684 THE VEINS. 



and to oppose its reflux from the heart towards the organs. Apphed, as tnej 

 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. Compared with the arteries, they have 

 only two distinct tunics. 



The internal tunic is composed of an endothelial layer of irregular and poly- 

 gonal cells, lying on a layer of connective tissue, limited by an elastic layer 

 — analogous to the internal elastic layers of arteries. 



The external tunic is composed of connective tissue, elastic fibres, and 

 smooth muscular fibres — the first predominating. 



The muscular fibres are transversal, longitudinal, or oblique, their aiTange- 

 ment varying with the vessels. Lorge states that the vena portal and intra- 

 lobular veins in the Horse have only longitudinal muscular fibres. 



In some veins or portions of a vein they may be absent. In the veins of 

 the bones, and in the sinuses of the dura mater, the walls are reduced to a layer 

 of endothelium laid on a layer of connective tissue. 



The venules differ from the arterioles in the dissemination of their muscular 

 fibres, which form a continuous layer in the latter. 



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

 work around them. The vena portfe alone is accompanied by nerve filaments 

 from 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 enclosed 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 vmcB comites, though the largest arteries have only one venous trunk. 

 The large and frequently repeated communications are undoubtedly intended to 

 compensate for the thinness of their parietes, which expose to obstruction and 

 dilatation ; this they cannot overcome, because of the slowness with which the 

 blood passes through them. The valves are accessory to these inosculations ; 

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

 correspond to the extent of the valve ; these pouches give the distended vessel 

 its nodulated appearance. Remak found longitudinal muscular fibres in the 

 adventitia of the large veins of the Ox and Sheep, but chiefly in the hepatic 

 portion of the posterior vena cava and the veins of the liver. 



It is also stated that these longitudinal fibres are found in the inner part of 

 the middle coat — for some authorities still describe three tunics— of some veins 

 — iliac, femoral, umbilical ; and the veins of the pia mater, brain, and spinal 

 cord, retina, and placenta, as well as those of the bones and dura mater sinuses, 

 have no muscular tissue at all. Valves are absent in the veins of the viscera, 

 of the cranium and vertebral canal, and umbilical vein, in addition to the precited 

 vessels. The endothelial cells on the side of the valves over which the blood 

 passes, are more elongated than on the cardiac side of the valves, where the 

 long axes of the cells are placed transversely — an arrangement which diminishes 

 friction.) 



