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TEXT-BOOK OF PHYSIOLOGY. 



tinuous and equable movement observed in the capillaries. This is accom- 

 plished in the following manner: With each contraction of the left ventricle 

 more blood is ejected into the aorta than the arteries can discharge into the 

 capillaries and veins during the time of the contraction. The portion not so 

 discharged exerts a lateral pressure against the walls of the arteries which at 

 t once dilate until a condition of equilibrium is established between the pres- 

 sure from within and the elastic reaction of the arterial walls from without. 



With the cessation of the contraction the elas- 

 tic walls recoil and propel the blood toward 

 the capillaries. The intermittent action of 

 the heart is thus succeeded by the continuous 

 reaction of the arterial wall. 



As the blood advances toward the periph- 

 ery of the arterial system and larger amounts 

 pass into the capillaries, both the distention 

 and the elastic recoil diminish, ancj, by the time 

 the blood reaches the capillaries its intermit- 

 tency of movement has been so far obliterated 

 by the elastic recoil that as it enters the capil- 

 laries the movement becomes equable and con- 

 tinuous. The elasticity thus serves the pur- 

 pose of equalizing the movement of the blood 

 throughout the arterial system. 



In youth the arterial walls are highly dis- 

 tensible and elastic; in advanced years they 

 are frequently relatively rigid and inelastic, 

 and in consequence the flow of blood toward 

 and into the capillaries approximates in its 

 characteristics the flow of a fluid through a 

 rigid tube under the intermittent action of a 

 pump; that is, the intermittent movement im- 

 parted by the heart is not so completely con- 

 verted into a continuous movement, and hence 

 the blood flows through the capillaries during 



the systole with greater velocity, and during the diastole with less velocity, 

 than is the case when the vessel is normally elastic. For these and other 

 reasons the tissues are not so well nourished and hence their nutrition 

 and functional activities decline. 



The contractility permits of a variation in the amount of blood passing 

 into a given capillary area in a unit of time. Normally each artery has a 

 certain average caliber due to a given contraction of the muscle coat. Be- 

 yond this average condition the artery can pass in one direction or the other 

 by either a relaxation or increased contraction of the muscle coat. During 

 the functional activity of any organ or tissue there is need for an increase in 

 the amount of blood beyond that supplied during functional inactivity or 

 rest. This is accomplished by a relaxation of the muscle-fibers. With 

 the cessation of activity the muscle-fibers again contract and reduce the 

 amount of blood to that required for nutritive purposes only. An increased 

 contraction of the muscle-fibers beyond the average, diminishes the outflow 



FIG. 143. COATS OF A SMALL 

 ARTERY, a. E n d o t h e 1 i u m. b. 

 Internal elastic lamina, c. Cir- 

 cular muscular fibers of the middle 

 coat. d. The outer coat. (Lan- 

 dois and Stirling.) 



