CIRCULATION II9 



ferent diameters, ranging from the large aorta, an inch in diameter in man, 

 to the microscopic capillaries barely large enough to admit a red blood 

 cell. Variations in the thickness of the walls roughly parallel the variations 

 in internal caliber, the wall of the aorta in man being about one-eighth 

 inch in thickness, the capillary walls being of microscopic size. The actual 

 internal structure as well as the thickness of the vessel walls are signif- 

 icantly different in different parts of the vascular tree. These structural 

 differences are of considerable importance to the physiologist, who finds 

 an intimate interrelationship here between structure and function. 



Much of the wall of the aorta and larger arteries consists of smooth 

 muscle. The spindle-shaped cells are arranged circularly around the vessel. 

 Contractions or relaxations of these fibers are capable of changing the 

 caliber of the vessels. 



The outermost coat of the arteries is made up largely of connective 

 tissue, which also invades the muscular layer to a certain extent. In the 

 connective-tissue layer proper there are a great many elastic fibers, which 

 give the vessel its elasticity and distensibility, so important in the circula- 

 tion. The connective-tissue layer renders the wall tough and resistant, 

 so that, though it "gives" somewhat under high internal pressures, it re- 

 sists rupture even by very high pressures. 



The arteries and veins are lined with thin, flat, epithelial cells which 

 always present a smooth surface to the moving blood. 



The extremely fine final terminal branches of the arteries just before 

 the capillary bed is reached are termed arterioles, or "little arteries." They 

 possess all the layers found in the larger arteries, although each coat is 

 much thinner. 



The veins are structurally like the arteries except that their walls are 

 thinner. Because there is less muscle than in arteries or arterioles, there is 

 considerably less possibility of active change in the cahber of the veins. 

 They are rather easily collapsed, as can be demonstrated on the super- 

 ficial veins of the skin. 



Connecting the arterioles with the veins is the capillary bed. As we 

 pass from arterioles to capillaries, two significant anatomical changes are 

 apparent. The walls now become exceedingly thin. The capillary vessels 

 have lost all the coats of the arteries except one — the thin, flat, innermost 

 cell layer. There are almost no connective tissue and no true muscle (ex- 

 cluding Rouget cells). For the most part the wall consists simply of a 

 tubular extension of the continuous smooth lining of the arteries. 



Also, the branching of the vascular tree is here more profuse than at 

 any other part of the system. One arteriole breaks up into a number of 

 capillaries whose individual diameters are only a little smaller than those 

 of the tiniest arterioles. 



Mention has been made of the fact that the veins are more easily col- 

 lapsible by externally apphed pressure than are the arteries. This is not due 



