VEINS. 



lamina connected with one another across the muscle layers which lie 

 between them, by elastic bands. The muscle fibers of the inner portion 

 have been described as short, broad and flattened elements joined to one 

 another so that they resemble cardiac muscle (Fig. 156). The outer muscle 

 is of a more ordinary form. The elastic ele- 

 ments greatly predominate and on section the 

 fresh aorta appears yellow, not reddish like 

 smaller vessels. The externa contains no 

 outer elastic membrane. It is relatively and 

 absolutely thinner than the externa in some 

 medium sized arteries. 



VEINS. The veins have thinner walls, 

 containing less muscle and less elastic tissue 

 than the corresponding arteries. Since the 



artery to any structure and the returning vein often are side by side, it is fre- 

 quently possible to make such comparisons in a given specimen. Because of 

 thinner walls the veins often collapse, or at least are not as circular as 

 the arteries; they may be distended with blood, and frequently have a 

 larger lumen than the contracted artery. In many large veins the media 



FIG. 156. BRANCHED SMOOTH MUS- 

 CLE CELLS FROM THE THORACIC 

 AORTA OP A CHILD AT BIRTH (a) 

 AND AT FOUR MONTHS (b). 

 (After Koelliker.) 



Nuclei of 

 smooth 

 muscle fibers. 



Connective 



tissue. 



FIG. 157. PART OP A CROSS SECTION OP A VEIN PROM A HUMAN LIMB. X 230. 



The elasticjelements are drawn very black. I, Intima; 2, media; 3, externa. (The middle 



of the.J3 objects labeled nuclei of smooth muscle is apparently, an elastic fiber.) 



is very thin or even absent, and the externa, containing large bundles of 

 longitudinal muscle fibers, becomes the principal muscular coat. 



Venules and precapillary veins are wider than the corresponding 

 arteries. Their endothelial cells are less elongated; the muscle fibers do 



