124 STRUCTURE OF VEINS. 



(2.) The media of the larger veins consists of alternate layers of 

 elastic and muscular tissue united to each by a considerable amount of 

 connective tissue, but this coat is always thinner than in the corres- 

 ponding arteries. This coat diminishes in the following order in 

 the following vessels popliteal, veins of the lower extremity, veins 

 of the upper extremity, superior mesenteric, other abdominal veins, 

 hepatic, pulmonary, and coronary veins. The following veins contain 

 no muscle veins of bone, central nervous system, and its membranes, 

 retina, the superior cava, with the large trunks that open into it, 

 the upper part of the inferior cava. Of course, in these cases, the 

 media is very thin. In the smallest veins the media is formed of 

 fine connective tissue, with very few muscular fibres scattered in 

 the inner part. 



(3.) The adventilia is thicker than that of the corresponding 

 arteries; it contains much connective tissue usually arranged longi- 

 tudinally, and not much elastic tissue. Longitudinally arranged 

 muscular fibres occur in some veins (renal, portal, inferior cava near 

 the liver, veins of the lower extremities). The valves consist of 

 fine fibrillar connective tissue with branched cells. An elastic net- 

 work exists on their convex surface, and both surfaces are covered 

 by endothelium. The valves contain many muscular fibres (Fig. 42). 

 [Ranvier has shown that the shape of the epithelial cells covering 

 the two surfaces of the valves differs. On the side over which the 

 blood passes, they are more elongated than on the cardiac side of 

 the valve, where the long axes of the cell are placed transversely.] 



The sinuses of the dura mater are spaces covered with endothelium. The spaces 

 are either duplicatures of the membrane, or channels in the substance of the tissue 

 itself. 



Cavernous spaces we may imagine to arise by numerous divisions and anasto- 

 moses of tolerably large veins of unequal calibre. The vascular wall appears to 

 be much perforated and like a sponge, the internal space being traversed by threads 

 and strands of tissue, which are covered with endothelium on their surfaces, that 

 are in contact with the blood. The surrounding wall consists of connective tissue 

 which is often very tough, as in the corpus cavernosum, and it not unfrequently 

 contains non-striped muscle. 



Cavernous Formations of an analogous nature on arteries, are the 

 carotid-gland of the frog, and a similar structure on the pulmonary 

 arteries and aorta of the turtle, and the coccygeal-gland of man (Luschka). 

 This structure is richly supplied with sympathetic nerve-fibres, and is a 

 convoluted mass of ampullated or fusiform dilatations of the middle 

 sacral artery (Arnold), surrounded and permeated by non-striped 

 muscle (Eberth). 



Vasa Vasorum. [These are small vessels which nourish the coats of the 

 arteries and veins. They arise from one part of a vessel and enter the walls of 



