538 



THE CELL AND MAMMALIAN HISTOLOGY 



vessels through to the capillaries is of course gradual. The walls 

 of veins are built on the same plan as those of the arteries, but 

 there is much variation. It is obvious in dissection that of a pair 

 of vessels running together to and from an organ the vein has a 

 larger lumen and a thinner wall than the artery, although the 

 outside diameters of the two are about the same. (The colour 

 of the blood can more readily be seen in a vein, while an artery 

 of any size is white or bluish, and an artery may usually be 

 safely held with forceps, whereas the veins are too easily torn.) 

 It is obvious that the larger lumen is made necessary by the fact 

 that the velocity of the current in a vein is less than that in the 



pavement epithelium 

 (endothelium) 



inner coat 



middle coat 



outer coat, 



inner coat, 



endothelium, 

 of vein 



nucleus of 

 muscle ce 



outer coat 



Fig. 421. — Diagrammatic transverse section of an artery and part of a vein of 

 a rabbit X c. 15. The endothelium and inner coat are thrown into folds 

 through contraction of the muscular layer. 



corresponding artery although the rate of flow (in volumes per 

 second) is about the same ; still waters run deep. In a typical 

 vein there is relatively less muscle and elastic tissue and more 

 collagenous tissue than in an artery, and the distinction between 

 the two connective tissue coats is poor or absent. In other veins 

 the amount of muscle ranges from much to nearly none, the 

 differences being presumably and sometimes demonstrably of 

 functional value. Where there is much muscle, as in the hepatic 

 veins and those of the penis, the vein can contract so that its 

 lumen is reduced and return flow of blood prevented. The valves 

 which are present in many veins are made of folds of the endo- 

 thelium with a little connective tissue between the layers. The 

 finest veins, or venules, consist, like arterioles, of endothelium with 

 a thin layer of supporting tissue on the outside. 



