196 STRUCTURE OF VEINS. [BOOK i. 



instance, a considerable quantity of white connective tissue is 

 present in the middle and indeed in the inner coat, as well as in 

 the outer coat. Leaving these smaller differences on one side we 

 may say, that while all three coats, but especially the important 

 middle coat, contribute to give an artery its characteristic elastic 

 qualities, by virtue of which it expands readily under internal 

 pressure, and shrinks again when the pressure is removed, it is 

 the middle coat which by means of the abundant circularly 

 disposed muscular fibres, now through the contraction of those 

 fibres narrows and constricts, now through their relaxation permits 

 the widening of the vessel. The importance of the inner coat is 

 probably centred in the epithelioid lining ; in treating of blood 

 ( 22) we saw reason to think that the blood vessels exerted a 

 marked, though obscure influence on the blood streaming through 

 them ; that influence in all probability is effected by the epithe- 

 lioid cells. The elastic elements of the inner coat are probably 

 chiefly of value in permitting this coat to follow the changes of the 

 more important middle coat. The outer coat, while increasing the 

 elastic power of the whole vessel, is especially useful, by means of 

 its small blood vessels, in conveying nourishment to the middle coat. 



110. The Veins. These vary in different parts of the body 

 so very widely, that it is difficult to give a general description of 

 structure suitable to all veins. It may be said however that they 

 differ from arteries in having much thinner walls, and in those 

 walls containing relatively much more white connective tissue, 

 and much less yellow elastic tissue. 



A large vein possesses like an artery an inner coat consisting 

 of an epithelioid lining, the cells of which are shorter and broader 

 than in the corresponding artery, resting on an elastic basis, which 

 is less conspicuous than in the corresponding artery, and consists 

 of a fine feltwork of fibres rather than a fenestrated membrane, 

 and contains more white connective tissue. 



In a medium sized vein such as the saphena vein it is possible 

 to distinguish outside the inner coat, a middle and an outer coat. 

 The former consists of white connective tissue, with a scanty supply 

 of elastic fibres; it contains, sometimes in considerable quantity, 

 plain muscular fibres, the bundles of which form a mesh work, with 

 the meshes disposed for the most part transversely. The latter 

 consists also of white connective tissue with some elastic fibres 

 running longitudinally and obliquely, plain muscular fibres being 

 sometimes present and when present disposed chiefly in a longi- 

 tudinal direction. Small vasa vasorum are present in the outer 

 coat and extend into the middle coat. In many large veins 

 there is no sharp distinction between a middle and outer coat ; the 

 whole wrapping round the inner coat consists of white connective 

 with a variable quantity of elastic tissue, and of muscular fibres 

 which run chiefly longitudinally or obliquely and which may be 

 very scanty, or which as in the vena portaR may be abundant. The 



