598 THE VEINS. 



face turned towards the heart when the valves are tense ; and a convex face 

 which, on the contrary, looks towards the roots of the veins. 



These valves are often isolated, and sometimes joined in twos or threes ; 

 according to some authorities, they are even found four or five together, 

 arranged in a circular manner. All the veins are not provided with them, 

 however, and where they exist they may be more or less numerous. They 

 are absent in the pulmonary system, and in the trunk of the vena cava ; 

 absent or quite rudimentary throughout the extent of the vena portae ; rare 

 and slightly developed in the vena azygos, the veins of the testicle, uterus, 

 and ovary ; and very numerous, very large, and very complete in the veins 

 of the limbs. 



The function of these valves is to favour the course of the blood, and 

 to oppose its reflux from the heart towards the organs. Applied, as they 

 are, to the walls of the veins by their concave face during the regular and 

 normal circulation, they are disposed as a transverse valve to sustain the 

 column of blood when any strain or pressure gives that column a retrograde 

 movement. 



STRUCTURE. The walls of the veins are thin, semi-transparent, and 

 elastic, and collapse when the vessels are empty. Like the arteries, they 

 have three tunics. 



The internal tunic is composed of an epithelial layer of elongated cells, 

 lying on an elastic membrane made up of longitudinal fibres. In the small 

 veins, besides the epithelium and elastic fibres, are striped layers with 

 elongated nuclei. This tunic is the most important ; it is persistent, while 

 the other two may be absent in certain veins. 



The middle tunic is much thinner than that of arteries, and has fewer 

 muscular and elastic fibres, while its tint is rather red than yellow. 

 The proportion of smooth fibres, disposed in a circular manner in the 

 midst of the connective tissue, varies with the volume and situation of 

 the veins ; being more considerable in the small than the large vessels, 

 and also in those through which the blood circulates with difficulty. 



The external, or adventitious tunic, is formed by connective tissue and 

 some longitudinal fasciculi of elastic and muscular fibres. 



In the veins of the bones, and in the sinuses of the dura mater, the two 

 latter tunics may be absent, and the walls of the vessels only consist of 

 epithelium. 



The veins have very numerous vasa vasorum, which form a complete 

 network around them. The vena portae alone is accompanied by nervous 

 filaments of the great sympathetic. 



(As remarked, the superficial veins are generally unaccompanied by 

 arteries ; they usually pass between layers of superficial fascia, and at the 

 most convenient situations generally those best protected pass through 

 the underlying fascia to terminate in the deep veins. These are most 

 frequently accompanied by arteries, being often inclosed in the same sheath 

 with them, particularly in the extremities. With a large number of arteries 

 there are two veins, one on each side, the vcnce comites, though the ^ 

 arteries have only one venous trunk. The large and frequently repea 

 communications are undoubtedly intended to compensate for the thinness 

 their parietes, which exposes to obstruction and dilatation ; this they can 

 overcome, because of the slowness with which the blood passes throu 

 them. The valves are accessory to these inosculations ; upon the cardi 

 face of each valve the vein is expanded into two sinuses, which correspon 

 with the extent of the valve ; these pouches give the distended vessel its 



