64: CIRCULATION OF THE BLOOD IN THE VESSELS. 



found in those vessels in which the muscular element of the middle coat pre- 

 dominates. The vaso-niotor nerves, as they are called, play an important 

 part in regulating the processes of nutrition. 



Course of the Blood in the Arteries. With every pulsation of the heart, 

 all the blood contained in the ventricles, excepting perhaps a few drops, is 

 forced into the great vessels. The valvular arrangement by which the blood, 

 once forced into these vessels, is prevented from returning into the ventricles 

 during their diastole, has already been described. The foregoing sketch of the 

 anatomy of the arteries indicates a complexity of phenomena in the circula- 

 tion 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 modifi- 

 cations 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 is particularly marked in the 

 largest vessels. If the aorta be forcibly distended with water, it may be di- 

 lated to more than double its ordinary capacity and will resume its original 

 size and form as soon as the pressure is removed, its elasticity being absolutely 

 perfect. This simple experiment shows 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 has been shown by direct 

 experiments ; although the immense capacity of the arterial system, as com- 

 pared with the small charge of blood which enters at each pulsation, renders 

 the actual distention of the vessels less than would be expected from the force 

 of the heart's contraction. 



Division of an artery in a living animal illustrates one of the important 

 phenomena due to the elastic and yielding character of its walls. It is ob- 

 served, even in vessels of considerable size, as the carotid or femoral, that the 

 flow of blood is not intermittent but remittent. With each ventricular sys- 

 tole there is a sudden and marked impulse ; but during the intervals of con- 

 traction, the blood continues to flow with considerable force. In the smaller 

 vessels, the impulse becomes less and less marked ; but it is not entirely lost, 

 even in the smallest vessels, the flow becoming constant only in the capillary 

 system. That the force of the heart is absolutely intermittent, is shown by 

 the following experiment : If the heart be exposed in a living animal, and a 

 canula be introduced through the walls into one of the ventricles, there is a 

 powerful jet at each systole, but no blood is discharged during the diastole. 

 The same absolute intermittency of the current is observed in the aorta near 

 the heart. The conversion of the intermittent current in the largest vessels 

 into a nearly constant flow in the smallest arterioles is effected by the physical 



