90 NORMAL HISTOLOGY. 



muscular tunic. Before the capillary is reached, the muscle is reduced to a 

 single layer of cells, which in turn gives place to groups of muscle-cells 

 that partially wrap around the vessel. After the disappearance of the muscle- 

 cells the blood-vessel has become a true capillary. The adventitia shares in 



Extemal elastic 



membrane 



Adventitia 



FIG. 126. Transverse section of artery of medium size, stained to show elastic tissue. X 100. 



the general reduction, and in the smallest arteries consists of only a few fibro- 

 elastic strands outside the scattered groups of muscle-cells. 



In the large arteries chiefly the intima and media thicken. Although 

 the inner coat greatly increases and contains a large amount of fibrous tissue 



FIG. 127. Small arteries in which muscular coat is reduced to single layer of cells. X 150. 



and elastica, a conspicuous internal elastic membrane, as seen in the smallest 

 vessels, is lacking. The character of the thickened media also changes, the 

 muscle being relatively reduced and overshadowed by the excessive amount 

 of fibro-elastic tissue, which confers a more compact and denser character to 

 the wall of the vessel. The adventitia, while proportionately thinner than 



