THE CIRCULATION" OF THE BLOOD. 



113 



lapsed, and often to contain blood. The veins are usually distributed 

 in a superficial and a deep set which communicate frequently in their 

 course. 



Fio 108 -Transverse section through a small artery and vein of the mucous membane of a 

 ch!ld%^^?S?SSSrt between the thick- walle/artery and the thin^aUed vein is well shown 

 A Artery the letter is placed in the lumen of the vessel, e. Endothehal cells with nuclei clearly vis- 

 ible-these cells appear very thick from the contracted state of the vessel. Outside it a double wavy 

 linl martS the ef a P stfc tunica intima. m. Tunica media forming the chief part of artenal wall and 

 consisting of unstriped muscular fibres circularly arranged: their nuclei are weU [seen, a Part of 

 the tunica adventitia showing bundles of connective-tissue fibre in section, with the circular nuclei 

 of the connective tissue corpuscles. This coat gradually merges into the surroundmg connective 

 tissue, V. In the lumen of the vein. The other letters indicate the same as in the artery The 

 muscular coat of the vein (m) is seen to be much thinner than that of the artery. X 350. (Klein 

 and Noble Smith.) 



FIG. 109. Diagram showing valves of veins. A, part of a vein laid open and spread out, with 

 two pairs of valves. B, longitudinal section of a vein, showing the apposition of the edges of 

 the valves in their closed state, c, portion of a distended vein, exhibiting a swelling in the situation 

 of a pair of valves. 



/Structure. In structure the coats of veins bear a general resemblance 

 to those of arteries (Fig. 108). Thus, they possess an outer, middle, and 

 internal coat. The outer coat is constructed of areolar tissue like that 

 of the arteries, but is thicker. In some veins it contains muscular fibre- 

 cells, which are arranged longitudinally. 

 8 



