THE CIRCULATION OF THE BLOOD. 323 



each of these three divisions of the vascular apparatus. Since these phenom- 

 ena, as well as the laws which govern them are similar to, though more com- 

 plex than the phenomena presented by relatively simple tubes with rigid or 

 elastic walls while liquids are flowing through them under a steadily acting 

 or an intermittently acting pressure, it will be conducive to clearness of 

 conception of the mechanics of the vascular apparatus, if there be 

 considered : 



1 . The flow of a liquid through a horizontal tube with rigid walls and of 

 uniform or variable diameter under a steadily acting pressure. 



2. The flow of a liquid through a series of branching and again uniting 

 tubes with rigid walls under a steadily acting pressure. 



3. The flow of a liquid through a tube with elastic walls under an inter- 

 mittently acting pressure. 



THE FLOW OF A LIQUID THROUGH A HORIZONTAL TUBE WITH 



RIGID WALLS. 



The phenomena and the laws which govern them, that attend the flow of a 

 liquid through a rigid tube of uniform diameter under a steadily acting pressure 

 may be readily observed in an apparatus similar to that represented in Fig. 150, 

 which consists of a reservoir or pressure vessel, P, provided with a horizontal tube 

 into which is inserted at equal distances a series of vertical tubes. If the reservoir 

 be filled with a liquid, water for example, the latter under certain conditions will 

 exert a downward pressure and act as a propelling or driving power, the degree of 

 which will depend on the height of the column and may be represented by H. If 

 the stopcock at O be opened the column of water, which has heretofore been exert- 

 ing ari equal pressure in all directions, will now exert a downward pressure only, 

 and in consequence it will be driven into and through the horizontal tube and 

 discharged from its free extremity with a definite velocity. At the same time the 

 fluid will rise in each vertical tube to a height directly proportional to the distance 

 of each tube from the free extremity. The velocity with which the fluid is dis- 

 charged can be determined by measuring the quantity, q, discharged in a unit of time, 



(i second) and dividing it by the area of the tube, Tzr 2 ; viz., v= 2 . Inasmuch as 



the tube is of uniform diameter the velocity through each cross-section will be the 

 same. 



As the water flows through the horizontal tube it meets with resistance, namely, 

 the cohesion and friction of its molecules, and the adhesion between the walls of 

 the tube and the water which must be overcome if the flow is to continue. Because 

 of the fact that water will moisten most surfaces with which it comes in contact 

 there will be an adhesion between the walls of the tube and the outer layer of the 

 column of water, in consequence of which it will become more or less stationary. 

 Between the outer stationary layer and the axis of the stream, there is an infinite 

 number of layers of molecules, the cohesion of which one for the other is more and 

 more overcome by the pressure in the vessel, P. The force of adhesion between 

 wall and fluid together with the force of cohesion between the molecules of the fluid 

 give rise to the resistance of the fluid to the flow. 



As a result of the resistance the forward movement of the water under the 

 pressure in P, is somewhat retarded, and as a consequence it will exert a lateral or 

 radial pressure against the walls of the tube. That such a pressure exists is shown 

 by the rise of the fluid in each of the vertical tubes, and the height to which it rises 

 in each tube is a measure of the pressure at its base. In the tube /, the fluid rises 



