CIRCULATORY SYSTEM. 



Endothelium. 



'" "_- .-"".;: Subendothelium. 



Fenestratedmem- 

 brane of Henle, 

 ' Media. 



Fig. 



, 



78. Cross section of small 

 artery. 



unnecessary and is greatly reduced, while elastic ele- 

 ments are unusually well developed. The muscle, too, 

 is deficient in large veins situated deep in the body, 

 as the vena cava. Many 

 of the smaller and more 

 superficial veins have 

 valves, folds of the in- 

 tima, so arranged as to 

 equalize the gravity press- 

 ure of the contained 

 blood. Without these 

 valves the thin-walled 

 veins would become 



greatly distended. If for any cause the veins ex- 

 pand so that the valves do not act, a permanent 

 distention with engorgement of blood follows. The 

 veins become distorted and are spoken of as varicose 



veins, a condition quite 

 common to the long 

 saphenous veins of the 

 lower limbs. 



Capillaries. These 

 are the finer organic 

 ramifications of the 

 circulatory system, and 

 unite arteries and 

 veins. Histologically, 

 the walls of capillaries 



Intima. 



Media. 



Adventilia. 



Vasa vasorum. 



Fig. 79. Cross section of vein. 



consist of a single layer 

 of flattened epithelial (endothelial or mesothelial) 

 cells. The blood courses very slowly through these in- 

 terlacing tubes. The white cells penetrate the walls 

 and under certain conditions even the red corpuscles 



