170 The Vertebrate Organ Systems 



the muscle is often arranged into two layers, a circular one and a longi- 

 tudinal one. The outermost layer of the blood vessel is the tunica 

 adventitia, which is formed of connective tissue. This tissue has a 

 gradual transition into the connective tissue cells that surround the 

 blood vessel. 



In general, the structure of the walls of the veins (Fig. 50) is 

 similar to that of the arteries, but there is less muscle and elastic tissue 

 in proportion to the amount of other connective tissue. Veins and 

 arteries are easily distinguished by the difference in the thickness of the 

 walls. Due to the fact that the blood flows more rapidly and under 

 greater pressure in the arteries, a heavier wall is needed better to with- 

 stand these strains. Usually, also, the lumen of an artery is smaller 

 than that of a vein of similar importance. This again is related to the 

 greater speed of flow. 



TUNICA 



INICA 

 ADVENTITIA 



ARTERY 



VEIN 



Fig. 50. — Section through an artery and vein. 



One Other important difference between arteries and veins is the 

 fact that veins have valves (Fig. 51) in their walls to prevent the back- 

 flow of blood. These are pocketlike valves which allow blood to flow 

 freely toward the heart, but prevent its backflow. 



The work of the blood is not done in the arteries and veins, rather 

 they are simply passageways to the small capillaries which form a dense 

 network throughout all tissues. The walls of these tiny vessels are 

 composed of a continuous layer of endothelial cells. The diameter of 

 the vessels is so small (0.008 mm.) that the blood cells must go through 

 single file. This may be easily observed in the webbing of the frog's 



