THE VEINS. 231 



This internal coat is more ostensible than the internal coat of 

 arteries, and is not, like the latter, disposed to ossification. It 

 is frequently so arranged as to form valves, which are plaits or 

 folds, of a semilunar form, that project from the surfaces into 

 the cavities of these vessels.* 



Two of these valves are generally placed opposite to each 

 other; and, when raised up, they form a septum in the cylin- 

 drical cavity of the vessel. The septum, thus composed, is 

 concave towards the heart, (see fig. 172, p. 208.) 



The valves have a great effect in preventing the contents of 

 the veins from moving in a retrograde course : they therefore 

 necessarily modify the effects of lateral pressure, in such a 

 manner, that it propels the blood forward, or towards the 

 heart. 



These valves are generally found in the veins of the muscular 

 parts of the body, especially in those of the extremities. They 

 are not found in those veins which are in the cavities of the 

 body, nor in the internal jugulars. They are placed at unequal 

 distances from each other. 



The coats of the veins are somewhat transparent; and there- 

 fore those veins which are subcutaneous have a bluish aspect, 

 which is derived from the color of the blood they contain. 



The color of the blood in the veins is different from that in 

 the arteries, being of a darker red. 



The situation and arrangement of the large trunks of veins 

 are much alike in different subjects ; but the branches, especially 

 those which are subcutaneous, are very variable in their situa- 

 tions. — The situation of the deeper seated veins of the extremi- 

 ties — the venae comites and the sinuses are subject to little 



circular fibres. 5. A coat of genuine elastic tissue. 6. The external cellular 

 lining. The veins, though thinner much in their walls, present the same struc- 

 ture, the true elastic coat only being absent. The valves of the veins consist 

 of a fibrous membrane covered by the epithelium, and are not mere duplica- 

 tures of the lining membrane. — p. 



* The valves of the veins were first described by Charles Etienne of Paris, 

 in 1546. In 1547, Amaius, a Portuguese, saw at Ferrara, those at the mouth 

 of the vena azygos. Sylvius, of Paris, announced them about the same time 

 in the jugular, brachial and crural veins. Fabricus ab Aquapendente claims 

 the discovery for himself in 1574. Lassus. 



