STRUCTURE OF VEINS. 367 



fibre-cells have for the most part as in the arteries a transverse direction, although 

 the layer which they form is not everywhere complete, being separated into bundles 

 by the intervention of connective tissue (fig. 425, c). But in many veins some of 

 the innermost fibres of the middle coat take a longitudinal course. This is the case 

 with the iliac, crural, branches of the mesenteric, umbilical of the foetus, and other 

 veins (Eberth). 



In many of the larger veins the middle coat is less developed, especially as 

 regards its muscular fibres, but in such cases the deficiency may be supplied by 

 muscularity of the outer coat. The middle coat is wanting altogether in the 

 thoracic part of the inferior vena cava, but is well marked in the hepatic part : in the 

 part below the liver the muscularity of the middle coat is less marked. In the internal 

 and external jugular veins there is but a slight development of the muscular tissue. 



External coat (fig. 425, d). This is often thicker than the middle coat ; but 

 the line of junction between them is not sharply marked. It consists of dense 

 areolar tissue and longitudinal elastic fibres. In certain large veins, as was first 

 pointed out by Remak, this coat contains a considerable amount of plain or non-striated 

 muscular tissue. Thus the muscular elements are well marked in the whole extent 

 of the abdominal cava, in which they form a longitudinal network, occupying the 

 inner part of the external coat ; and they may be traced into the renal, azygos, 

 spermatic and external iliac veins. The muscular tissue of the external coat is also 

 well developed in the trunks of the hepatic veins and in that of the vena portae, 

 whence it extends into the splenic and superior mesenteric. It is found also in the 

 axillary vein. 



Other veins present peculiarities of structure, especially in respect of muscularity, 

 as follows. 1. The striated muscular tissue of the auricles of the heart is prolonged 

 for some way on the adjoining part of the vense cavse and pulmonary veins. 2. The 

 (plain) muscular tissue is largely developed in the veins of the gravid uterus, in 

 which, as well as in some other veins, it is described as being present in all three 

 coats, and as having for the most part a longitudinal arrangement. 3. On the 

 other hand, muscular tissue is wanting in the following veins, viz., a, those of the 

 maternal part of the placenta ; b, most of the veins of the pia mater ; c, the veins 

 of the retina ; d, the venous sinuses of the dura mater ; e, the cancellar veins of the 

 bones ; /, the venous spaces of the corpora cavernosa. In most of these cases the 

 veins consist merely of an epithelium (endothelium) and a layer or layers of connective 

 tissue more or less developed ; in the corpora cavernosa the epithelium is applied to 

 the trabecular tissue. It may be added that in the thickness of their coats the super- 

 ficial veins surpass the deep, and the veins of the lower limbs those of the upper. 



The coats of the veins are supplied with nutrient vessels, vasa vasorum, in the 

 same manner as those of the arteries. In some of the larger veins they penetrate 

 into the middle coat and even approach the inner surface. Nerves are distributed 

 to them in the same manner as to the arteries, but in far less abundance. 



Valves. Most of the veins are provided with valves, a mechanical contrivance 

 adapted to prevent the reflux of the blood. The valves are formed of semilunar 

 folds of the internal coat, strengthened by included connective tissue, and projecting 

 into the vein. Most commonly two such folds or flaps are placed opposite each other 

 (fig. 426, A) ; the convex border of each (which, according to Haller, forms a par- 

 abolic curve) is connected with the side of the vein ; the other edge is free, and 

 points towards the heart, or at least in the natural direction of the current of the 

 blood along the vessel, and the two flaps incline obliquely towards each other 

 in this direction. Moreover the wall of the vein immediately on the cardiac side of 

 the curved line of attachment of the valves, is dilated into a pouch or sinus 

 (fig. 426, B), so that, when distended with blood or by artificial injection, the vessel 

 bulges out on each side, and thus gives rise to the appearance of a knot or swelling 



