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a particularly well-developed band. In the larger arteries a great 

 deal of yellow elastic tissue, together with some white, fibrous 

 tissue, pervades the middle coat. At the inner and outer border of 

 this coat the elastic fibres fuse, to form an internal and external 

 fenestrated membrane a marked feature of an artery. This coat 

 endows the arteries with lability (extensibility and elasticity) and 

 contractility. The outside coat consists mostly of white fibrous tissue, 

 and not only protects the arteries, but by its rigidity prevents over- 

 distension. The connective tissue, like the leather case of a football, 

 allows extension of the elastic layers up to a certain point, and 

 then becomes taut. In the veins where the middle coat is some- 

 what thinner and contains less elastic tissue, the outer coat consists 

 largely of muscle-fibres. There is more white connective tissue in the 

 walls of the veins. The valves of the veins are formed of fibrous 

 and elastic tissue covered with endothelium. The walls of the larger 

 bloodvessels are supplied with blood through the vasa vasorum. 

 As the arterioles branch into capillaries the muscular and elastic 

 elements become less and less, until in the capillaries themselves 

 there is left only the layer of endothelium, supported by some stellate 

 connective-tissue cells. There is some evidence that the cells lining 

 the capillaries can alter their shape, and so contract the lumen of 

 these vessels. A phagocytic action is also ascribed to these cells 

 e.g., in the liver. In early embryonic life these cells give origin to 

 red corpuscles (see p. 88). The capillaries form networks, which 

 accommodate themselves to the structure of the organs e.g., longi- 

 tudinal networks in muscle, loops in the papillae of the skin, close- 

 meshed networks round the alveoli of glands, cells of liver, etc. In 

 the liver the blood penetrates into the substance of the liver cells, 

 the capillaries forming sinusoids. In the spleen the capillaries open 

 into the pulp. The lumen of the capillaries can be widened or narrowed 

 by varying contractility. As the capillaries join together to form 

 the venules, muscle fibres again appear and coat the wall of the 

 litter. The bloodvessels are supplied with vaso-motor nerves, which 

 regulate their calibre and the supply of blood according to the needs 

 of the body. The nerves end in a plexus of fibrils among the muscle 

 fibres. Ganglion cells occupy the larger nodes of the nerve plexus. 

 The ends of a torn artery retract, coil up within the external coat, 

 and so prevent haemorrhage. The excised arteries e.g., of an ox or 

 sheep contract when mechanically irritated, and remain capable of 

 contraction for some days after excision. They may be relaxed by 

 freezing, or by poisoning with a solution of fluoride of sodium. 



The elastic t'ssues of the arteries successfully withstand the strain 

 of the pulse some seventy "times a minute throughout the years of 

 a long life. 



The elastic co-efficients of the several layers of the coat of an artery 

 increase from within out, and thus great strength is obtained with 

 the use of a small amount of material. The elasticity of a healthy 

 artery is almost perfect, while the breaking strain both of arteries and 

 veins is very great, and far above that exerted by the blood-pressure 



