest chink along the line of contact be perceived. But if 



Venous valves, a, femoral vein : b, saphena interna, 

 Human, ccvi". 



