. Quain's elements of anatomy . d veins.—In vessels a littlelarger than the capillaries, there is added outside the epithelial layer(fig. 189, a, a), a layer of j)lain muscular tissue, in form of the usuallong contractile fibre-cells, which are directed across the length of thevessel (fig. 189, m, m). The elongated nuclei of these cells may bebrought into view by means of acetic acid or by staining fluids. Thislayer corresponds with the middle coat of the larger vessels. Inthe smallest vessels in which it appears the muscular cells are few andapart, and a single long cell may turn spirally rou
. Quain's elements of anatomy . d veins.—In vessels a littlelarger than the capillaries, there is added outside the epithelial layer(fig. 189, a, a), a layer of j)lain muscular tissue, in form of the usuallong contractile fibre-cells, which are directed across the length of thevessel (fig. 189, m, m). The elongated nuclei of these cells may bebrought into view by means of acetic acid or by staining fluids. Thislayer corresponds with the middle coat of the larger vessels. Inthe smallest vessels in which it appears the muscular cells are few andapart, and a single long cell may turn spirally round the tube (Lister);in larger vessels, especially those of the arterial system, the muscularcells are more closely arranged. Outside the muscular coat is theareolar or connective tissue coat, containing fibres and connectivetissue corpuscles, with longitudinally placed nuclei. In vessels of -gL- of an inch in diameter, or even less, the elastic layersof the inner coat may be discovered (fig. 190, a, 8), in the form generally. Fi-^. 190.—A SMALL ARTERY A, WITH A CORRESPONDING VEIN B, TREATED WITH ACETIC ACID, AND MAGNIFIED 350 DIAMETERS (after KolUker). o, external coat with elongated nuclei; fi, nuclei of the transverse muscular tissue of themiddle coat (Avben seen endwise, as at the sides of the vessel, their outline is circular) j7, nuclei of the epithelium-cells ; 5, elastic layers of the inner coat. of homogeneous or fenestrated membrane, more rarely of longitudinalreticulating elastic fibres. The small veins differ from arteries of corres-ponding size, chiefly in the inferior development of their muscular tissue jthe lining cells of the arteries also are very much longer and narrowerthan those of the veins. These differences, as weU as the comparative DEVELOPMENT OP BLOOD-VESSELS. 197 size of corresponding vessels, are well shown in the accompanyingfigures (189 and 190). The only open communication between the arteries and the veins, is by means ofcapillary vessels as
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