THE VASCULAR SYSTEM. 1131 



as in the arteries, but its cells are more oval, less fusiform. It is supported by a 

 connective-tissue layer, consisting of a delicate network of branched cells, and 

 external to this is a layer of longitudinal elastic fibres, but seldom any appearance 

 of a fenestrated membrane. This constitutes the internal co&t. The middle coat is 

 composed of a thick layer of connective tissue with elastic fibres, intermixed, in some 

 veins, with a transverse layer of muscular fibres. The white fibrous element is 

 in considerable excess, and the elastic fibres are in much smaller proportion in the 

 veins than in the arteries. The outer coat consists of areolar tissue, as in the 

 arteries, with longitudinal elastic fibres. In the largest veins the outer coat is from 

 two to five times thicker than the middle coat, and contains a large number of 

 longitudinal muscular fibres. This is most distinct in the inferior vena cava, and 

 at the termination of this vein in the heart, in the trunks of the hepatic veins, in 

 all the large trunks of the vena portae, in the splenic, superior mesenteric, external 

 iliac, renal, and azygos veins. In the renal and portal veins it extends through 

 the whole thickness of the outer coat, but in the other veins mentioned a layer of 

 connective and elastic tissue is found external to the muscular fibres. All the large 

 veins which open into the heart are covered for a short distance with a layer of 

 striped muscular tissue continued on to them from the heart. Muscular tissue is 

 wanting in the veins (1) of the maternal part of the placenta ; (2) in the venous 

 sinuses of the dura mater and the veins of the pia mater of the brain and spinal 

 cord ; (3) in th'e veins of the retina ; (4) in the veins of the cancellous tissue of 

 bones ; (5) in the venous spaces of the corpora cavernosa. The veins of the above- 

 mentioned parts consist of an internal endothelial lining supported on one or more 

 layers of areolar tissue. 



Most veins are provided with valves, which serve to prevent the reflux of the 

 blood. They are formed by a reduplication of the inner coat, strengthened by 

 connective tissue and elastic fibres, and are covered on both surfaces with endo- 

 thelium, the arrangement of which differs on the two surfaces. On the surface 

 of the valve next the wall of the vein the cells are arranged transversely ; whilst 

 on the other surface, over which the current of blood flows, the cells are arranged 

 vertically in the direction of the current. The valves are semilunar. They are 

 attached by their convex edge to the wall of the vein ; the concave margin is free, 

 directed in the course of the venous current, and lies in close apposition with the 

 Avail of the vein as long as the current of blood takes its natural course ; if, how- 

 ever, any regurgitation takes place, the valves become distended, their opposed 

 edges are brought into contact, and the current is interrupted. Most coimnonlv 

 two such valves are found placed opposite one another, more especially in the 

 smaller veins or in the larger trunks at the point where they are joined by smaller 

 branches ; occasionally there are three and sometimes only one. The wall of the 

 vein on the cardiac side of the point of attachment of each segment of the valve 

 is expanded into a pouch or sinus, which gives to the vessel, when injected or dis- 

 tended \vith blood, a knotted appearance. The valves are very numerous in the 

 veins of the extremities, especially of the -lower extremities, these vessels having 

 to conduct the blood against the force of gravity. They are absent in the very 

 small veins i. e. those less than ^ of an inch in diameter ; also in the venae 

 cavos, the hepatic veins, portal vein and most of its branches, the renal, uterine, 

 and ovarian veins. A few valves are found in the spermatic veins, and one also 

 at their point of junction with the renal vein and inferior vena cava, respectively. 

 The cerebral and spinal veins, the veins of the cancellated tissue of bone, the 

 pulmonary veins, and the umbilical vein, and its branches, are also destitute of 

 valves. They are occasionally found, few in number, in the venae azygos and 

 intercostal veins. 



The veins are supplied with nutrient vessels, vasa vasorum, like the arteries. 

 Nerves also are distributed to them in the same manner as to the arteries, but in 

 much less abundance. 



The lymphatic vessels, including in this term the lacteal vessels, which are 

 identical in structure with them, are composed of three coats. The internal is an 



