THE CIRCULATORY SYSTEM. 113 



In the larger arteries the muscle-fibres of the media are grouped 

 in bundles, which are separated by white and elastic fibrous tissue 

 (Fig. 97). The muscle-fibres themselves are less highly developed 

 than in the smaller arteries, so that the vessels are less capable of 

 contracting, but are more highly elastic, because of the greater 

 abundance of elastic fibres. In these larger arteries the boundary 

 between the media and the intima is less sharply defined than 

 in the smaller arteries, the elastic tissues of the two coats being 

 more or less continuous. In cross-sections of the smaller arteries 

 this boundary is very clearly seen, the elastic lamina of the intima 

 appearing as a prominent line of highly refracting material, which 

 assumes a wavy course around the artery when the latter is in a 

 contracted state. In such sections the nuclei of the endothelial 

 layer of the intima appear as dots at the very surface of the intima. 



3. The Capillaries (Fig. 25). As the arteries divide into progres- 

 sively smaller branches the walls of the latter and their individual 

 coats become thinner. In the smallest arterioles the elastic tissue 

 of the wall entirely disappears, and the muscular coat becomes so 

 attenuated that it is represented by only a few transverse fibres 

 partially encircling the vessel. These in turn disappear, and the 

 branches of the vessel then consist of a single layer of endothelium 

 continuous with that lining the intima of the larger vessels. These 

 thinnest and smallest vessels are the capillaries. They form a net- 

 work or plexus within the tissues, and finally discharge into the 

 smallest veins the blood they have received from the arteries. It 

 is chiefly through the walls of the capillaries that the transudation 

 giving rise to the lymph takes place, but some transudation prob- 

 ably also occurs through the walls of the smaller arteries and 

 veins. 



4. The veins closely resemble the arteries in the structure of their 

 walls, but relative to the size of the vessel the wall of a vein is 

 thinner than that of an artery. This is chiefly because the media 

 is less highly developed. The elastic lamina of the intima is also 

 thinner in veins than in arteries of the same diameter. 



The valves of the veins are transverse, semilunar, pocket-like 

 folds of the intima, which are strengthened by bands of white 

 fibrous tissue lying between the two layers of intima that form the 

 surfaces of the valves. The valves usually occur in pairs, the 

 edges of the two coming into contact with each other when the 

 valvular pockets are filled by a reversal of the blood-current. 



