THE CIRCULATION OF THE BLOOD 333 



in pairs and formed by a reduplication of the internal coat, strengthened 

 by fibrous tissue. They are always directed toward the heart and in close 

 relation to the walls of the veins, so long as the blood is flowing forward. 

 An obstruction to the flow causes the valves to turn backward until they 

 meet in the middle line, when they act as a barrier to regurgitation. Under 

 these circumstances the elastic tissue permits the veins to distend and accom- 

 modate the blood. With the removal of the obstruction the recoil of the 

 elastic tissue, and perhaps the contraction of the muscle-tissue, forces the 

 blood quickly onward. The veins of many viscera, of the bones, of 

 the cranial and vertebral cavities are devoid of valves. The function of 

 the veins is therefore to collect the blood from the capillary areas and trans- 

 mit it to the heart. 



THE FLOW OF THE BLOOD THROUGH THE VESSELS 

 HYDRODYNAMIC CONSIDERATIONS 



The blood flows through the arteries, capillaries and veins in accordance 

 with definite laws. During its transit certain phenomena are presented by 

 each of these three divisions of the vascular apparatus. These are mainly 

 velocity and pressure and in the arteries alone an alternate expansion and 

 recoil of the arterial wall with each heart beat, termed the pulse. Since these 

 phenomena, as well as the laws which govern them are similar to, though 

 more complex 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 con- 

 sidered : 



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 UNDER A STEADILY ACTING PRESSURE 



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. 148, 

 which consists of a horizontal tube into which is inserted at equal distances a series 

 of vertical tubes , and which is connected with a reservoir or pressure vessel, P. If 

 the reservoir be filled with a liquid, water for example, the latter under certain con- 

 ditions 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 

 exerting an equal pressure in all directions, will now show its downward pressure, 

 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, Trr 2 ; viz., v= ~ r Inasmuch as 



