CHAP, iv.] THE VASCULAR MECHANISM. 199 



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 portse may be abundant. The 

 structure of the veins in fact varies very widely; on the whole they 

 may be said to be channels, the walls of which are elastic enough 

 to adapt themselves to considerable variations in the quantity of 

 blood passing through them, without possessing, as do the arteries, 

 a great store of elastic power to meet great variations in pressure, 

 and which are not so uniformly muscular and contractile as are the 

 arteries. And we shall see that this general character of passive 

 channels is adapted to the work which the veins have to do. 

 This general character however is modified in certain situations to 

 meet particular wants ; thus while the veins of the bones and of 

 the brain are devoid of muscular fibres, others such as the vena 

 portae may be very muscular ; and in some veins such as those of 

 the extremities a considerable quantity of elastic tissue is present. 



A minute vein just emerging from capillaries differs very little 

 from an artery of corresponding size ; it is of rather wider bore, 

 has decidedly less muscular and elastic tissue, and the epithelioid 

 cells are shorter and broader. 



Many veins, especially those of the limbs, are provided with 

 valves, which are pouch-like folds of the inner coat, the mouth of 

 the pouch looking away from the capillaries towards the heart. 

 The wall of each valve consists of a lining of epithelioid cells on the 

 inside and on the outside, and between the two, a layer of white 

 connective tissue strengthened with a few elastic fibres and some- 

 what thicker than the connective tissue basis of the epithelioid 

 lining of the veins generally. The valves may occur singly or 

 may lie two or even three abreast. The veins of the viscera, those 

 of the central nervous system and its membranes, and of the bones, 

 do not possess valves. 



111. The details of the structure of the peculiar muscular 

 tissue forming the greater part of the heart we shall reserve to a 

 later section ; but we may here say that the interior of the heart is 

 lined with a membrane (endocardium) corresponding to the inner 

 coat of the blood vessels, and consisting of a layer of epithelioid 

 cells, which however are shorter and broader than in the blood 

 vessels, being polygonal rather than fusiform, resting on a con- 

 nective tissue basis in which are present elastic fibres and in 

 places plain muscular fibres. 



The valves of the heart, like those of the veins, are folds of this 

 lining membrane, strengthened by a considerable development of 

 connective tissue. In the middle of the thin free border of each 

 of the semilunar valves of the aorta and pulmonary artery bundles 



