THE VEINS. yi 



in arteries of medium size, is also increased, and consists of robust fibres and 

 plates of elastica, many of which are longitudinal, and strong bundles of fibrous 

 tissue. Exceptionally, among the lower animals if not in man, scattered 

 bundles of involuntary muscle are found within the external tunic. In the 

 roots of the aorta and pulmonary artery, the media consists chiefly of striated 



Iiitima 



Media 



S2S^**Cs^-^^feSS^w 





FIG. 128. Transverse section of abdominal aorta. X 90. 



muscle resembling the myocardium with which it is continuous, both vessels 

 having been derived from the anterior segment of the primary heart-tube. 

 The Veins. Although the walls of the veins are thinner than those 

 of the corresponding arteries, their thickness for veins of a given diameter is 

 not constant, owing to the frequent irregularity in the composition of the 

 tunics. In consequence of the smaller amount of muscular and elastic tissue 

 that they contain, veins are generally more flaccid and less contractile than 

 the arteries which they accompany. In veins of medium size, the intima 

 consists of the lining endothelium, the cells of which are broad and short, a 



