VEINS. 79 



scanty connective tissue representing the remains of the 

 adventitia. 



In small arteries, viewed longitudinally (or from the side), 

 the transverse muscle-cells, with their conspicuous rod-shaped 

 nuclei, present a very characteristic appearance by which 

 vessels of this size are easily recognized. 



When, finally, all the coats have disappeared except the 

 endothelium, the artery has merged into a capillary. 



Veins : The structure of veins is much the same as that of 

 arteries ; but they have a larger calibre, thinner walls, more 

 white fibrous tissue, and less muscle than the corresponding 

 arteries, and they are provided with valves. 



The walls of veins, as of arteries, consist of three coats, 

 the tunica intima, tunica media, and tunica adventitia. 

 These coats, however, are often indistinctly marked off 

 and merge into one another without well-marked dividing- 

 lines. 



The tunica intima is lined internally with a single layer of 

 endothelium-cells, which are more polygonal and less elongated 

 in shape than are those of arteries. The subendothelial layer 

 consists of white and elastic tissue. The internal elastic 

 lamina in veins is often poorly defined. 



The tunica media consists of white and elastic fibrous tissue 

 and involuntary muscle-cells, which are mostly arranged 

 circularly, but in some situations partly longitudinally also. 

 The media is much thinner and contains less muscle than in 

 arteries. 



The tunica adventitia is a fibro-elastic outer sheath, often 

 thick and well marked, containing vasa vasorum and in some 

 situations longitudinal non-striated muscle-cells. 



The valves of veins are folds of the tunica intima, strength- 

 ened bv additional connective tissue. 



At the junction of the large rein* with the cardiac auricles 

 the peculiar muscle of the heart is continued for a short dis- 

 tance in the venous walls. The walls of the veins of the 

 lower extremities are thicker and more muscular than those of 

 the upper part of the body. 



The venous N/////.NVX of the interior of the skull consist 

 of an endothelial layer resting on fibrous tissue, and occupy 



