PHYSIOLOGY OF THE CIRCULATION. 73 



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As the causes impeding the flow of blood through 

 the arteries exist throughout the whole extent of 

 that system of tubes, increasing in their amount of 

 opposition until it reaches a maximum in the minute 

 vessels immediately continuous with the veins, it 

 follows that the act of accumulation or detention of 

 a quantity of blood behind each impeding surface 

 will also continue in operation from the origin of the 

 aorta to a point corresponding with the most con- 

 tracted portion of the capillaries. 



The blood thus detained in the arterial system, 

 being prevented from moving readily forwards by 

 the impediments so often referred to, while it at the 

 same time receives an onward impulse from each 

 successive wave of blood which leaves the ventricle, 

 must necessarily exist in a state of compression, and 

 will, consequently, exert a certain amount of outward 

 or lateral pressure. If the obstruction to the blood's 

 passage through the arteries were complete, then the 

 accumulated fluid would of course be stagnant, and, 

 like all other stagnant fluids, would transmit the 

 whole of the impelling pressure equally in all direc- 

 tions. But as the obstacles occurring in the arteries 

 present only a partial obstruction to the blood's pas- 

 sage through those vessels, the distending force of 

 the arterial blood cannot be estimated till we have 

 previously determined in what proportion the lateral 

 pressure of a fluid is increased by the existence of 

 any impediment to its free discharge from the con- 

 taining tube. The following experiments, though 

 rude and not sufficiently extended for the finer pur- 

 poses of physics, may perhaps enable us to form 



