246 CIRCULATION. 



motor " nerves play an important part in regulating the 

 function of nutrition. 



Course of the Blood in the Arteries. At every pulsation 

 of the heart, all the blood contained in the ventricles, except- 

 ing, perhaps, a few drops, is forced into the great vessels. 

 We have already studied the valvular arrangement by which 

 the blood, once forced into these vessels, is prevented from 

 returning into the ventricles during the diastole. The sketch 

 we have given of the anatomy of the arteries lias prepared us 

 for a complexity of phenomena in the circulation in these 

 vessels, which would not obtain if they were simple, inelastic 

 tubes. In this case the intermittent force of the heart would 

 be felt equally in all the vessels, and the arterial circulation 

 would be subject to no modifications which did not come 

 from the action of the central organ. As it is, the blood is 

 received from the heart into vessels endowed, not only with 

 great elasticity, but with contractility. The elasticity, which 

 is the prominent property of the largest arteries, moderates 

 the intermittency of the heart's action, providing a continuous 

 supply to the parts ; while the contractility of the smallest 

 arteries is capable of increasing or diminishing the supply in 

 any part, as may be required in the various functions. 



Elasticity of the Arteries. This property, particularly 

 marked in large vessels, has long been recognized. If, for 

 example, we forcibly distend the aorta with water, it may be 

 dilated to more than double its ordinary capacity, and will 

 resume its original size and form as soon as the pressure is 

 removed. This simple experiment teaches us, that if the 

 force of the heart be sufficient to distend the great vessels, 

 their elasticity during the intervals of its action must be 

 continually forcing the blood toward the periphery. The 

 fact that the arteries are distended at each systole is abun- 

 dantly proven by actual experiment; though the immense 

 capacity of the arterial system, compared with the small 



