118 



THE VASCULAR SYSTEM 



monary arteries from the heart, the walls may be composed mainly 

 of cardiac muscle. The fibroelastic tissue forming the outer coat of 

 the adventitia in all vessels merges with the connective tissue 

 supporting the vessel. 



The main function of arteries is to conduct blood away from the 

 heart. The contraction of ventricles forces blood into the great 

 arteries originating from them, and these vessels already filled with 

 blood are distended by the added supply. When the force of ven- 

 tricular contraction is spent, the fluid tends to return to the ven- 

 tricles but is prevented by the closing of the valves at their entrance. 

 Then the distended elastic walls contract as the stretched elastic 

 tissue recoils. The blood is thus sent on into the medium-sized 

 arteries where the smooth muscle takes a part and active peristaltic 

 waves propel the blood onward. 



THE VEINS. 



Passing from a capillary toward the heart, there are postcapillary 

 veins, venules, small veins, medium-sized veins, and large veins, 

 showing varying structural additions and modifications. Outside 

 the endothelial lining smooth muscle and fibroelastic connective 



Adventitia 



Media 



.J , VEIN 



Intima 



Internal elastic membrane 



. ^r^T^T^^r External elastic membrane 



ARTERY 



Fig. 69. — Diagram of small artery and vein. 



tissue form the added tissue in the wall of veins, but there is such 

 a great variation in structure that each vein nuist be studied for its 

 particular type of organization. On the whole, however, it can be 

 said that the wall of any vein is thinner than that of its accompany- 

 ing artery, and consequently veins often appear collapsed in micro- 

 scopic preparations. (Fig. t)9.) Elastic lamiiue are not usually 

 present and in many cases it is difficult to distinguish all three 



