THE MAIN FACTS OF THE CIRCULATION. 155 



same time through the section c as passes through a or b. But if just as 

 many particles of water have to get through the narrow section a in the 

 same time as they have to get through the broader section c, they must 

 move quicker through a than through c, or more slowly through c than 

 through a. For the same reason water flowing along a river impelled bv 

 one force viz., that of gravity rushes rapidly through a " narrow " and 

 flows sluggishly when the river widens out into a " broad." The flow 

 through B will be similarly slackened if B, instead of being simply a single en- 

 largement of the tube A, consists of a number of small tubes branching out 

 from A, with a united sectional area greater than the sectional area of A. 

 In each of such small tubes, at the line c, for instance, the flow will be 

 slower than at a, where the small tubes branch out from A, or at b, where 

 they join again to form a single tube. Hence it is that the blood rushes 

 swiftly through the arteries, tarries slowly through the capillaries, but 

 quickens its pace again in the veins. 



An apparent contradiction to this principle that the rate of flow is de- 

 pendent on the width of the bed is seen in the case where, the fluid having 

 alternative routes, one of the routes is temporarily widened. Suppose a tube 

 A dividing into two branches of equal length x and y which unite again to 

 form the tube V. Suppose, to start with, x and y are of equal diameter ; 

 then the resistance offered by each being equal, the flow will be equally 

 rapid through the two, being just so rapid that as much fluid passes in a 

 given time through x and y together as passes through A or through V- 

 But now suppose y to be widened ; the widening will diminish the resistance 

 offered by y, and in consequence, supposing that no material change takes 

 place in the pressure or force which is driving the fluid along, more fluid 

 will now pass along y in a given time than did before ; that is to say, the 

 rapidity of the flow in y will be increased. It will be increased at the ex- 

 pense of the flow through x, since it will still hold good that the flow through 

 x and y together is equal to the flow through A and through V. We shall 

 have occasion later on to point out that a small artery, or a set of small 

 arteries, may be more or less suddenly widened without materially affecting 

 the general blood-pressure which is driving the blood through the artery or 

 set of arteries. In such cases the flow of blood through the widened artery 

 or arteries is for the time being increased in rapidity, not only in spite of, 

 but actually in consequence of, the artery being widened. 



It must be understood in fact that this dependence of the rapidity of the 

 flow on the width of the bed applies to the general rate of flow of the whole 

 circulation, and that, besides the above instance, other special and temporary 

 variations occur due to particular circumstances. Thus changes of pressure 

 may alter the rapidity of flow. The cause of the flow through the whole 

 system is the pressure of the ventricular systole manifested as what we have 

 called blood-pressure. At each point along the system nearer the left ven- 

 ;ricle, and therefore further from the right auricle, the pressure is greater 

 lan at a point further from the left ventricle and so nearer the right auricle ; 

 it is this difference of pressure which is the real cause of the flow from the 

 one point to the other ; and other things being equal the rapidity of the flow 

 will depend on the amount of the difference of pressure. Hence, temporary 

 or local variations in rapidity of flow may be caused by the establishment 

 of temporary or local differences of pressure. For example, at any point 

 ilong the arterial system the flow is increased in rapidity during the tem- 

 porary increase of pressure due to the ventricular systole, i. e., the pulse, 

 and diminished during the subsequent temporary decrease, the increase and 

 decrease being the more marked the nearer the point to the heart. And we 

 shall probably meet later on with other instances. 



