214 HANDBOOK OF PHYSIOLOGY. 



Thus by the exercise of the elasticity of the arteries, all the force of 

 the ventricles is expended upon the circulation; for that part of the 

 force which is used up or rendered potential in dilating the arteries is 

 restored or made active or kinetic, in full when they recoil. There is 

 no loss of force; neither is there any gain, for the elastic walls of the 

 artery cannot originate any force for the propulsion of the blood they 

 only restore that which they received from the ventricles. It is by this 

 equalizing influence of the successive branches of every artery that at 

 length the intermittent accelerations produced in the arterial current 

 by the action of the heart, cease to be observable, and the jetting stream 

 is converted into the continuous and equable movement of the blood 

 which we see in the capillaries and veins. In the production of a con- 

 tinuous stream of blood in the smaller arteries and capillaries, the re- 

 sistance which is offered to the blood-stream in these vessels is a neces- 

 sary agent. Were there no greater obstacle to the escape of blood from 

 the larger arteries than exists to its entrance into them from the heart, 

 the stream would be intermittent, notwithstanding the elasticity of walls 

 of the arteries. 



By means of the elastic and muscular tissue in their walls again the 

 arteries are enabled to dilate and contract readily in correspondence 

 with any temporary increase or diminution of the total quantity of 

 blood in the body; and within a certain range of diminution of the 

 quantity, still to exercise due pressure on their contents. The elastic 

 tissue further assists in restoring the normal channel after diminution 

 of its calibre, whether this has been caused by a contraction of the mus- 

 cular coat, or by the temporary application of a compressing force from 

 without. This action is well shown in arteries which, having contracted 

 by means of their muscular element, after death regain their average 

 potency on the cessation of post-mortem rigidity. 



The office of the muscular coat also is employed to adjust the flow 

 of the blood locally, to regulate the quantity of blood to be received by 

 each part or organ, and to adjust it to the requirements of each, accord- 

 ing to various circumstances, but, chiefly, according to the activity with 

 which the functions of each are at different times performed. The 

 amount of work done by each organ of the body varies at different times, 

 and the variations often quickly succeed each other, so that, as in the 

 brain, for example, during sleep and waking, within the same hour a 

 part may be now very active and then inactive. In all its active exer- 

 cise of function, such a part requires a larger supply of blood than is 

 sufficient for it during the times when it is comparatively inactive. It 

 is evident that the heart cannot regulate the supply to each part at dif- 

 ferent periods; neither could this be regulated by any general and uni- 

 form contraction of the arteries; but it may be regulated by the power 

 which the arteries of each part have, in their muscular tissue, of con- 



